ADAPTIVE CODING, MODULATION, AND POWER CONTROL FOR POSITIVE TRAIN CONTROL SYSTEMS

Abstract

To improve the throughput of packets transmitting positive train control (PTC) messages over an asynchronous wireless network supporting train motion control, data rates are adjusted based on one or more link conditions. Modulation and coding schemes with less overhead can be used to increase the data rate when the estimated link quality or conditions are relatively good. Conversely, when the estimated link quality is relatively poor, more robust modulation and coding schemes can be used to maintain link performance but at the cost of reduced data rates.Optionally, if the estimated link quality is higher than that required to achieve a predefined maximum transmission rate at a given predefined power rate, the transmission power can be reduced to that required to transmit at the predefined maximum transmission rate.

Description

ADAPTIVE CODING, MODULATION, AND POWER CONTROL FOR POSITIVE TRAIN CONTROL SYSTEMS Field of Invention The invention relates to a wireless radio data network used with railway control systems, particularly positive train control systems. Background of the Invention Railroads in the United States and Canada have implemented centralized traffic control (CTC) systems that allow a dispatcher at a central office or central dispatch office to monitor and control interconnections and traffic flow within a designated territory. Interconnections generally refer to signaling arrangements that prevent conflicting train movements through crossings and junctions. A dispatcher may, in some circumstances, directly control the signal indications given by train movement authorities for a track block. Additionally, a dispatcher may sometimes need to be able to directly control switches that, for example, allow a train to move onto a through track, cross over to an adjacent track, or divert to an alternate track or route.A CTC system can also ensure that trackside devices or equipment, such as switches, are properly configured before and during a train's movement through a track block. In addition to receiving status information from signals and switches, the CTC system can also collect status information from other types of trackside devices, such as rail integrity / track circuits and hazard detectors. A complex, interconnected collection of wireless and wired networks is typically used by a central office to communicate with trackside devices and trains. Wireless networks are usually distributed over large geographical areas and consist of radio base stations linked to each other and to central offices by communication links that are typically wired but not necessarily limited to wired communication links. Base stations are used to establish and maintain wireless communication links with locomotives, service vehicles, trackside devices, and operating systems within the base station's coverage area. A positive train control (PTC) system is designed to prevent train-to-train collisions, derailments due to speeding, incursions into work zone boundaries, and the movement of a train through a switch left in the correct position. Like a CTC system, messages in a PTC system rely on wireless communication links to transmit messages between the functional subsystems used to control movement on railways. These functional subsystems include trackside units such as crossing signals, switches, and interlockings; mobile units, such as locomotives and other equipment that travel on railways, and their onboard controllers; and dispatch units located in central offices.Each functional subsystem consists of a collection of physical components comprising computers or other types of information processing equipment programmed to perform control processes, data storage components for storing databases and other information, and communication interfaces through which messages are exchanged. A PTC system is “interoperable” if it allows locomotives from a host railroad and a tenant railroad to communicate with and respond to the PTC system, while supporting uninterrupted movement across property lines. Interoperability for PTC systems has been mandated for some railroads under the Railroad Safety Improvement Act of 2008 (Public Law 110-432 of 2008). To support the implementation of positive train control, Class I freight railroads formed PTC-220 LLC to secure the 220 MHz spectrum as a data radio infrastructure for carrying PTC data between base stations and mobile units and at the trackside. Designing and operating a communications system for a transportation industry to support interoperability, particularly one as complex as the railroad system in the United States, requires addressing many constraints. In the railroad industry, for example, a reliable and efficient communications system must be able to handle different types of information, including data transmitted from the central rail office and trackside systems to the locomotive's onboard computers, as well as voice transmissions between train crews and the central office.Wireless communication systems that support interoperable positive train control (IPTC) must also meet the requirements and objectives of the Railroad Safety Improvement Act of 2008 and the transmission band requirements mandated by the Federal Communications Commission (FCC), including, for example, those related to frequency band allocation, channel width, and spacing. Furthermore, an IPTC system must also meet all the engineering demands imposed on any system deployed in the harsh operating environment of railroad tracks. An example of a wireless communication protocol that supports message exchange to provide interoperable train control is ITCnet®, which was developed by Meteorcomm, LLC of Renton, Washington. ITCnet® is capable of supporting, for example, messages for CTC, IPC, IPTC, and other systems used by railroads in North America. U.S. Patents Nos. 8,340,056, 8,602,574, and 10,710,620, which are incorporated herein by reference for all purposes, disclose and describe various aspects of communication processes enabled by ITCnet®. Figure 1 is a high-level schematic representation of basic functional subsystems or components of a railway control system. In this representative example, the railway control system 100 supports wireless communications between a central office (or network operations center) 101 and locomotives 102 and other track vehicles located at various points around a railway system, as well as direct communications between the locomotives 102 and the trackside electronic surveillance subsystems. In the communications systems 100, the central office 101 communicates with packet radios on locomotives 102 through a wired telecommunications network and a series of packet radio base stations dispersed over thousands of square miles of geographic area through which the railway system operates. Figure 1 illustrates only two, representative radio base stations 103a and 103b. The communications system 100 also includes a series of trackside monitoring subsystems, which monitor trackside systems such as signals, switches, and track circuits, and communicate the monitored information directly to locomotives 102 within the corresponding wireless coverage area, as well as to the central office 101, via base stations 103a and 103b. FIG. 1 shows two representative trackside monitoring subsystems 104a, 104b, and 104c. As examples of typical uses of trackside monitoring subsystems 104, trackside monitoring subsystem 104a is shown monitoring a switch 105 and a signal 106, and trackside monitoring subsystem 104b is shown monitoring a manual switch 107. Also, for illustrative purposes, two parallel track sections 108a and 108b, and a connecting section 109, are shown in FIG.1, which represents only a very small part of the global track system. In the following discussion, a “remote radio” refers to a radio that is not at a base station. Remote radios are, for example, radios mounted on locomotives 102 and other rail vehicles, trackside radios 104a, 104b, and 104c, and other radios geographically separated from Central Office 101, and which are not radios at base stations 103a and 103b. Mobile remote radios refer to remote radios mounted on locomotives 102 and other rail vehicles, or any other remote radio that can change location. Base station and remote radios can be implemented using software-defined radio (SDR). An SDR offers several advantages, including multi-channel capability. For example, a multi-channel remote radio on a locomotive can receive information from a base station and a trackside monitoring subsystem simultaneously. Additionally, with an SDR, locomotives and base stations can receive status messages from multiple trackside monitoring subsystems simultaneously. This capability supports communication across a high density of trackside monitoring stations in urban areas. One challenge with interoperable train control applications, such as IPTC applications, is the need to maintain multiple communication paths between various communication nodes within the system. Furthermore, these multiple communication paths must support the exchange of different types of information while still complying with all wireless regulatory requirements imposed by the FCC. For example, a communication path must be maintained between mobile remote radios on locomotives and a central office to support the exchange of information such as locomotive location reports, locomotive status and diagnostic data, movement authorities, files, and network management data. A separate communication path must be established between mobile remote radios on non-locomotive rail vehicles (not shown) and the central office. Data traffic on this route includes vehicle location reports, work reports, email, and material requests. Another set of communication paths is required to maintain communications with the fixed remote radios located along the sides of the railway tracks. In this case, a communication path is required between the trackside radios and the central office to support the signaling system condition and status monitoring, centralized control of checkpoints, and alarms and data from the trackside fault detection system. An additional communication path is required between the mobile radios on locomotives and the trackside radios, which supports trackside status updates provided to locomotives in the vicinity of a given set of tracksides. In a PTC system, trains typically require a status update for each approaching trackside. For each trackside within 5.63 kilometers (3.5 miles ahead of a train, the trackside condition age should not exceed 12 seconds with six 9s (i.e., 99.9999%) reliability. It is also desirable that trackside condition updates be sent to the central office. Finally, another communication path is required between mobile remote radios on locomotives and non-locomotive rail vehicles and mobile remote radios on other locomotives and non-locomotive rail vehicles. This path supports peer-to-peer proximity position reporting so that a mobile radio knows the locations of nearby mobile radios. IPTC systems utilize channels within a group of RF frequencies in the 220 MHz band, with the channel plan specified by the FCC in 47 CFR §90.715. The FCC channel plan describes the 5 kHz channels. However, when a licensee is authorized on adjacent channels, the 5 kHz channels can be aggregated in the contiguous spectrum. The bandwidth of a channel for IPTC is currently specified at 25 kHz. It comprises five (5) adjacent 5 kHz channels in the FCC channel plan. This makes at least four pairs of 25 kHz channels in the 220 MHz band currently available for IPTC. IPTC systems use each 25 kHz frequency channel in half-duplex mode, meaning that a single channel is used as a communication path in both directions between two connected radios, but only in one direction at a time. In other words, each frequency channel supports transmissions from both a base station radio and remote radios, but not simultaneously. If more than one radio transmits on the channel at the same time, a signal collision occurs, resulting in the loss of all transmissions. Mobile radios can transmit on channels in the 221-222 MHz range or on channels in the 220-221 MHz range. Base stations are currently only permitted to transmit on channels in the 220-221 MHz range. For IPTC applications, frequency channels in the 220 MHz band are paired with the frequency channels used by base station radios and the frequency channels used by mobile radios. Each base station radio transmission frequency is taken from the 220-221 MHz range and paired with a mobile radio frequency in the 221-222 MHz range. According to current FCC regulations, a mobile radio can transmit or receive on either a mobile radio or a base station radio frequency, while a base station radio can only transmit on a base station radio frequency.In the future, the FCC may also allow base station radios to transmit on a mobile radio frequency, subject to certain antenna height and power restrictions. For example, a base station radio transmitting on a mobile radio frequency may be limited to antennas less than 7 meters tall or power levels below 50 Watts ERP. In a wireless IPTC network such as ITCnet®, the available 25 kHz frequency channels are divided into two groups: local channels and common channels. A common channel is shared by all radios in the base station and remote radios. A local channel is used to support traffic from all users within a base station's coverage area and is centrally controlled by that base station using a master-slave architecture. Each base station typically controls only one local channel but could control more than one. Each local frequency channel is controlled and managed by a base station. Each remote radio can listen to multiple 103 base stations, but a remote radio can select only one 103 base station as its master; other 103 base stations are considered neighboring base stations of the remote radio. Different local channels can be assigned to adjacent 103 base stations to prevent interference, and the same local channel can be reused by multiple 103 base stations that are widely separated to increase spectral efficiency. A set of 25 kHz channels on the base station frequency is established as primary local channels. Since 103 base stations can transmit at higher power on the base station radio frequency, using channels on the base station radio frequency for local channels provides greater coverage than using channels on a mobile radio frequency. Based on the currently available 220 MHz IPTC spectrum, at least three 25 kHz channels on a base station radio frequency can be established as primary local channels. In high-density areas where three primary local channels are insufficient to support traffic, additional local channels can be used. In ITCnet®, a 25 kHz channel is preferably reserved for a common channel. The common channel should be on a base station radio frequency that allows both 103 base stations and remote radios to transmit on the channel. This common channel is shared by all users employing the CSMA scheme described above. A packet transmitted on the common CSMA channel is typically a short packet carrying very high-priority data. The common channel can also be used to transmit base station beacon signals, which carry information necessary for remote radios to identify and select a base station radio, as well as to configure their receive frequencies. Brief Description of the Invention Briefly, the following describes methods