Indoor positioning Doppler effect compensation method based on adaptive filtering

Abstract

The invention discloses an indoor positioning Doppler effect compensation method based on adaptive filtering, and relates to a compensation method. The objective of the invention is to provide a method which can ensure Doppler compensation in a more flexible manner without the cost of transmission capacity. The indoor positioning Doppler effect compensation method based on adaptive filtering is used for compensating data in a frame transmitted through wireless transmission between a mobile body and a base station, and the mobile body and the base station respectively comprise a transceiver connected with an antenna used for wireless transmission. The method comprises the following steps: for a given moving speed and direction of the moving body relative to the base station, when a frame is transmitted as a signal over the wireless transmission, determining a Doppler effect acting on the frame; the method further comprises the step of applying, by the transceiver, a sampling process by assigning different time delays to the signal, the different time delays being selected following a rule adapted to compensate for the determined Doppler effect.

Description

Technical Field

[0001] The invention relates to the technical field of Doppler effect, and in particular to an indoor positioning Doppler effect compensation method based on adaptive filtering. Background Art

[0002] It is known that the spectrum of a wireless transmission between a moving vehicle and a base station fixed on the ground undergoes some changes due to the Doppler effect. This effect corresponds to a phase noise that the electromagnetic waves from such a wireless link suffer when the direction of motion of the moving object is parallel to the direction of propagation of the electromagnetic waves. For practical wireless transmission technologies based on well-separated frequency channels, this phase noise or signal degradation can usually be ignored.

[0003] However, for technologies based on multi-carrier transmission that attempt to use its full capacity, this may no longer be the case. This is a typical case for, for example, orthogonal frequency division multiplexing, which is a technology compatible with IEEE standard 802.16 used for wireless transmission, wherein the standard is also known as the acronym WIMAX for Worldwide Interoperability for Microwave Access, which appears to be one of the most promising technologies being discussed today for mobile two-way communications. For this technology, the sensitivity to the Doppler effect may no longer be negligible because the frequency channels are selected to have very narrow frequency intervals. For example, in OFDM, the frequency interval is set to cause the correlation between the modulated frequency band signal transmitted by the nth subcarrier of the multi-carrier transmission and the modulated frequency band signal transmitted by the n+1th subcarrier to become zero. The device based on the OFDM scheme on the transmitting side uses a serial / parallel converter to convert serial data into parallel data including multiple symbols. And an inverse fast Fourier transform operation unit is used on the frequency data to be converted into a time signal, wherein the subcarrier frequency components have been multiplexed. The real and imaginary parts of the result are used for quadrature modulation, and the transmitter up-converts the modulated signal to a high frequency signal. At the receiving side, the N symbols transmitted by N subcarriers over this wireless link are modulated and output by the reverse operation of the operation performed at the transmitting side.

[0004] According to OFDM, frequency allocation with overlapping frequency bands becomes possible, thereby being able to obtain an improvement in spectrum efficiency. OFDM is different from other multi-carrier transmission schemes that modulate its carriers independently, and because modulation / demodulation is completed once by FFT, an orthogonal relationship can be established between each carrier. In addition, by adding a guard interval signal on the transmitting side, inter-symbol interference caused by multipath delay can be eliminated. If the IFFT output signal following one OFDM symbol is used as a unit, the insertion of the guard interval signal means that the tail end portion of the signal is copied to its head end. In this way, using OFDM, multipath equalization is basically unnecessary. However, in order to avoid causing performance degradation, a guard interval larger than the maximum multipath delay time expected in the system must be set in a way that inter-symbol interference does not occur. Although the insertion of the guard interval makes it possible to eliminate the interference effect caused by multipath, the trade-off brought about is that the guard interval also reduces the transmission efficiency. In order to mitigate the decline in transmission efficiency, the OFDM symbol duration must be as long as possible, that is, the protection ratio is as small as possible. From this point of view, the carrier spacing within a given bandwidth should be smaller, that is, the number of carriers should be increased.

[0005] However, due to the fading, the received signal changes not only in the time direction, but also in the frequency direction, the latter being the Doppler shift. The Doppler shift, which is directly proportional to the speed of the moving body, is generated within the range of the maximum Doppler frequency. If the carrier spacing is small, this change is greater than one carrier, and it is difficult to achieve carrier synchronization on the receiving side. As a result, frequency selective fading occurs and the performance at the receiver is reduced, in which the maintained changes vary depending on the frequency. The reason for this result is that because the frequency fluctuations from one carrier to another (or more particularly, from one carrier group to another within the coherent bandwidth) are independent of each other, interference between carriers is generated. In order to suppress the degradation of performance caused by this interference, the carrier spacing must be as large as possible. In this way, there is a compromise on transmission efficiency.

