Distributed Cyclic Delay Diversity for OFDM Reliability
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Solution Overview
Problem
Existing communication systems using OFDM with fixed cyclic prefixes face challenges in achieving reliable and efficient data throughput, especially in multipath fading channels, where the cyclic prefix length must be sufficient to prevent intersymbol interference (ISI) but fixed lengths can reduce bandwidth efficiency and are not adaptable for simultaneous transmission from multiple transmitters.
Innovation Solution
The implementation of a distributed cyclic delay diversity (CDD) scheme, where transmitters circularly rotate symbols with a unique shift and copy the last symbol to the first position, allowing receivers to detect and combine signals from CDD transmitters, and using a unique identification symbol to distinguish CDD transmissions, while synchronizing transmitters with a universal clock to manage propagation delays and tap delays across channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed cyclic prefix is used in OFDM to eliminate intersymbol interference, then reliability is improved, but throughput is reduced due to significant bandwidth consumption
Solution Approach 1:
The patent applies dynamics by transitioning from a fixed cyclic prefix length to a variable cyclic prefix length that adapts to channel conditions. The system dynamically adjusts the cyclic prefix length based on measured channel characteristics, allowing the system to optimize between reliability (longer prefix) and throughput (shorter prefix) depending on the actual multipath delay spread of the channel.
Solution Approach 2:
The patent changes the parameter of cyclic prefix length from a fixed value to a variable parameter that can be adjusted based on channel conditions. By measuring the channel's delay spread and adjusting the cyclic prefix length accordingly, the system eliminates the need to use excessively long fixed prefixes that waste bandwidth, while still ensuring sufficient protection against ISI when needed.
2Device complexity
If a fixed length cyclic prefix is used, then device complexity is reduced, but adaptability to different channel conditions and simultaneous transmissions from multiple transmitters is insufficient
Solution Approach 1:
The system dynamically adjusts the cyclic prefix length based on measured channel characteristics, allowing adaptation to different multipath fading channel conditions and simultaneous transmissions from multiple transmitters. This dynamic adjustment maintains manageable complexity while significantly improving adaptability.
Solution Approach 2:
The patent implements feedback mechanisms where the system measures channel delay spread and uses this information to adjust the cyclic prefix length. This feedback loop enables the system to adapt to varying channel conditions and the presence of multiple transmitters while maintaining reasonable system complexity through automated adjustment.
3Reliability
If the cyclic prefix length is increased to cover the delay spread of multipath channels, then reliability is improved, but the bandwidth efficiency is reduced
Solution Approach 1:
The patent changes the cyclic prefix length parameter dynamically based on the actual delay spread measured in the channel. By measuring the channel characteristics and adjusting the prefix length to match the actual delay spread, the system uses the minimum necessary bandwidth for reliability, avoiding the excessive bandwidth consumption that would occur with a universally long fixed prefix.
Solution Approach 2:
The system applies local quality by tailoring the cyclic prefix length to the specific channel conditions at each receiver. Instead of using a uniform long prefix for all conditions, the system measures the local channel delay spread and adjusts the prefix length accordingly, optimizing the balance between reliability and bandwidth efficiency for each specific transmission scenario.
Data Source
AI summary
Systems and methods for a communication system including a set of transmitters, wherein operations of the set of transmitters are synchronized with an accuracy bound by a synchronization error. A controller forms a message with ordered symbols including data symbols and at least one identification symbol, and controls transmitters from the set of transmitters to transmit the message using a cyclic delay diversity (CDD). Wherein each transmitting transmitter prior to transmitting, circularly rotates the ordered symbols of the message with a unique shift, then copies some symbols located at an end of the message. Wherein a number of the copied symbols is based on a predetermined cyclic prefix length, into a first position in the rotated message, to form a transmitter identifiable message, and transmits via each transmitting transmitter the transmitter identifiable message.