for improving the performance of communication links in an asynchronous wireless network used to transmit messages for train control, particularly supporting positive train control systems on railways. These methods allow for adjusting the data rate transmitted from one radio to another over a wireless communication link based on one or more link conditions. For example, modulation and schemes with less overhead can be employed, thereby increasing throughput or data rate when estimated link quality or conditions are relatively good, while maintaining the relatively high performance levels required for reliable train control.Conversely, when the estimated link quality is relatively low, more robust modulation and coding schemes can be used to transmit messages while maintaining the desired or required level of link performance, but at the cost of reduced throughput, meaning a lower data rate. Alternatively, if the estimated link quality is higher than required to achieve a predefined maximum data rate at a given predetermined power level, the transmission power is reduced. Brief Description of the Drawings Figure 1 is a high-level schematic representation of basic functional subsystems or components of a railway control system. FIG. 2A is a schematic representation of a multiple access scheme for an IPTC wireless network. FIG. 2B is a schematic representation of a DSB cycle of the IPTC wireless network multiple access scheme represented by FIG. 2A. FIG. 2C is a schematic representation of a DTDMA cycle of the IPTC wireless network multiple access scheme represented by FIG. 2A. FIG. 3 schematically represents a structure for an RF packet for an IPTC wireless network. FIG. 4 is a flowchart that represents a link quality estimation process with an estimated average signal-to-noise ratio generated from received signals. FIG. 5 is a flowchart depicting a process of a remote radio determining a data rate and determining a base station to use the data rate. FIG. 6 is a graph that represents relating error rates and signal-to-noise ratios at given data rates. FIG. 7 is a flowchart that illustrates processes performed by a base station and a remote radio to adapt data rates on a channel when the remote control initiates a transmission. FIG. 8 is a flowchart that illustrates processes performed by a base station and a remote radio to adapt data rates on a channel when the base station initiates a transmission. FIG. 9 is a flowchart of a process for a remote radio to determine a requested transmission power. FIG. 10 is a signal constellation diagram for DQPSK. FIG. 11 is a signal constellation diagram for D8PSK. FIG. 12 is a constellation diagram for 16DAPSK. FIG. 13 is a schematic representation of the basic elements of a base station radio and a remote radio that are used to provide adaptive coding and modulation and adaptive power control for the radios. Detailed Description of the Invention In the following description, similar numbers refer to similar items. The ITCnet® network is a radio network from Meteorcomm Communications, LLC that is currently used by Class I railroads, short-track and commuter rail companies, systems integrators, and Positive Train Control (PTC) hosting providers in the United States to enable interoperable train control communication. U.S. Patents Nos. 8,340,056, 8,602,574, and 10,710,620 describe various details of the various protocols used by the ITCnet® wireless network. Each of these patents is incorporated herein for all purposes. Briefly, ITCnet® is an example of a suite of protocols suitable for use in networks that support IPTC. The following description of the adaptive coding, modulation, and power processes in a wireless network that supports IPTC systems modifies the ITCnet® protocols and uses the modified ITCnet® protocols as an example for implementing the processes in an IPTC wireless network. ITCnet® protocols already exist, defining data communication processes in an IPTC network or communication system at the application, network, data link, and physical layers. Data link layer protocols, when implemented by two nodes on the same communication link, allow each node to transmit and receive data from the other. These protocols also define processes by which nodes can detect and, in some cases, correct transmission errors, as well as processes by which a node can detect a new neighbor or whether a neighbor is offline. These processes implement forward error correction (FEC), cyclic redundancy check (CRC), and packet acknowledgment.Additionally, the data link layer protocol supports over-the-air channel access based on TDMA (Time Division Multiple Access) and CSMA (Carrier Sense Multiple Access) schemes. At the physical layer, the ITCnet® protocols specify methods for sending raw bits across a physical wireless channel from one radio to another. For example, they define modulation properties, bit rate, bandwidth, frequency, synchronization, and multichannel processing. With reference also to Figures 2A, 2B, and 2C, which schematically represent multiple access schemes used by ITCnet® for communication links between neighboring radios in a wireless network. A multiple access scheme defines processes that allow the base station and remote radios to cooperate in sharing available channel resources. ITCnet® is a representative example of a multiple access scheme for an IPTG-compliant wireless network. Like the ITCnet® protocol in general, the access schemes represented in the figures are intended as representative, not exhaustive, examples of multiple access schemes suitable for a wireless link supporting interoperable train control messages. The principles of the processes described below can be implemented with other multiple access schemes. The ITCnet® scheme combines two basic types of multiple access schemes: time-division multiple access (TDMA) and carrier-sense multiple access (CSMA). For a particular local frequency channel 200, three different multiple access cycles are grouped into a cyclic or periodically repeating superframe 201 of fixed duration. The superframe duration is the same for each local frequency channel and is set within a transmission interval on the side of the path. Superframes 201 are therefore synchronized across all local channels. Each superframe 201 has two partitions: a fixed time-division multiple access (FTDMA) partition 202; and a dynamic time-division multiple access (DTDMA) partition 206. In FTDMA partition 202, local frequency channel 200 is time-scheduled. The FTDMA slot size can vary from slot to slot, but the channel time allocation to each user is fixed. FTDMA partitioning can be used to transmit constant, periodic traffic from remote radio users. For example, a fixed number of FTDMA slots, each with a fixed slot size, are periodically reserved for use by a remote radio at a fixed repeat rate. In the ITCnet® implementation, one FTDMA slot is used to support one FTDMA packet. Each user can be assigned multiple FTDMA slots to transmit multiple RF packets (each of which is also referred to as a "packet" in this description). The FTDMA configuration is pre-set based on the channel capacity and the channel frequency required to send FTDMA data for each user on the network.For example, FTDMA partition 202 might contain one FTDMA 204 cycle, which can be used to support periodic traffic such as periodic WIU (Wayside Interface Unit) status messages. When transmitting on the FTDMA partition, a remote radio can use a GPS pulse slot synchronization, making it independent of the base station's radio transmission. Each FTDMA 204 cycle on a channel would have the same length, determined by the anticipated or expected amount of periodic traffic on the channel. However, if more than one FTDMA cycle is used on the FTDMA 202 partition, the FTDMA cycles can have different lengths. Furthermore, the fixed length of the FTDMA cycle on each local channel can be the same as, or different from, the fixed lengths of the FTDMA cycles on other local channels. The DTDMA partition is the local frequency channel 200, also with time slots, but a base station controls the allocation of time slots for use by the base station and remote radios (or “users”) to transmit RF packets. The partition consists of one or more consecutive Dynamic Short Broadcast (DSB) cycles followed by one or more consecutive DTDMA cycles. The DTDMA partition on a given local frequency channel can be controlled by a single base station or shared by multiple base stations. If the local frequency channel 200 is controlled by a single base station, that base station controls the entire DTDMA partition. In this case, the duration of the DTDMA partition at the base station is set equal to the duration of a superframe minus the duration of the FTDMA cycle.When local channel 200 is shared by two or more adjacent base stations, those base stations coordinate their transmissions within the DTDMA partition. The entire duration of the DTDMA partition is divided among the sharing base stations. For example, with N adjacent base stations sharing local channel 200, the DTDMA partition is divided into N parts, one for each of the N base stations. At the beginning of each DSB 210 cycle, the base station that controls the local frequency channel for that DSB cycle transmits a DSB 212 control packet. Following the control packet are one or more DSB 214 slots. The slots can have variable lengths. Each DTDMA 208 cycle is controlled by a 103 base station. A DTDMA 208 cycle is used to support traffic from a 103 base station and remote radios. All traffic within a DTDMA 208 cycle is organized by the 103 base station controlling that DTDMA cycle. The length of a DTDMA 208 cycle depends on the amount of traffic, which can vary from one DTDMA cycle to the next. Therefore, DTDMA can be of variable length in this example. The start and end times of the DTDMA partition are configurable parameters that can be set on the base station radio. The base station radio uses GPS synchronization as the reference time to start and end the DTDMA partition in each 201 superframe. Each slot in a DTDMA 208 cycle is allocated for a single RF packet transmission from a base station or a remote radio. A base station can allocate a slot for a remote radio transmission to a specific remote radio or configure it as a contention slot. Contention slots are not assigned to a specific remote radio. All remote radios access the channel during a contention slot using a slotted carrier sensing multiple access (CSMA) scheme. At the beginning of each DTDMA cycle, the base station in control of the local channel transmits a control packet in a variable-length control packet slot 216 at a predetermined rate. The slot following the control packet is the variable-length base station transmit slot 218, during which the base station can, for example, transmit a unicast message intended for a particular remote radio. Next is a variable-length remote radio transmit slot 220, which is allocated to a specific remote radio for transmissions. The two remaining slots, the HP CSMA slot 222 and the AP CSMA slot 224, are each a variable-length contention slot accessed using a CSMA scheme. Slotted CSMA is a variation of a basic contention-based access scheme where a physical channel is shared by users (i.e., base station and remote radios) with a mechanism to prevent collisions between multiple users attempting to access the channel simultaneously. CSMA requires users to listen to the channel before transmitting to avoid potential collisions with other ongoing transmissions. Generally, when a user has a packet to transmit, they wait a random period of time during which the channel is detected. If the channel is idle, the user transmits the packet immediately. If the channel is busy, the user reschedules the packet transmission to a future time (chosen somewhat randomly), at which point the same process is repeated. In slotted CSMA, packet transmission must begin at the start of a time slot. The slot size can be shorter than the time required to transmit a packet. When a user has a packet to transmit, they choose a random integer and wait for that number of slots to occur before scheduling a transmission. The user then checks the channel, and if the channel is idle, transmits their packet at the beginning of the current slot. If the channel is busy, the user chooses another random integer and reschedules the packet transmission, as in basic CSMA. The maximum backoff time (i.e., the interval of random integers) is configurable. Timeouts for different data priorities can also be set to different values ​​to improve latency performance. A DTDMA slot allocation in each 208 cycle can be performed by a scheduler at the base station based on transmit queue information from the base station radio and associated (connected) remote radios. The DTDMA slot allocation is transmitted by the base station radio in the DTDMA control packet sent in control slot 216. To enable the scheduler to obtain information about the remote radio transmit queues of the associated radios, each associated remote radio sends an update of its transmit queue information to base station 103 as needed. The remote radio can transmit its queue information in remote radio transmit slot 220 or in a contention slot.At the end of each DTDMA cycle, the scheduler uses the currently available queue information from each user (that is, base station radio or remote radio) to determine slot allocation in the next DTDMA cycle. Base stations and remote radios transmit data in RF packets, which are blocks of binary data structure. Figure 3 schematically illustrates the physical layer structure for packet transmissions in ITCnet®. Different packet types can be specified in the ITCnet® network. However, all ITCnet® packet types share a common structure to allow newer radios to be backward compatible with older radios. A packet (302) has two parts: a packet header and the data. The packet header in this example includes fields for a preamble (304), a physical layer or “layer 1” header (“L1 header”) (306). The remaining length of the packet is its payload (308), which can include additional message data and overhead. The preamble is used by the receiver to synchronize and detect bits of information in the packet. In the mode used by ITCnet®, the preamble is 8 bytes long. The L1 header (306) is used for packet detection and is three bytes long. Link layer overhead is used by a receiving radio, specifically its receiver, to decode and extract data from the packet. Link layer overhead may include, for example, data indicating the packet type, address identifiers, a cyclic redundancy check (CRC) code, and a forwarding error correction (FEC) code. In the ITCnet® packet, the L1 header includes a 3-bit field (310) for a CRC used to validate the L1 header decoding. The CRC is followed by the Type field (312), which contains a value (8 bits in this example) indicating the packet type.The “MOD and FEC” field 314 is a 4-bit value that indicates the modulation type and FEC being used. The Mod and FEC field value points to an entry in a lookup table containing information about the modulation and encoding used to transmit the payload 308. In this example, the table could have up to 16 possible entries, meaning that up to 16 different combinations of, for example, modulation type, FEC type, encoding rate, and / or interleaving could be specified. The Data Length field 316 is the number of bytes in your payload 308, excluding the L1 header 306 and preamble 304, before any applied FEC. In this example, it is an 8-bit field. The