[0006] For the IEEE 802.16 standard, which is one of the most popular technologies being discussed today for mobile two-way communications, the following estimates apply: up to 100 km / h, the effect is negligible; up to 200 km / h, the signal quality degrades slightly, but is still acceptable; above 200 km / h, the effect becomes increasingly important and seriously degrades the signal quality. At 400 km / h, which is the target speed for modern long-distance trains, Doppler compensation is absolutely necessary. This situation may be even worse when the technology used is based on smaller frequency channel spacing to increase the transmission capacity, i.e. the speed limit beyond which the Doppler effect becomes increasingly important may become smaller. Summary of the invention

[0007] In view of the above, it is an object of the present invention to find some alternative methods to the use of multiple antennas so that Doppler compensation can be ensured in a more flexible way without sacrificing transmission capacity.

[0008] The present invention discloses an indoor positioning Doppler effect compensation method based on adaptive filtering, which is used to compensate for data in a frame transmitted via wireless transmission between a mobile body and a base station, wherein the mobile body and the base station each include a transceiver connected to an antenna for the wireless transmission, and the method includes the following steps:

[0009] for a given speed and direction of movement of the mobile body relative to the base station, determining a Doppler effect acting on the frame when the frame is transmitted as a signal via the wireless transmission;

[0010] The method further comprises the steps of:

[0011] A certain sampling process is applied by the transceiver by assigning different time delays to the signal, the different time delays being selected following a rule suitable for compensating the determined Doppler effect.

[0012] The present invention provides an indoor positioning Doppler effect compensation method based on adaptive filtering, wherein for Doppler compensation on the receiving side, the sampling process is performed when a signal is received from the wireless transmission and before multiplexing the subsequently delayed samples to regenerate the frame received from the wireless transmission.

[0013] The present invention provides an indoor positioning Doppler effect compensation method based on adaptive filtering, wherein for Doppler compensation on the sending side, the sampling process is performed after multiplexing the generated frames and before transmitting the signal via the wireless transmission.

[0014] The present invention is an indoor positioning Doppler effect compensation method based on adaptive filtering, wherein the sampling process is completed by transmitting the signal through a conductive path with different outputs along the path, and the samples are tapped out at each output according to the time delay to be allocated.

[0015] The present invention provides an indoor positioning Doppler effect compensation method based on adaptive filtering, wherein the sampling process is completed by transmitting the signal through a conduction path that is a delay line with different propagation time properties.

[0016] The present invention is an indoor positioning Doppler effect compensation method based on adaptive filtering, wherein the sampling process is completed by using a first-in-first-out (FIFO) buffer to cache the signal in the FIFO at a write rate Rw, and the write rate Rw is different from the read rate Rr of reading samples from the FIFO, and the ratio between the rates Rr and Rw follows a rule suitable for compensating for the determined Doppler effect.

[0017] The invention provides an indoor positioning Doppler effect compensation method based on adaptive filtering, wherein a new determination of the Doppler effect is performed at equal time intervals.

[0018] The Doppler effect compensation method for indoor positioning based on adaptive filtering of the present invention is different from the prior art in that the Doppler effect compensation method for indoor positioning based on adaptive filtering of the present invention can apply the Doppler compensation method according to the present invention when receiving a signal from wireless transmission, before multiplexing the samples that are subsequently delayed. It can also be applied on the sending side as a "pre-compensation" method, that is, the sampling process is performed after multiplexing the generated frames and before transmitting the signal via wireless transmission. In a similar manner, the wireless frequency device according to the present invention can be implemented in a base station or in a mobile device, both of which participate in the wireless transmission. It is also conceivable that both the base station and the mobile device are equipped with such a wireless frequency device and / or the Doppler compensation method according to the present invention is applied. DETAILED DESCRIPTION

[0019] The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0020] A "telescope" delay line is inserted between an antenna mounted on a moving vehicle (here a train) and an access device on this vehicle. In this way, a variable time delay can be added to the received signal depending on the actual length of the telescope. If the vehicle moves towards a ground station (BS) during the transmission of a frame from the vehicle to the ground station (BS), the telescope is adjusted to a minimum length. And during the reception of the frame, the length of the telescope is continuously increased in order to add additional time delay corresponding to the amount of time delay that is reduced in the signal between the ground station and the vehicle due to the movement of the vehicle. In this way, it is possible to keep the total delay constant, thus compensating for any Doppler effect that degrades the transmitted signal.