information in the payload 308 comprises overhead and data. The length of the information can vary, depending on the packet type and the data within the packet. The L1 header, in this example, is always modulated and encoded using a fixed scheme. For example, the L1 header can be modulated using differential quadrature phase-shift keying (DQPSK) and encoded with a convolutional code at a coding rate of 0.5. In this example, the encoded L1 header would have a length of 6 bytes (the input to the convolutional encoder will be 24 bits and the output 48 bits). However, as mentioned above, the information in payload 308 is modulated or encoded using schemes indicated by the value in the MOD and FEC 314 field. This allows the use of adaptive modulation and encoding (ACM), which may be different from the modulation and encoding used for the L1 header. The processes below describe the implementation of ACM's IPTC-enabled wireless networks, such as ITCnet®, to increase the overall performance, efficiency, and reliability of at least one, and preferably each, communication or radio link between a base station and a remote radio on a local channel, based on the communication link condition. By dynamically changing the modulation and forward error correction for packets sent on the link in response to measured link conditions, the processes effectively convert link margin into increased data throughput. When link conditions are favorable, higher-order modulations and forward error correction (FEC) schemes with lower overhead can be used to increase data transfer.Conversely, under poor link conditions, robust modulation and coding can be used to maintain the link, but with reduced performance. In fading channels that model wireless propagation environments, adaptive modulation radio transmitters implementing the processes described herein have been found to perform better than non-adaptive radio transmitters, exploiting channel knowledge within the transmitter. To adapt the modulation and coding, the transmitter needs information about the radio link or channel. The transmitter can obtain this information in two ways. It can assume that the channel it is using for transmission is in a similar state to a similar channel on which it is receiving transmissions from other radios. Alternatively, it can receive information about the radio link status from the radio receiving its transmissions on that link. This feedback can be transmitted on the same local frequency channel or on a different one. In an example of an ACM implementation in an IPTC wireless network, each remote radio receiver measures or estimates the signal-to-noise ratio (SNR) of one or more transmissions from a base station to which it connects. A high SNR indicates a good link, and a low SNR indicates a poor link. Each remote radio measures the SNR by estimating it from the signals received over a certain period of time. The remote radio then informs the base station radio to which it connects of the link condition, either by sending an indication of the SNR or a recommended modulation and coding combination. Tailoring the coding and modulation of transmissions across a radio link will result in a predictable data throughput rate and provides the opportunity to increase the rate where possible. The base station and remote radios in an IPTC wireless network can implement adaptive power control (APC) methods, either instead of or in addition to APC. In APC, a radio automatically reduces its transmission power below a predetermined level if it is not required to maintain a connection of a certain quality between the base station and a remote radio. For example, the power level might be reduced to equal or greater than that required to achieve a desired transmission speed on a link with a desired performance level. Transmitting at a power level no higher than necessary to achieve a given quality can reduce signal interference in the IPTC wireless network. Dynamically adapted power levels can be used with coding and modulation adaptation based on the estimated link quality, as represented by, for example, the estimated SNR. If the estimated SNR is greater than that required to achieve the maximum transmission rate, then the transmission power is reduced to the level required to achieve the maximum transmission rate. Otherwise, the radio transmits at maximum power for the selected modulation. In an example of an ACM implementation in a wireless train control network, such as ITCnet, a local channel is controlled and managed by a base station. A remote radio connected to the base station can send a request for data rate and transmission power level to the connected base station. The base station will then inform the remote radio of the data rate and power to transmit. Each remote radio estimates the quality of the communication link between itself and its connected base station by using packets transmitted by the base station on the local channel. For each packet received from the base station, the remote radio estimates the signal-to-noise ratio (SNR). The remote radio then determines the average SNR by averaging the estimated SNR values ​​of multiple received packets. For example, in the ITCnet protocol described above, there are several broadcast packets from the base station sent at a predetermined rate that can be used by the remote radio to estimate link quality. These packets include, for example, any one or a combination of two or more of the following: the base station radio control packet transmitted in DSB control slot 210, the base station control packet transmitted in DTDMA control slot 216, and base station beacon broadcasts sent periodically at a configured or predetermined fixed interval. In addition to broadcast packets, a remote radio can optionally use unicast packets that a base station directs to the remote radio to estimate the communication link quality. Unicast packets are sent in base station transmission slot 218 of the DTDMA cycle. Figures 4 through 8 illustrate non-limiting examples representative of process modes for radios within an IPTC wireless network that adapt the encoding and modulation and adapt the power transmission levels based on the estimated link quality. The flowchart in Figure 4 illustrates the basic steps of a remote radio link quality estimation process. The process can be implemented, for example, by hardware circuits programmed on special-purpose digital signal processors, or software running on general-purpose microprocessors, or a combination of both, used to implement the radio. Process 400 is intended to produce a value that reflects the average signal-to-noise ratio (SNR), which is the noise level relative to the average received signal level, regardless of channel conditions. As indicated by decision block 402, the process is performed for each packet received from a base station to which the remote radio connects during a specified, configurable period.The packets can be one of the broadcast packets, any or more or all of the broadcast packets, the unicast packets, or all packets of any type transmitted by the base station and received by the remote radio. The signal received by the remote radio is processed in stage 404 through an RF chain, with the processed signal being converted into a baseband signal. Although optional, it is preferred that the amplitude and phase distortion caused by fading channels be removed in stages 406 and 408 so that they do not affect the estimated SNR value. The radio then generates a value in stage 410 that is an estimate of the SNR of the received signal using, for example, a mean square error estimation process. The SNR estimate is stored in stage 412. To obtain the average SNR, the radio performs a process in step 414 to generate an average of the estimated SNR values ​​over a given period. In one mode, the average SNR is a weighted motion average of the estimated SNR values ​​obtained by multiplying the weights by the estimated SNR values ​​of multiple packets received during a predetermined period, which is a motion window of a predetermined length. The window length is configurable. The default window length, for example, can be set to 8 seconds, which is equivalent to 2 ITCnet superframes. If the window length is T seconds, the average SNR can be generated according to the following equation: τ SNRWI, = w.SNR,11Equation (1) Where Nτ is the number of base station packets received within T seconds before the average time, SNR¡ is the estimated SNR of the imo received packet, and w¡ is the weight of the imo received packet. The set of weights w¡ is configurable. Examples of default weights are w¡=1 / NT for all i. Nτ can be different for each window, depending on the number of base station packets received in that window. With reference now to FIG. 5, a flowchart depicting the basic steps of a remote radio's 500 process during which it connects to a base station, estimates the link quality, determines the appropriate data rate for the link to meet the packet success rate requirements, and sends a request for data transmission using the determined data rate. In stage 502, the remote radio connects to a base station on a local channel. When a remote radio wants to connect to a base station, it initiates the connection process by sending a packet, referred to herein as an “acquired” packet, to the base station using the network's default data rate. The default data rate, for example, can be set to a rate that will meet the required performance metrics under most, if not all, conditions. An example of a default data rate for a remote radio on a locomotive radio is 24 kbps in the ITCnet versions in use at the time of this writing. Therefore, communication between the base station and the remote radio on a communication link begins with incoming messages from the remote radio. The remote control sets the data rate for the link in stage 504 to the default preset rate for the wireless network. A data rate reading is stored in the remote radio's memory. In step 506, the remote control begins a link quality estimation process. Step 400 in Figure 4 is an example of this process. The process will continue. Any changes in channel condition are stored. Based on the estimated or measured link quality, the process determines a data rate for the link in stage 508. If the radio determines that the data rate should be set to a rate different from the currently stored data rate, the memory is updated with the determined data rate, and this becomes the data rate that the remote radio will request from the base station to use for transmission over the link. The determination or update of the stored data rate based on the estimated link quality can occur periodically, in response to the activation of a timer, in response to a change in the stored value of the channel condition or quality metrics—for example, the SNR—determined by the radio in stage 504, in response to receiving data for transmission (or another signal indicating the need to transmit data), or a combination of any two or more of these events.These are not exhaustive examples. In this example, the estimated average SNR generated using, for example, process 400 (FIG. 4) is used to select a maximum data rate that can be achieved while still meeting the link's performance requirements. In one implementation example, this determination is made using a lookup table that associates data rate values ​​with an average SNR. The data rate values ​​in this example are maximum data rate values ​​determined based on modeling that takes into account the link's performance requirements and, in this mode, other possible real-world conditions. The data rate values ​​for a given estimated average SNR in the lookup table can also be established (and optionally dynamically updated) using, in whole or in part, field measurements. One metric for the performance of a communication link between a base station and a remote radio on a given local channel to support train control is the packet error rate. Because communication must be highly reliable for train control, particularly PCT, the percentage of successful packet transmissions to support train control must be relatively high, such as 90% or greater. At 90%, the packet error rate would be 10%. Furthermore, a wireless network supporting PCT must support moving trains, which in some cases are capable of speeds up to 160 mph. Channel fading is possible. Therefore, in addition to the performance requirements for positive train control, this example accounts for channel fading due to voltage movement, establishing a maximum data rate for a given SNR when using modeling. With brief reference to Figure 6, this graph plots the result of modeling packet error rates as a function of SNR at different data rates. Each curve represents predicted packet error rates (y-axis) as a function of SNR (z-axis) at a given data rate. Using this information, the estimated link quality metric—the average SNR in this example—can be associated with a predetermined maximum SNR for a given modulation scheme and coding rate. The following is an example of a table (Table 1) that associates achievable data rates using the specified modulation parameters and encoding rates with the SNR required to maintain a 90% packet success rate at those data rates. The required SNR also includes a margin to account for implementation loss and estimation errors that might occur in actual operation. Table 1 SNR (dB) Modulation Coding Rate Data Rate (kbps) 15 DQPSK 1 / 3 10.7 17 DQPSK 1 / 2 16 19 DQPSK 3 / 4 24 20 DQPSK 7 / 8 28 24 D8PSK 3 / 4 36 27 D8PSK 7 / 8 42 30 16DAPSK 3 / 4 48 33 16DAPSK 7 / 8 56 44 64DAPSK 3 / 4 72 46 64DAPSK 7 / 8 84 Table 1 is a representative, non-limiting example of an adaptive modulation and coding scheme for increasing or decreasing data rates. With each modulation scheme, two or more coding rates are used; the higher the coding rate, the higher the data rate achieved using the same modulation scheme. However, alternatively, a single data rate could be used for a given modulation scheme, or some modulation schemes may have only one coding rate. Each of the modulation formats or schemes in this example is based on modulating the phase of a carrier signal using phase-shift keying (PSK), with the symbols encoded differentially. Such schemes are generally referred to as differential phase-shift keying (DPSK) and allow for non-coherent data transmission.At lower data rates, the most reliable forms of DPSK are used. At lower data rates, the DPSK modulation scheme uses 4 phases (known as differential quadrature phase-shift keying or DQPSK) and 8 phases (D8PSK). At higher data rates, the amplitude of the carrier signal is also modulated using a modulation scheme known as differential amplitude phase-shift keying (DAPSK). In this example, the 16DASPSK and 64DAPSK formats are used to achieve the higher data rates. In alternative modes, different modulation schemes can be substituted. Software-defined radios are used to implement transmitters and receivers capable of modulating and demodulating packets sent using different modulation and encoding schemes. Referring again to FIG. 5, when the remote radio wants to transmit data, as indicated in decision step 510, it sends a request to the base station in step 512 using a packet containing a data rate request. The requested data rate is set to the stored data rate determined in step 508. The packet may optionally also contain an indication of the amount of data to be transmitted and / or a priority. In step 514, the remote control transmits the request to the base station at the default rate for the wireless network, not at the requested data rate. In an ITCnet wireless network, for example, the remote radio sends an incoming slot request in which to transmit in the form of a QSTAT packet, which is sent in CSMA slots 222 or 224 of a DTDMA 208 cycle (see FIG. 2A-2C). The QSTAT packet includes information indicating one or more of the following: the requested data rate determined in step 508; the amount of incoming data to be transmitted; and the data priority. Figure 7 is a flowchart for a 700 process performed by the base station and remote radio after the remote radio sends a request for an incoming transmission slot, as in step 514 of process 500 (Figure 5). After the base station receives the packet, such as a QSTAT packet, in step 702 from the remote radio, the base station records the requested data rate from the remote radio and resets a timer for that requested data rate in step 704. If the base station receives any requested data rate information from the remote radio before allocating a time slot to the remote radio, the base station updates the requested data rate from the remote radio and resets the timer. In step 706, when it is time for the base station to allocate channel time for the incoming transmission from the remote radio, the base station selects the data rate for the remote radio to transmit and allocates the time slot based on the selected data rate and the amount of data requested by the remote radio. As indicated by steps 708, 710, and 712, the base station selects the requested data rate from the remote radio if the timer for that data rate has not expired. Otherwise, the base station selects the default data rate for the network. The base allocates a time slot, as indicated by step 714, and in step 716 transmits a control packet to the remote radio containing information about the allocated transmission slot and the selected data rate. In one mode, the packet is transmitted at the default data rate for the network.For ITCnet, the base station control packet is transmitted at the beginning of the DTDMA 208 cycle. DTDMA base station control packets, in one mode, are always transmitted at the default rate. After the remote radio receives the control packet from the base station at stage 718, it can transmit the data or payload portions of the packets at stage 720 using the default modulation and coding scheme and parameters defined for the selected data rate for the channel specified by the base station in the control packet. However, the remote radio can instead transmit other types of packets, including, for example, a control packet (such as an ITCnet QSTAT packet) in the allocated time slot to provide the base station with updated information, such as a change in the requested data rate, the amount of data to be transmitted, and / or the data priority. The control packet is transmitted at the default data rate, using the default modulation and coding scheme defined for that rate. With reference to FIG. 8, if communication between a base station and a connected remote radio begins with an outgoing message from the base station, in step 802, the flowchart depicted describes a representative process between the base station and the remote radio to adapt the data transmission rate based on link conditions. As indicated by steps 804, 806, 808, and 810, the base station selects a data rate for outgoing transmission to the remote radio by checking the stored data rate for the remote radio, which should be the last requested data rate received from the remote radio. If the timer that started when the requested data rate did not expire, the base station selects the stored data rate as the rate for outgoing transmission to the remote radio.If the time expires, the base station selects the default data rate. The base station also allocates a time slot after the outgoing slot in which it transmits the message in stage 814 so that the remote radio can acknowledge receipt of the outgoing packet. After receiving the outgoing packet from the base station in step 816, the remote radio determines an updated requested data rate for communication with the base station using an estimate of the link quality and data rate, which it determines with, for example, processes 400 (FIG. 4) and 500 (FIG. 500). In step 818, the remote radio transmits an acknowledgment packet to the base station in its assigned acknowledgment slot. The acknowledgment packet may also include one or more of the following types of information: the amount of data in the remote radio's transmit queue; and the remote radio's updated requested data rate. When the base station receives the acknowledgment packet in step 820, it stores the remote radio's requested data rate and resets the timer for that stored data rate. As previously mentioned, in the ITCnet protocol, the local channel is managed by the base station. Radios associated with that base station transmitting in their assigned slots do not interfere with each other. However, with base station frequency reuse, radios associated with different base stations using the same frequency channel can transmit simultaneously and interfere with each other. By using lower transmit power, if channel conditions allow, the base station and remote radios have the option to reduce transmit power to minimize interference from other radios simultaneously transmitting on the same frequency channel. When done only when the maximum permissible data rate can still be maintained, power can be reduced without compromising channel throughput. In one mode, the transmit power is reduced from a predetermined transmit power level only when the base station or remote radio is transmitting at a highly permitted data rate. At lower data rates, the base station and remote radios transmit at maximum power and use an ACM process, such as those described above, to adjust the transmitted data rates on a link based on the link quality, as described above in conjunction with Figures 4 through 8. When the link quality between a base station and a remote radio is good enough to allow transmissions on the link at the highly permitted data rate with lower transmit power while still meeting the link performance requirement, the radios on the link can reduce their transmit power below the predetermined power level. Figure 9 is a flowchart depicting an example of process 900 for adapting the transmit power of a remote radio, along with the processes described above and Figure 5. As indicated by steps 902 and 904 in Figure 9, if the data rate estimate determined in step 508 (Figure 5) is less than the highest data rate, the transmit power for the remote radio is set to the predetermined transmit power in step 902. 904. If, as indicated by step 906, the SNR is not greater than an SNR threshold, which may be the SNR used to select the highest data rate plus a margin, the transmit power is also set to the default transmission in step 904. An example of a margin that can be used is 3 dB. Otherwise, a setting to the transmit power is stored by the remote control in step 908 based on the estimated SNR minus the SNR threshold minus the margin. Therefore, in step 508, both a data rate and a transmit power are set and stored by the remote radio. In the processes described above, when the remote radio transmits a packet to the base station containing a data rate request, it also includes a requested transmit power. However, it can alternatively omit the requested transmit power from the packet if the requested data rate is not the highest available data rate, in which case the base station can assume that the requested transmit power is the default transmit power. Thus, for example, when the remote radio sends an incoming slot request to the base station using the default data rate and default transmit power, which is step 514 in FIG. 5, the request also includes a requested transmit power in addition to a data rate request and, optionally, other information such as the priority and amount of data to be transmitted to the base station. This information, in an exemplary ITCnet mode, is formed into a QSTAT packet that is sent in the CSMA slots of the DTDMA cycle as discussed earlier. Therefore, the QSTAT packet includes the following information: amount of incoming data to be transmitted; data priority; requested data rate; and requested transmit power. In process 700 of FIG. 7, after the base station receives the packet from the remote radio requesting an incoming slot in stage 702, the base station records, in addition to the requested data rate, the requested transmit power from the remote radio and resets the timer for the stored data rate and transmit power in stage 704. As previously described in conjunction with FIG. 7, if the base station receives any requested data rate and transmit power information from the remote radio before the base station allocates a time slot to the remote radio, the base station updates the remote radio's requested data rate and transmit power and resets the timer. When the base station transmits a control packet in stage 716, it also includes the selected transmit power and data rate. Therefore, if the timer has not expired, the base station selects the stored data rate and transmit power that the remote radio last sent and includes them in the control packet. Otherwise, the base station selects the default data rate and transmit power for the incoming transmission and includes them in the control packet. The control packet is always transmitted at the default data rate and transmit power. In stage 720, the remote radio transmits in the assigned slot using the selected data rate and transmit power specified in the control packet.If the remote radio chooses to send a packet, for example an ITCnet QSTAT packet, in the allocated time slot to provide updated base station information, it includes the requested transmit power in addition to the requested data rate. In process 800 of FIG. 8, when the base station selects a data rate in stage 806, it also selects a transmit power for the outgoing transmission to the remote radio by verifying the stored requested data rate and the remote radio's transmit power. If the timer for the requested data rate has not expired, the base station selects the stored requested data rate and transmit power for the outgoing transmission to remote radio stages 812 and 814. If the timer expires, the base station instead selects the default data rate and power. Upon receiving the outgoing packet from the base station in stage 816, the remote radio uses the selected data rate and transmit power from the base station for communication with the base station. In the allocated acknowledgment slot, the remote radio sends an acknowledgment packet to the base station.The reconnaissance packet may include, in addition to the amount of data in the remote radio's transmission queue and the requested data rate from the remote radio, the requested transmission power from the remote radio. When the base station receives the reconnaissance packet, it updates and stores the requested data rate from the remote radio and resets the timer, as indicated by step 820, and also updates and stores the requested transmission power from the remote radio. For direct peer-to-peer communications between two remote radios, the remote radios can establish a link over one or more pre-designated common local channels and use a contention access scheme, such as CSMA, to access the channel. In the ITCnet protocol, direct peer-to-peer communications are supported on common channels (also known as DirectRF channels). Common channels are unorganized and shared by any ITC radio on the network. Each remote radio listens to the common channels. A remote radio (the transmitting remote radio) can transmit to another remote radio on one of the common local channels when the transmitting remote radio finds the channel idle. If the remote radio to which the packet is sent (the receiving remote radio) successfully receives the transmitted packet, it responds with an acknowledgment packet.The remote radio can also send another data packet immediately after sending the reconnaissance packet. The concepts of adaptive coding and modulation, along with adaptive power control processes described in Figures 4, 502 through 510 of Figure 5, and 9, can be applied to direct peer-to-peer communications between two remote radios. Such processes are effective when the two remote radios have multiple packets to transmit to each other and the communication is not intermittent. The remote radios always begin transmitting at a predetermined data rate and power. After peer-to-peer communications continue for a period of time, the radios are able to estimate the link quality based on the received signals and select transmission rates and / or power levels accordingly. If communication becomes intermittent or packets are lost, the radios restart the process.To determine the appropriate data rate and transmission power, each remote radio can apply process 400 using signals received from the peer remote radio. The time period for estimating link quality can be configured. ITCnet in general, and in particular the multiple access schemes and packet structures represented by Figures 2A-2C and 3, are intended as non-limiting, representative examples of multiple access schemes and packet structures that can be used with the processes described herein. Although they can be used advantageously with wireless networks based on ITCnet® protocols according to the processes described above, the processes are adaptable for implementation in other types of wireless networks that support train control. They are not limited to use with ITCnet® protocols except to the extent expressly indicated. The following describes further details and examples of adaptive coding and modulation (ACM) and adaptive power control (APC) methods that adaptively adjust to the transmission scheme (TS) used in a channel link based on the channel link quality for wireless networks used to support rail and similar transportation systems, and in particular wireless networks that support train control. The methods are described with reference to those using relevant or substantially similar ITCnet protocols. Aspects of the methods may find use in other types of wireless networks that support real-time applications. The TS values ​​when using ACM and APC in the examples below are the combination of modulation, forward error correction (FEC) coding scheme, and transmit power for the channel link. However, if only ACM is used, the TS value is the combination of modulation and FEC scheme used by the channel link. If only APC is used, it is the transmit power. The possible modifications, alternatives, and examples are given in the context of a wireless network implementing the ITCnet protocol, but they could be used in networks with similar protocols, including future versions of the ITCnet protocol. For ITCnet or other ITCnet-like wireless networks, ACM and APC apply to unicast traffic. However, this does not preclude ITCnet or other wireless networks from applying ACM, APC, or both, as described herein or in other ways, to other types of traffic. Unicast traffic can be sent between a base station and a remote control on a local channel or between remote controls on a DirectRF channel. The following description focuses primarily on ACM and APC for unicast communications between the base station and remote controls on the local channel. However, each can be used on a DirectRF channel. ACM and / or APC can only be applied to select