[0021] The mechanical model described above as an academic model can be implemented in a way that there is no longer a mechanical (telescope) variable delay line. This can be done according to the invention in the following way. The signals received by the antenna are fed in parallel in arrays of time delay lines, each with an increased length and thus an increased delay. The delay lines are of similar length but have different propagation time properties, for example using different material combinations. In any case, the outputs of all delay lines are connected to a corresponding number of inputs of a multiplexing and sampling device. The delay line output is selected, and its delay is added to the signal delay between the moving vehicle and the ground station so as to provide a total constant delay. This can be obtained by controlling different time delay arrays, possibly using a decision unit that switches the time delay line arrays according to the analyzed Doppler effect. If the delay is constant, then no Doppler effect will occur.

[0022] In an alternative embodiment according to the invention, instead of a switched time delay line array, a time delay unit made of a single conducting path is used. When a signal is transmitted through the conducting path, samples are tapped off at different outputs of the conducting path to obtain different time delays to be assigned to the signal.

[0023] Furthermore, it is important to sample the signals from the delay line unit outputs (array or single path) in such a way that exactly one sample of each delay line output is used to form the output of the multiplexer, otherwise the signal will be affected by the Doppler effect. In general, embodiments according to the present invention may be designed as separate wireless frequency RF devices, which are only applied at the RF input of standard user equipment instead of standard antennas. However, for practical reasons (e.g., for synchronization between the multiplexer and the frame structure of the wireless access system), it is advantageous to integrate it into the RF front end of the access device.

[0024] The mobile body, here again a train, for example a fast train, moves towards the ground station BS at a defined speed. The radio frequency device according to the invention now comprises a delay unit made of a first-in-first-out FIFO buffer. The sampling device takes samples from the output signal of the antenna of the mobile body using a sampling rate Rw and stores them in a FIFO register. After the frame elements of this vehicle are all stored in the FIFO buffer, the FIFO is read at a slightly different sampling rate Rr. If Rw>Rr, then the content of the FIFO is extended in time (the tail end of the frame element is subject to more added delay than the beginning of the frame element). On the contrary, if Rr>Rw, then the frame element is compressed in time (the tail end of the frame element is subject to less added delay than the beginning of the frame element). By selecting the ratio Rw / Rr in an appropriate manner, the total delay between the ground station (BS) and the access device (radio frequency device) on the vehicle can be kept constant during the reception of the frame element.

[0025] In any embodiment according to the invention, different samples of the received packet are processed in such a way that they are stretched or compressed into frames, the stretching or compression factors being defined according to the speed and direction of movement of the mobile relative to the BS determined. Those stretching or compression factors correspond to different time delays assigned to the signal, in order to follow a rule suitable for compensating the Doppler effect analyzed. In order to implement the method according to the invention, samples are taken from the received signal and are respectively delayed according to a certain rule before they are transferred to other uses of the signal. The samples can be taken with constant time distance values, but they can also be taken with increasing or decreasing time distance values.

[0026] A rule suitable for compensating the Doppler effect causing degradation of a signal transmitted by such a wireless transmission can be defined as follows. For the sampling frequency fs and the time delay increase di (defined as the difference in time delay between two adjacent delay line outputs - the incremental time delay) used to compensate for the Doppler effect caused by the velocity vr of the mobile relative to the BS (c is the wireless propagation velocity in a vacuum, i.e., about 300,000 km / s):

[0027] di = vr / (fs×c) and fs = vr / (c×di).

[0028] Depending on the maximum frequency fmax of the transmitted signal, the sampling frequency fs must be defined to be greater than a minimum value fsmin, which is given by the Nyquist criterion: fsmin>=2×fmax.

[0029] The Doppler effect is present whenever the transmitter or the receiver moves so that the distance between them changes. Furthermore, for example for OFDM transmissions, the relevance of the Doppler effect depends on the number of carriers and the bandwidth of the subchannels, both of which define the sensitivity of the Doppler effect. For Wimax technology compliant with IEEE standard 802.16, which is one of the most popular technologies under discussion today for bidirectional communication with mobile bodies, we can see from the above that, at least above 200 km / hour, the effect becomes increasingly important and seriously degrades the signal quality. Thus, a minimum speed vrmin of the mobile body can be defined, below which the Doppler effect can be ignored so that the compensation mechanism according to the invention does not have to be applied. The incremental delay value di is given using the parameters in the above formula:

[0030] di = vrmin / (fsmin×c);

[0031] And, the sampling frequency required to compensate for the Doppler shift for the relative velocity vr can be calculated as follows:

[0032] fs=fsmin×(vr / vrmin).