packets used by the wireless system. Table 2 below is an example of applying ACM and APC to packets in ITCnet. The table lists the packet types on the left and indicates whether ACM and APC apply to them. Table 2 Applied Package Name ACM / APC Base Signaling No QSTAT (optional) ACK No ACQ No TOD No CNTL No DSB CNTL No DSB SBM No FSB SBM No LBM No UNICAST Yes hrnzzn / zznz / q / uιλι Apply ACM / APC only to unicast messages (UCMs) to ensure there is no negative impact on Positive Train Control (PTC) operation when using the ACM / APC scheme on ITCnet. However, for further optimization, ACM / APC can be applied to other packets such as CNTL and ACK. ACM and / or APC can also be applied to a QSTAT packet (a remote transmit queue status message) transmitted in the R-TX section of the DTDMA cycle. If ACM / APC is not applied to QSTAT and a base station is scheduling a high-speed UCM, a QSTAT message (not high-speed) may not fit in the required slots. The slots must always be large enough for QSTAT messages, or the QSTAT messages must be able to be accepted. It is preferred that ACM / APC not be applied to QSTAT messages transmitted in the CSMA section of the DTDMA cycle. If all connected remotes have a suitable SNR, a CTL packet could apply ACM / APC at the level of the worst SNR of the connected remotes, unless HP or AP CSMA is being allocated. For communications in the 220 MHz interoperable band using the ITCnet protocol, the modulations and coding schemes for ACM identified in Table 3 below have been found to be effective. Table 3 shows the modulations and coding for ACM and the percentage change from the default rate used in radios currently used for ITCnet. The default rate for ACM refers to full rate, which is 24 kbps, using DQPSK modulation with a 3 / 4 FEC coding rate (convolutional coding). The FEC scheme for other rates in the table is convolutional coding. The lowest data rate for ACM specified below is the same as that used in current PTC operation, since one use of ACM / APC is to increase channel capacity. The ACM and APC methods described herein could be applied at lower data rates to increase signal coverage.Table 3 includes only three modulations: DQPSK, D8PSK, and 16DAPSK, for baseline purposes. Higher modulations such as 64DAPSK and QAM could also be employed to further increase channel capacity. Table 3 Modulation Coding Rate (Convolutional) Data Rate (kbps) % Change DQPSK 3 / 4 24.0 0.0 DQPSK 7 / 8 28.0 17% D8PSK 3 / 4 36.0 50% D8PSK 7 / 8 42.0 75% 16DAPSK 3 / 4 48.0 100% 16DAPSK 7 / 8 56.0 133% DQPSK modulation refers to Differential Quaternary Phase Shift Keying (DQPSK) modulation. For DQPSK, the encoded bit sequence is paired into sets of two-bit binary data, where k is the symbol index. These bits are mapped to the k-th complex-valued symbol, where k is the phase transition factor. The phase transition factor is calculated by applying Gray encoding to the two binary bits, according to the DQPSK modulation in Table 4. Table 4 (T Ck / 0) Lk 0 0 π / 4 0 1 3 ^ / 4 1 1 3 ^ / 4 1 0 -π / 4 The modulation symbol K is formed by applying a phase shift to the previous symbol , ,. ¿¿i, — sirdif—i — — 1 xix and is defined as κκ κ i κ i , where . Alternatively, phase transitions can be represented as + ^k. The corresponding signal constellation diagram for DQPSK is shown in FIG. 10. D8PSK modulation refers to differential phase-shift keying modulation. The encoded bit sequence is grouped into three-bit binary data sets ckckck, where k is the symbol index. Similar to DQPSK modulation, these bits are mapped to the k-th complex-valued symbol Λ, where k is the phase transition factor. The phase transition factor Λkck(2) is calculated by applying Gray encoding to the three binary bits Λk, Λk, according to the D8PSK modulation in Table 5. Table 5 cir (1Ί ck <01 ck' ΔΦ* 0 0 0 π / 8 0 0 1 3π / 8 0 1 1 5π / 8 0 1 0 7π / 78 1 1 0 -7π / 8 1 1 1 -5π / 8 1 0 1 -3π / 8 1 0 0 -π / 8 The modulation symbol K is formed by applying a phase shift to the previous symbol 1 = 1 = 1^^^^ ^0 — 1 λ i+ χ· . . * 1 and is defined as * * * 1 κ i , u Alternatively, the phase transitions can be represented as The corresponding signal constellation diagram for D8PSK is shown in FIG. 11. 16DAPSK (Differential Phase-Shift Amplitude Manipulation) is a modulation scheme that combines 8-DPSK (Differential Phase-Shift 8 π / 8 Manipulation) and 2-D8PSK (Differential Amplitude-Shift Manipulation). The constellation diagram for 16DAPSK is shown in FIG. 12. The constellation is composed of two rings, each containing two sets of eight constellation points, each corresponding to an alternating phase shift of π / 8 between consecutive symbols. A ring factor α is defined as: where au and aH (aL < aH) are the amplitude levels for the inner and outer rings, respectively. Analysis shows that the optimal value for a is 2. The encoded bits are grouped into four-bit binary data sets c(0)f(3) k, k, k, y k, where k is the symbol index. These bits are mapped to the k-th complex-valued symbol Sk: c — r ¿k—'k^ where rk is the amplitude transition factor, and A0k is the phase transition factor. The phase transition factor dec(0)c(l)c(2) is calculated by applying Gray encoding to the three binary bits sk,k, yk, tie according to the 16DAPSK phase transitions in Table 6. Table 6 ck <2> Ck(1) Ck(<” ΔΦΚ 0 0 0 π / 8 0 0 1 3π / 8 0 1 1 5π / 8 0 1 0 7π / 8 1 1 0 9π / 8 1 1 1 11 π / 8 1 0 1 13π / 8 1 0 0 15π / 8 The remaining binary bit k determines which of the two possible 8-DPSK rings is used: the inner ring with amplitude ai or the outer ring with amplitude aH. The amplitude of the current symbol ak is found by multiplying the amplitude of the previous symbol ak-i by the amplitude transition factor rk, as defined in Table 7. The transmitted symbol dk is therefore equal to: dk= akej^ = skdkA = rkej^ * ak_}eJ^ = rkak_ / ^+^ Equation 1 where dk-i is the previously transmitted symbol. Table 7 (3Ί Ck' rk 3k-1 = 3l 3k-1 = 3h 0 1 0 1 α 1 / α Table 8 is an example of a list of transmission schemes for the ACM and APC methods described above suitable for ITCnet or a protocol that is, in the relevant part, substantially similar. Table 8 Transmission Scheme (TS) Data Rate (kbps) Tx Power Modulation Coding Rate 1 24.0 Pmax DQPSK 3 / 4 2 28.0 Pmax DQPSK 7 / 8 3 36.0 Pmax D8PSK 3 / 4 4 42.0 Pmax D8PSK 7 / 8 5 48.0 Pmax 16DAPSK 3 / 4 6 56.0 Pmax 16DAPSK 7 / 8 7 56.0 Pmax-^P 16DAPSK 7 / 8 8 56.0 Pmax-2AP 16DAPSK 7 / 8 Additional modulations and transmission schemes may be used, including those with higher or lower values, or both. Pmax is the maximum power a radio is allowed to transmit. A base station must inform remote controls of the maximum power they can transmit. This can be done by including Pmax in a base signal transmission and allowing the remote control to transmit within the Pmax level. ΔP is a power adjustment stage in decibels (dB). ΔP should be set and adjusted based on testing. Power is controlled only in transmission schemes 7 and 8. If APC is not used or is disabled, the list of available transmission schemes would be limited to the first six. In this example, each radio (base station and remote control) is programmed or configured with a set of transmission schemes it can support. The base station and remote control exchange and agree on the set of permitted transmission schemes when the remote control initiates a connection with the base station. This set of transmission schemes, for example, could be sent in an L3 message. Further details of the message exchange between the base station and the remote control are provided below. For communication between two remote controls on a DirectRF channel, the remote controls must also exchange their set of transmission schemes when they first communicate with each other. In a transmission link established between two radios, each radio also sends a transmission scheme indication to the other radio (a "link member") during communications. The transmission scheme indication provides information about the transmission pattern for the communication link. Specifically, when a radio receives a directed packet from the link member, it estimates the quality of the communication link. The radio can either send the link quality estimate back to the link member or use the link quality estimate to determine the transmission scheme and send an indication (a TS indication) to the link member.Non-limiting and representative examples of transmission scheme indications that could be used to send to a link member include one or more of the following: an estimated SNR value, which may be the actual value or a code representing the estimated SNR or that the SNR is within one of two or more predefined ranges; an estimated link quality; an achievable transmission scheme value; an indication (a value) representing a step up or down from a current transmission scheme; an indication (for example, a binary value) indicating that the current transmission scheme is less than an achievable transmission scheme; and an indication (a value) representing an action to be taken, such as no change, step up, step down, or switch to a predetermined transmission scheme. Any or more of these TS indications may be used in the methods described in conjunction with Figures 1 through 9. Each of the TS indication examples has advantages and disadvantages. The last three examples may require the radio to have the current transmission scheme used by the link member, and therefore may be less desirable in situations where the radio may not have accurate knowledge of the transmission scheme currently used by the link member's radio. Although not required, sending only the estimated SNR may limit the optimization of the transmission scheme using other parameters for link quality estimation. Requiring the use of a TS indication other than SNR (or an indicative SNR value) or allowing multiple TS indications may allow for better optimization of the transmission scheme for the link. If a link quality estimate is sent, it will need to be defined, which may require the ability to update or accommodate previously deployed radios if the definition needs to change. The methods described for ACM and APC that allow a radio to determine a reachable TS and send it to its link member may be modified to allow different methods due to the protocol being used (including changes to ITCnet). This may require adding a method for exchanging ACM and APC information. This would allow, for example, backward compatibility with deployed radios, and additional packet types (or changes to existing packet types) could be implemented to enable the exchange of ACM and APC-related information. Representative examples of how this information can be exchanged as part of any of the methods described herein are shown below. In one example, ACM / APC information is included in a base signaling message. This information could include, for example, information (a value) indicating whether the base station transmitting the base signaling packet is ACM / APC compliant, a maximum power level that a remote control can transmit, or both. Another alternative is to place this information in a field appended to one or more pre-existing packet types used by the access scheme employed by the wireless network for other purposes. In the ITCnet protocol example, the field could be appended to a packet such as the ACK packet. However, adding a field would increase the packet's length, requiring more time for a radio to transmit it. Depending on the field's length, the duration of a slot in which such a packet can be transmitted—the DTMA slot unit (DSU) and / or FTMSA slot unit (FSU) in the ITCnet access scheme—might not be long enough, and therefore, it might be necessary to extend it. Other examples include adding a field to a unicast data packet, such as the UCM packet in ITCnet, or to a control packet. Alternatively, a new control packet, such as a Transmission Scheme (TS) control packet transmitted by a base station, could be used. The attachments for a TS control packet could include one or more of the following: a remote ID (for example, hrnzzn / zznz / q / uli, for each remote control that has a TS change) and a TS assigned to the remote control. This packet, for example, could be sent at a predefined rate. On ITCnet, it could be sent in the B-TX slot following the control packet. Alternatively, these fields could be attached to an existing base station control packet. However, using a TS control packet avoids backward compatibility issues. An alternative to adding a field to an ACK or unicast packet that would make it incompatible with deployed radios that are not updated or cannot be updated, is to use a new packet type to carry the TS indication. This would be an information packet type instead of a control packet type. Note that they could have the same format and therefore be a single control packet type. Another alternative is a Layer 3 message type that carries the list of transmission schemes used by the sending radio. This message could be sent between ACM-compatible radios when they are initially connected. Using a message with a list of transmission schemes allows different transmission schemes to be used on different radios. The message would be sent at the default rate. In ITCnet, the default time slot for FSU, DSU, and CDMA (CSU) slot units can be optionally changed to a 1 ms unit, reducing the time allocated for packet transmission and thus increasing capacity. CGR time units would be designed for original control packet sizes and full / half speed. Because NGR with ACM will support various speeds, using a 1 ms time unit for FSU, DSU, and CSU for all packet types will be advantageous. ACM / APC applies to unicast communications between radios. This includes unicast traffic between base stations and remote stations using DTDMA on the local channel, and unicast traffic between remote stations using DirectRF channels. The radio can be a base station or a remote station. The base station's link member is the remote station that connects to the base station. The remote station's link member can be the base station that connects to another remote station (DirectRF channel communication). The following are representative modes and examples of methods for ACM and APC in a wireless network such as ITCnet. When two radios initially connect, they exchange and agree on the set of transponders (TS) to be used for communication. After the initial connection, each radio sends a TS indication to the other radio, its link member, when it has an updated TS indication. Specifically, when the radio receives a directed packet from the link member, it estimates the link quality and determines the TS indication based on that quality, then sends the TS indication to the link member. If the TS indication is in the form of a reachable TS, the radio determines the reachable TS based on the estimated link quality and sends it to the link member. When the radio has a directed packet to transmit to the link member, it uses the received TS indication to decide which TS to use for the transmission. One or more methods can be used to measure and estimate link quality. One method for estimating link quality and determining TS is based on the average received signal-to-noise ratio (SNR) as a reference value. Figure 13 is a schematic representation of the basic elements of a base station radio 1302 and a remote radio 1304 used to provide adaptive coding and modulation and adaptive power control for the radios. (These are not complete schematic diagrams of the radios.) The base station radio 1302 and the remote radio 1304 each have a receiver 1306, an SNR estimator 1308, a link quality estimator 1310, and a TS indication determination logic or module 1312 to determine or decide on a transmission scheme and produce a signal or value representing a TS indication 1314. The receivers can be implemented as software-defined radios using, for example, a gate array or a digital signal processor. Each of the modules can be implemented as programmed processes or logic using gate arrays, digital signal processors, general-purpose processors, or a combination thereof.The use of the same part number for modules or other elements of the base and remote radios does not imply that the hardware and software for an element is or should be the same in each radio. Elements may be implemented differently in a base station radio and a remote radio. However, each would be configured or adapted to perform at least the methods described in this document. The SNR estimator 1308 estimates the signal-to-noise ratio of a packet received by receiver 1306. Its input is the received signal corresponding to the packet. Its output is an estimated SNR value. The link quality estimator 1310 estimates the communication link quality from the link member to the radio from the estimated SNR values—and optionally, other information that might be available about the link quality—and provides an average SNR (or link quality indicator) to the link quality estimator 1310. The TS indication determination logic or module 1312 determines a transmission scheme that uses (or responds to) the output of the link quality estimator 1310 and outputs the TS indication 1314. The TS indication can specify an achievable TS or be one or more of other types of TS indications as described above.The TS indication determined by a radio (radio A) indicates which of the permitted transmission schemes will have, based on the link quality estimate measured by radio A using a transmission from radio B to radio A, a desired margin in the channel link for a transmission from radio B to radio A. Each radio also includes a TS selection module (1316). The TS selection module for a given radio receives as input the current transmission scheme used by the radio and the TS indication from its link member radio. This allows it to compare the link member's TS indication with the one it is currently using. In response to the radio's request to select a transmission scheme, it selects the transmission scheme based on at least these two inputs. It can also take into account additional information the radio may have about the link quality, such as the packet loss count. The packet loss count is, for example, the number of packets transmitted unsuccessfully based on the number of addressed packets sent to the link member and not acknowledged by the link member. A value or signal is provided to the 1318 transmitter indicating the selected transmission scheme.In response, the transmitter configures itself to use the selected transmission scheme for the next transmission on the channel link. For example, when a directed packet is received on the radio, the SNR estimator estimates the received SNR of each packet received from the link member. Next, the link quality estimator estimates the link quality of the received packets. In one example, the link quality estimator obtains an average SNR from the estimated SNR values. Then, the TS indication is determined from the average SNR based on the estimated link quality. If the achievable TS is used as the TS indication, it is the highest TS the link member can use for transmission while still meeting link performance requirements. Once the TS indication is determined, the radio sends it to the link member. When the radio has a directed packet to transmit, the TS selection module is called to select the TS for transmission.TS selection takes the received TS indication and packet loss count as inputs, selects the TS based on these inputs, and then uses the outputs of the selected TS that the radio will use for transmission. The received TS indication is what the radio receives from the link member, determined based on the quality of the link between the radio and the link member. The following are additional representative examples of ACM and APC usage methods with a base station and remote radios in a wireless network to support messaging and communications in railway applications. The example is in the context of a wireless network using ITCnet protocols but can be adapted to other protocols. The methods can also be modified by the options described above. The process assumes that the base station is capable of ACM and / or APC, and that the base station has indicated this to remote radios that might connect to it, such as by using a transmitted signaling message that includes information about ACM / APC. This information might include, for example, an indication that the base station is ACM / APC compliant. If the base station is APC compliant, the base station's signaling might include the maximum power level at which a remote communication with it can transmit. A packet transmitted by the remote radio to the base station, such as a packet to initiate a connection with the base station radio—an acquisition (ACQ) packet on ITCnet—indicates whether the remote radio is ACM and / or APC compliant.The base station radio has a preconfigured or default set of transmission schemes that is backed up, and the remote radio also has a set of transmission schemes that it supports, which it sends to the base station when the remote connects. The base station determines which set of transmission schemes will be used in the link between the base station and the remote, and then sends this information to the remote. This ensures that any selected transmission scheme is known to and compatible with both the base station and the remote. However, other methods can be used in addition to or as a substitute for this method to agree on a list of allowed transmission schemes. More specifically, for a wireless network using the ITCnet protocol or one substantially similar in relevant aspects, a base station periodically broadcasts a base station signal. The base station signal advertises its ACM and / or APC capability using an indication in the signals. A remote radio that hears the base station signals decides whether to connect to the base station. If either the base station radio or the remote control lacks ACM or APC capability, ACM and / or APC are not used for channel bonding. More specifically, if the remote control lacks ACM / APC capability and receives a base signal containing ACM / APC information, it will not process the ACM / APC information in the base signal. To initiate a connection, it will send an ACQ to the base station without any indication that the remote control is ACM or APC compliant. The base station will notice this and use standard communication procedures or CGR with the remote control.If the remote control has ACM / APC capabilities but the base does not advertise these capabilities, the remote control stores information that the base station does not have this capability and uses CGR procedures or standard communication schemes with the base station. If the remote radio is ACM and / or APC compliant, it will store information indicating that the base station radio is ACM / APC compliant, along with the maximum allowable power level included in the base station signaling. The remote control sets the maximum transmit power for all communications under the base station within the maximum allowable power level: Pmax = min(Pmax_remote, Pmax_allowed). Pmax is the maximum power the radio uses for transmission; Pmax_remote is the maximum power the remote control can transmit (based on the radio specifications); and Pmax_allowed is the maximum permitted power (e.g., based on regulations). The remote radio sends an ACQ packet to the base station. The ACQ includes an indication that the remote radio is ACM / APC compliant. The base station and remote radios exchange messages to agree on the set of transmission schemes that can be used. Once the remote radio and the base station radio have stored the same set of selected or allowed transmission schemes, each selects the most reliable transmission scheme from the set as its default transmission scheme (TS_Default). The reliability of each transmission scheme is known. The process for agreeing on permitted transmission schemes takes place after the best station receives an ACQ from the remote control to connect to the base station. This can be done, for example, using the following method. First, the base station allocates a slot to the remote radio. The remote radio sends a packet containing the set of transmission schemes it can support to the base station. If the base station receives the packet, it acknowledges it in the next control frame. Otherwise, the base station allocates another slot to the remote radio. If the remote radio does not receive the acknowledgment, it attempts to retransmit the packet. Once the base station receives the set of transmission schemes from the remote radio, it determines which transmission schemes will be used between the base station and the remote radio. One option is to select all the base station's transmission schemes that are also compatible with the remote radio. The base station sends a packet containing the selected set of transmission schemes to the remote radio in a DTDMA B-TX slot. The remote radio records the set of transmission schemes and acknowledges receipt of the packet. If the base station receives the acknowledgment, it records the selected set of transmission schemes for the remote radio. Otherwise, the base station retransmits the packet in the next DTDMA cycle. The following applies to communications between a base station radio and a remote radio, as well as communications between remote controls. The radio described below may be a base station or a remote radio. Some of the ACM / APC methods described below are implemented on every radio, whether a base station or a remote control, and therefore can be applied to communications between a base station radio and a remote radio, as well as communications between remote radios. If only "radio" is mentioned, it can refer to either a base station or a remote radio. (1) Set the current TS to the default TS: TS_Default. (2) When a directed packet is received from the link member, (a) estimate the link quality, the communication link quality from the link member to the radio; and (b) determine the TS indication (call the TS indication determination module) and queue the TS indication. The queue is deep; overwrite anything previously queued. If reachable TSs are used for the TS indication, determine the reachable TSs and queue the reachable TSs.(3) If there is a TS indication in the received packet, extract the TS indication. Overwrite any previously received TS indication. This is the TS indication for the radio link to the link member. When a directed packet needs to be transmitted to the link member's radio, the following method can be used. (1) Select the TS (call the TS selection module). Set the current TS to the selected TS. (2) If the TS indication is queued and can be carried by the packet, include the TS indication. (3) Transmit the packet, using the current TS. When a radio has an updated TS indication in the queue, the radio should attempt to deliver the TS indication as soon as possible. After the TS indication is successfully transmitted (for example, an ACK packet is received or a reply is received), the radio removes it from the queue. When a remote radio has a TS indication to deliver, one example of a delivery method is as follows: (1) Add the TS indication to a UCM packet if one is pending for the intended destination. (2) Otherwise, append the TS indication to an ACK packet if one is pending. (3) Otherwise, send a TS indication packet at the earliest opportunity. For communication between the base and remote control, this can be sent in CSMA slots instead of sending QSTAT just to request time for this shorter packet. Sending a TS indication in response to the UCMs the radio receives from the link member can be the most efficient way to send this information if the link member is sending UCMs to the remote control. Therefore, it should be done every time a UCM is received. When a base station has a TS indication to deliver, it can: (1) Add the TS indication to a UCM packet if one is pending for the intended destination. (2) Otherwise, add the TS indication to a CTL packet that includes a slot allocation for the desired remote, if one is pending. (3) Otherwise, send a TS indication packet in the B-Tx section of a DTDMA cycle. When ACM and APC protocols are implemented on a wireless network using ITCnet protocols, the communication schedules between a base station and a remote control on local channels, as well as the local channel access schedule, can optionally be adjusted for the base and remote controls that are ACM / APC compliant. It may be necessary to allocate additional time for a field to carry the TS indication in the ACK and UCM packets. Since these slots are quantized over a specific time period—4 ms on ITCnet—no additional time may be required. However, to fit the packet containing the TS indication information into a slot, the size of the allocated slots could vary based on the TS indication sent by the remote radio. For example, if the base receives a TS indication indicating a lower or slower transmission scheme, the base system begins allocating longer slots.However, if the base station sends a TS indication of a higher or faster transmission scheme, it should not switch to allocating shorter slots until it detects that the remote station has sent packets using the higher or faster transmission scheme. On the other hand, if the time elapsed since the last successful slot allocation by the base station to the remote station exceeds a certain threshold (approximately equal to the timeout for automatic downgrade), the base station will begin allocating longer slots to allow for downgrade to a slower transmission scheme. If the remote station responds to an allocation message that increases the length of the allocated slots without changing the transmission scheme, the base station will optionally revert to the previous slot size. The following describes an example of a method for estimating SNR that can be used by a base station radio and a remote radio. In this example, interference is not distinguished from noise. When interference is present, the algorithm provides an estimate of the received signal-to-noise ratio and the interference ratio (SINR). In the ITCnet protocol, there are several base station broadcast packets that can be used by remote control to estimate link quality. These packets include one or more of the following packet types. The first type of broadcast packet that can be used is the DSB (Dynamic Short Broadcast) Base Control Packet, which is broadcast by the base station radio at the beginning of each DSB cycle. The DSB Base Control Packet is transmitted at a predetermined data rate. A second type is the DTDMA Base Control Packet, which the base station radio broadcasts at the beginning of each DTDMA cycle. The DTDMA Base Control Packet is also transmitted at a predetermined data rate. Refer to the access scheme shown and described in Figures 2A, 2B, 2C, and 3 for details regarding the cycles. A third type is Base Signaling.The base station radio broadcasts it periodically at a predetermined time interval, usually configurable. In addition