[0033] It should be noted that the sampling frequency must be varied to adjust the Doppler compensation mechanism to the desired Doppler shift.

[0034] Here are some data for example, with the condition that the sampling frequency fs is at least twice the maximum frequency of the sampled signal (frame generation) as a constraint. The number n of storage cells at the output of the time delay line or FIFO buffer can then be calculated as follows:

[0035] n = (length of frame element) / (distance of sample).

[0036] If the signal has a 10MHz bandwidth around a 1GHz carrier, then the maximum frequency is then 1.005GHz, so the sampling frequency (minimum) is 2.01GHz. For a frame length of 5ms, n would be defined as follows:

[0037] n=5ms / 0.49ns=10050000

[0038] These are still high requirements, but they are feasible. For a better practical implementation, the signal can be mixed down to a suitable intermediate frequency range after the antenna, for example around a 20MHz IF carrier. Then, the maximum frequency in the above example would be 25MHz, and the sampling frequency (minimum) would be 50MHz. In that case, n is defined as follows:

[0039] n=5ms / 20ns=250000

[0040] It is easier to implement.

[0041] It may be advantageous to perform sampling at the input of the analog-to-digital converter, which results in the conversion being assigned different time delays to compensate for the determined Doppler effect. In addition, the Doppler compensation method according to the present invention can be applied before the samples that are subsequently delayed are multiplexed when the signal is received from the wireless transmission. It can also be applied on the transmitting side as a "pre-compensation" method, i.e., the sampling process is performed after the multiplexing of the generated frames and before the signal is transmitted via the wireless transmission. In a similar manner, the radio frequency device according to the present invention can be implemented in a base station or in a mobile device, both of which participate in the wireless transmission. It is also conceivable that both the base station and the mobile device are equipped with such a radio frequency device and / or the Doppler compensation method according to the present invention is applied.

[0042] Although the present invention has been described in detail above by general description and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made to the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all belong to the scope of protection claimed by the present invention.

Claims

1. A method for compensating the Doppler effect of indoor positioning based on adaptive filtering, for compensating data in a frame transmitted via wireless transmission between a mobile body and a base station, wherein the mobile body and the base station each include a transceiver connected to an antenna for the wireless transmission, the method comprising the following steps: for a given speed and direction of movement of the mobile body relative to the base station, determining a Doppler effect acting on the frame when the frame is transmitted as a signal via the wireless transmission; The method further comprises the steps of: A certain sampling process is applied by the transceiver by assigning different time delays to the signal, the different time delays being selected following a rule suitable for compensating the determined Doppler effect.

2. The indoor positioning Doppler effect compensation method based on adaptive filtering according to claim 1, wherein for the Doppler compensation on the receiving side, the sampling process is performed when a signal is received from the wireless transmission and before multiplexing the subsequently delayed samples to regenerate the frame received from the wireless transmission.

3. The indoor positioning Doppler effect compensation method based on adaptive filtering according to claim 1, wherein for the Doppler compensation on the transmitting side, the sampling process is performed after multiplexing the generated frames and before transmitting the signal via the wireless transmission.

4. The indoor positioning Doppler effect compensation method based on adaptive filtering according to claim 1, wherein the sampling process is completed by transmitting the signal through a conductive path with different outputs along the path, and the samples are tapped out at each output according to the time delay to be allocated.

5. The indoor positioning Doppler effect compensation method based on adaptive filtering according to claim 1, wherein the sampling process is completed by transmitting the signal through a conductive path that is a delay line with different propagation time properties.

6. The indoor positioning Doppler effect compensation method based on adaptive filtering according to claim 1, wherein the sampling process is completed by using a first-in-first-out (FIFO) buffer so that the signal is cached in the FIFO at a write rate Rw, and the write rate Rw is different from the read rate Rr of reading samples from the FIFO, and the ratio between the rates Rr and Rw follows a rule suitable for compensating for the determined Doppler effect. 7 . The method for indoor positioning Doppler effect compensation based on adaptive filtering according to claim 1 , wherein a new determination of the Doppler effect is performed at equal time intervals.