to broadcast packets, the remote control can also use one or more of the unicast packets that the base station radio sends directly to the remote control to estimate the quality of the communication link. The unicast packet is sent during the Base Tx portion of the DTDMA cycle. At the base station, the base station radio can use one or more of the packet types it receives from the remote radio to estimate the received SNR. These include unicast data packets and other ITCnet packet types such as QSTAT, ACQ, and ACK. One goal of the SNR estimation algorithm is to generate a value that reasonably accurately reflects the average SNR (the noise level relative to the average received signal level) regardless of channel conditions. Ideally, the presence of amplitude and phase distortion caused by fading channels should have no effect on the estimated SNR value. To make the estimation algorithm insensitive to fading, the method can estimate and remove the amplitude and phase distortion. SNR estimation is performed, in one mode, at the baseband level. The radio receives the signal, processes it through the RF chain, converts the processed signal to baseband, and then performs the SNR estimation method using, for example, a gate array programmed or executed by a digital signal processor or central processing unit. The SNR estimation algorithm can be implemented by first removing phase distortion, then removing amplitude distortion, and then estimating the SNR using the mean squared error estimation. After the remote radio estimates the SNR of the received packet, it stores the estimated SNR value for further SNR averaging. A detailed SNR estimation algorithm and performance analysis are provided in the appendix. As previously mentioned, each remote control estimates the communication link quality between itself and its connected base station by using packets transmitted by the base station on the local channel. Similarly, the base station radio also estimates the communication link quality between the base station and each connected remote control by using packets received by the base station from the remote control on the local channel. There are different ways to measure and estimate link quality. One example of a method for measuring and estimating link quality uses an average signal-to-noise ratio (SNR). The link quality estimate can optionally be further improved by taking into account additional information such as packet error rate (PER), base station-to-remote distance, or both. For base-to-remote control communications, on the remote radio, an average SNR is determined for the downlink from the remote control to its connected base. On the base radio, average SNRs are determined for the uplink from each remote control connected to the base. For remote control communications on DirectRF channels, on each remote radio, the average SNR is determined for the communication link from the liaison member to the remote control. For each packet received from the link member, the radio estimates the SNR value of the received signal, using a method such as one described above. The radio then determines the average SNR by averaging the estimated SNR values ​​of multiple received packets. To obtain the average SNR, the radio performs a moving average of the estimated SNR values. The SNR estimate preferably includes packets of fixed length, such as CTL packets. However, it can also include packets of variable length, such as UCMs, using only the header portion of the packet. APC does not have a significant impact on the SNR estimate because power is only adjusted when the radio is operating at the highest data rate. The radio averages the estimated SNR values ​​over a period of time, T. The time window T is configurable and can be adjusted to optimize the estimation test data. The default time window is set to a predetermined interval, such as 8 seconds, which is equivalent to 2 ITCnet superframes. The average SNR can be determined by averaging the estimated SNR values ​​for all packets received during the time window. Further optimization can be performed later, for example, by using average weight, low-pass filtering, etc. The averaging process receives the estimated SNR values ​​as input and provides the average SNR as output. The average SNR at time t is obtained as follows. (1) Count the number of values ​​of Nt Estimated SNR of the desired link within T seconds from (tT) to t seconds. Let 1 be the number of estimated SNR values. (2) Determine the average SNR if there is a sufficient number of Nt estimated SNR values. If <J_min, finaliza el algoritmo. No se proporciona una SNR promedio. De otra manera, continúe con la Etapa 3. (3) Determine la SNR promedio de acuerdo a la siguiente ecuaciónΝτEcuación 2 SNR dB where i_ is the / -th estimated SNR value in dB. Nt Note that J depends on how many packets the radio receives during T seconds and may be different for each T-second window. It is considered valid when 1 NTNt Nt is at least _min. The J_min is configurable, and the default value is 2. J_min will be updated periodically when internal test data and / or field test data become available. The Nt update 1_min will be included in later specification releases where applicable. The average SNR can be used in a method that determines an achievable transmission scheme. In one example of the method, a data rate is first selected as the maximum rate that can be achieved while still meeting the link performance requirement. Then, if power control is enabled and the radio can transmit at the highest data rate with the lowest transmit power and still meet the link performance requirement, a transmission scheme with a transmit power lower than the maximum is selected. Therefore, the TS determination method takes the average SNR as the input and provides the achievable TS as the output. The method is as follows: (1) Note the average SNR, (2) Determine the achievable speed if the average SNR is valid. (2)(a) If the average SNR is valid, the achievable speed is the maximum speed that can be achieved: R = maxñj, con SNR.dB^J < (2)(b) Otherwise, terminate the algorithm. No achievable TS is provided. (3) Select full transmit power if APC is disabled. (3)(a) If APC is disabled, select full transmit power P = Pmax, and go to Step 5; (3)(b) otherwise, continue to Step 4 to determine the achievable power. (4) Determine the achievable power if APC is enabled. (4)(a) If the achievable rate is less than the highest data rate, select full transmit power: P = Pmax; (4)(b) If the achievable data rate is the highest data rate (R = Rmax), check if the SNR is greater than the SNR threshold plus the margin and determine the transmit power accordingly. The margin is configurable. The default margin is 3 dB. (4)(b)(i) If the SNR is not greater than the SNR threshold plus the margin, select maximum power: P = Pmax.(4)(b) (ii) Alternatively, the transmission power can be reduced by the following PowerAdjust: if R <Rmax, PowerAdjust = 0; de lo contrario PowerAdjust SNR dB = afg- _ SNR_dB(Rmax) - margen. Seleccione la potencia P = Pmax-kAP donde ΔΡ es la etapa de ajuste de potencia y k es el número entero máximo en el que kAP es menor que PowerAdjust. (5) Genere el TS alcanzable correspondiente a la velocidad alcanzable R y la potencia de transmisión P. La etapa de ajuste de potencia ΔΡ es un parámetro configurable que se puede ajustar con base en pruebas internas y de campo. To support train control, the performance requirement for communication links between a base station radio and a remote radio is set at a 90% successful transmission rate, which is equivalent to a 10% packet error rate. In PTC, the communication system is also required to support train speeds of up to 160 mph. Taking into account the requirements for packet success rate and train speed, the data rates and corresponding required SNR are then determined based on the transmission and reception performance over a vanishing channel. Using field test data and / or simulation testing, the data rates and required SNR to maintain the same packet success rate for each transmission are determined and placed in a table, as shown in Table 9. The required SNR preferably includes the implementation margin for lost counts and the estimated error that could occur in actual operation. Table 9 Data Rate (kbps) Modulation Coding Rate Single Antenna SNR (dB) Diversity SNR (dB) PAPR (dB) 24 DQPSK 3 / 4 19 12 3.8 28 DQPSK 7 / 8 20 13 3.8 36 D8PSK 3 / 4 24 19 4.3 42 D8PSK 7 / 8 27 21 4.3 48 16DAPSK 3 / 4 30 24 5.5 56 16DAPSK 7 / 8 33 27 5.5 PAPR is the Peak to Average Power Ratio and is obtained from laboratory measurements. The method for selecting a transmission scheme applies to both base and remote radios. A software-implemented process is called to run on a processor in the radio when the radio has a packet destined to be transmitted to its link member. The radio selects which transmission stage (TS) to use based on the TS indication the radio previously received from the link member. The radio may also consider a packet loss count when making the decision. In one mode of the method, when the link is good, the TS is slowly increased one step at a time from the default TS. The TS selected by the radio cannot be higher than the achievable TS. The default TS is the most reliable TS. The method also reverts to the default TS when no packets are received for a predetermined or set period of time and / or when packets transmitted at a higher TS are not successfully delivered for N consecutive packets.An example of a default value for N is 2. However, it is optionally a configurable parameter, as are the time periods. The radio optionally keeps track of packet loss for the unicast traffic it sends to the link member. In ITCnet, for example, the packet loss count is accounted for during the TS section for packets transmitted in the B-TX and R-TX sections but not in the CSMA sections of DTDMA cycles. Initially, the packet loss count is 0. After a radio transmits a directed packet, if the packet is acknowledged (for example, an ACK or reply is received), the radio resets the packet loss count to 0. Otherwise, it increments the packet loss count by one. The following is a representative, non-limiting example of a transmission scheme (TS) selection method that can be used on a base station, a remote station, or both. The TS selection algorithm takes the TS indication and the current TS as inputs and selects which TS to use. The TS selection algorithm generates the selected TS. The algorithm is called when a radio has a packet addressed to transmit. If more than N consecutive packets are lost, select the default TS: TSselect = TSdefault. Otherwise, if no TS indication is received for a predetermined or configurable time, select the default TS by setting the value of TSselect to TSdefault. Otherwise, store the TS indication received from the link member. The method also stores the current TS, TScurrent, which is the TS the radio is currently using.If the TS indication is the reachable TS, TSachiev, the method selects the TS as follows: If TScurrent < TSachiev, the selected TS is one stage higher than the current TS: TSselect = TScurrent+1. If TScurrent >= TSachiev, the selected TS is the same as the reachable TS: TSselect = TSachiev. The selection criteria above can be adjusted if another parameter is used as the TS indicator. The method's output is the selected TS. An example of an SNR estimation method for estimating the received signal-to-noise ratio (SNR) of received π / 4-DQPSK symbols is given below. The estimation algorithm is blind in that it does not require prior knowledge of the modulating data; it does not need to know the preamble bit pattern or header information, and it does not require the presence of any known pilot symbols, etc. Furthermore, the method is capable of providing accurate results over a wide range of channel conditions, including AWGN and flat frequency fading. For the estimation algorithm to be insensitive to fading, it needs to estimate and remove amplitude and phase distortion. The following steps are performed: a) remove phase distortion, b) remove amplitude distortion, c) estimate the SNR. To eliminate phase distortion after downconversion to baseband, the received symbols undergo several signal transformations. The first produces a sequence of QPSK symbols scaled by the channel's fading amplitude from a sequence of received symbols. Differential demodulation eliminates (or reduces to a minimum) the random phase of the channel, leaving only the amplitude fading and (though not included in the equations above) noise. The transformation is described by the following equation, where Any Φη are the amplitude and phase of the channel, respectively, at symbol index n: ^ = ^(4+14) farn=0 until N-1 Equation 3 where Φπ is one of the four possible phases, corresponding to the π / 4-DQPSK signal alphabet: Jπ 3π Equation 4 φ = ±—, ± — 4 Once the channel phase is removed, the next step is to separate the amplitude fading component from the random noise. Since the SNR estimation method is unaware of the data modulation, the underlying data is removed by doubling the signal constellation so that all received symbols occupy the upper RHS quadrant. This is achieved by converting all the / and Q components to positive values ​​(eliminating the negative signs): zn = abs{realjyn)) + j xabslima Equation 5 where y are the differentially demodulated symbols described in the previous section and the resulting symbols are z. The next step is to rotate the bend over the symbols by 45 degrees, so that the centroid of the received symbols lies on the real axis ( / ). The phase rotation is achieved by performing the following multiplication of complex values ​​for each output symbol of the equation 5. z' = z η n Equation 6 The fading component then moves to the real axis (the in-phase component), and the imaginary axis (the Q component) contains only noise. The next step is to normalize the signal level so that the average of the in-phase component ( / ) is scaled to a value of unity. This normalization step allows the SNR estimation algorithm to operate over a wide range of signal levels. The average of the / component (equal to unity after the normalization step) is then subtracted, so that the received “cloud” symbol is centered on the origin (l,Q = 0,0). The normalization and translation to (0,0) are performed as follows: z-=_____<__I Equation 7 ¿ni K-l1— 1=(1 where n is the symbol index, and K is the total number of symbols. By discarding the / component of the z!' symbols, the fading is eliminated and all that remains is the noise in the Q component: z'' = imag(z') Equation 8 After completing the various signal transformations described above, the remaining signal is a zero-mean signal with a variance equal to the variance of the received noise. The mean squared error is estimated as follows: «g Equation 9 mse = — N zz=O The output is then converted to SNR in decibels (dBs): snríIB= io logl()(i / mse) Equation 10 which is equivalent to: SNRdB= -10 log1()(mse) Equation 11 The foregoing description is of exemplary and preferred embodiments. The invention, as defined by the appended claims, is not limited to the described embodiments. Alterations and modifications to the described embodiments may be made without departing from the invention. The meaning of the terms used in this specification is, unless expressly stated otherwise, intended to have their ordinary and common meaning and is not intended to be limited to the details of the illustrated or described structures or embodiments.

Claims

1. A method for adapting one or more transmission parameters to transmit packets containing train control data over a wireless link between a first radio and a second radio in a wireless network that supports positive train control, comprising: estimating with the first radio a link quality metric for the wireless link based on packets received by the first radio from the second radio over the wireless link;determining with the first radio a data rate to transmit data over the wireless link to the second radio based, wherein determining the data rate comprises selecting a data transmission rate from a plurality of predetermined data rates based on the link quality metric for the wireless link, each of the plurality of predetermined data rates having a corresponding predetermined modulation and coding scheme capable of transmitting data at the predetermined data rate over a wireless link that has the estimated link quality metric while meeting at least one or more predefined performance metrics;and transmit with the first radio a packet consisting of at least a header portion and a payload portion over the link to the second radio, the payload portion being transmitted at the transmitted data rates using the predetermined modulation and coding scheme corresponding to the transmitted data rates.

2. The method according to claim 1 wherein the link quality metric is a signal-to-noise ratio.

3. The method according to claim 1 or 2, wherein at least one or more predefined performance metrics comprise a maximum error rate.

4. The method according to any one of claims 1 to 3, wherein the header portion is transmitted at a predetermined speed.

5. The method according to any one of claims 1 to 4, wherein the first radio is a remote radio and the second radio is a base station radio, the base station radio controlling the use of a local wireless network channel in a geographical area covered by the base station.

6. The method according to claim 5, wherein the base station radio controls access to the local channel with a predetermined multiple access scheme, the base station radio allocating slots to remote radios using the local channel to transmit packets containing data.

7. The method according to claim 5 or 6, wherein the remote radio estimates the link quality metric using unicast or wider transmissions from the base station over a period of time.

8. The method according to claim 7, wherein the time period comprises a sliding time window.

9. The method according to claim 7, wherein the link quality metric is an average of link quality metric estimates made over the time period.

10. The method according to claim 1, wherein transmitting the predetermined data rate to the second radio by the first radio at a predetermined data rate comprises transmitting the predetermined data rate in a control packet requesting the second radio to allocate to the first radio a transmission slot in a multiple access scheme over a local channel used by the link to transmit the train control data.

11. The method according to any one of claims 1 through 10, further comprising determining with the first radio a transmit power for the link, the transmit power being set equal to a predetermined transmit power or, if the estimated link quality metric exceeds the link quality metric required to select a higher data rate from among the predetermined data rates, to a lower transmit power that permits data transmission over the link at the higher data rate while meeting one or more predetermined performance metrics.

12. The method according to claim 11 wherein transmitting over the link a packet containing the determined data rate from the first radio to the second radio further comprises including in the packet the determined transmission power with the first radio.

13. The method according to claim 11 wherein transmitting over the link a packet containing the determined data rate from the first radio to the second radio further comprises including in the packet an indication of the determined transmission power with the first radio.

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