Cooperative Coding Technology in Cooperative Communication Network

Cooperative Coding Technology in Cooperative Communication Network

In 2003, ITU-R completed the programmatic document M.1645 of the next generation mobile communication system, which clearly requires that the next generation mobile communication system supports the transmission rate of low-speed users 100Mb / s and high-speed users 1Gb / s. Due to the shortage of spectrum resources, how to achieve large-scale network coverage and support higher-speed wireless data transmission under the condition of occupying limited frequency band resources is the basic problem that mobile communication networks must try to solve in the future. Recent studies have shown that distributed wireless communication technology is expected to become a key technology for solving high-speed data transmission and large-scale network coverage.

Distributed wireless communication not only disperses the protocol control function to multiple control terminals (such as Relay stations) in the cell, but also realizes the multiplexing of spectrum, time, space and other multi-dimensional resources in a smaller coverage unit. It also allows mobile terminals to Communicate directly without a base station or control terminal. Multi-user cooperative communication technology can improve the utilization rate of wireless spectrum resources and system power efficiency through the cooperation between users, and realize the effective reuse of multi-dimensional resources such as frequency, time, space, etc. of wireless communication systems, thereby achieving and supporting higher rates, Higher bandwidth wireless transmission improves network coverage performance. It can be seen that the application of distributed wireless communication technology will not only affect the design of cellular network protocols, but will also have a fundamental and profound impact on basic wireless communication methods and even communication methods.

It is undoubtedly one of the basic technical features of future wireless communication network terminals, relay stations and even base stations to have the capability of cooperative communication.

As an important type of cooperative communication technology, cooperative coding greatly expands the connotation of cooperative communication. It expands the cooperative physical signal transmission and / or cooperative signal processing between multiple users on the pure physical level of cooperative diversity technology to the correction of the data link level. Miscoding and decoding help to further improve and improve communication performance and collaboration efficiency. This article summarizes the current status of cooperative coding technology research in cooperative communication networks, explains the impact of terminal and relay station working methods on cooperative coding technology solutions, introduces cooperative coding solutions based on different error correction coding technologies under the classic relay communication model, and reveals different The impact of collaborative communication network environment on cooperative coding technology solutions, and discuss the problems faced by the improvement of cooperative coding technology, and the future development direction of adaptive cooperative coding technology and network coding cooperation technology based on adaptation.

1 Cooperative coding technology

In recent years, multi-user cooperative communication technology represented by collaborative diversity has become the research focus of distributed wireless communication. Multi-user cooperative diversity is a new communication system that allows users to share antennas and obtain spatial diversity gain through different cooperation methods and corresponding signal processing. It can realize the so-called "virtual antenna array" (AVV) function, greatly Improve the anti-fading performance of wireless communication systems, improve resource efficiency and system capacity [1-2].

Cooperative communication based on cooperative diversity technology can use coding technology to further improve system performance [3].

Based on the detection and forwarding cooperation strategy, combined with the coding idea, the cooperative (or relay) node can decide whether to participate in the cooperation according to the detection result, which effectively guarantees the effectiveness of cooperative communication and avoids error propagation under error detection conditions . In short, the cooperative coding mechanism is the product of the organic fusion of channel coding ideas and cooperative communication ideas.

The biggest difference from cooperative diversity is that in cooperative coding, the data processing steps of the cooperative (relay) node include three basic steps: first decoding, then coding, and forwarding: the data of the source node is transmitted through error correction coding and broadcast; When the relay node receives the data from the source node, it first performs error correction decoding, receives the data from the source node, re-encodes it, and forwards it to the target node; the target node will receive the codewords from the source node and the relay node, respectively. After the merge process, the decoding recovers the information from the source node.

First, the source node data is sent after error correction coding, which can effectively improve the probability of the relay node and the target node receiving data correctly.

Secondly, in order to avoid the interference and destruction of cooperative communication under the condition that the relay node can decode incorrectly, the relay node should usually perform error check on the decoded data (for example, through cyclic redundancy check), only the relay node is correct Under the condition of decoding, the cooperative communication mechanism is started. Due to the participation of relay nodes, through the reasonable selection of cooperative nodes to ensure effective cooperation, and at the relay nodes, the coding methods and methods that are complementary to and complementary to the source node's coding methods and methods are adopted. On the basis of making full use of the cooperative space diversity gain, additional coding gain is obtained to achieve better cooperative communication performance.

2 Cooperative coding under full-duplex and half-duplex communication

There are two basic working modes for communication terminals in a cooperative communication environment, namely half-duplex mode and full-duplex mode. In half-duplex mode, the node receives and transmits data in different time slots or through different frequency bands, but the node cannot send and receive data at the same time; while in full-duplex mode, the node can send and receive data at the same time. In the actual cooperative communication environment, the nodes of the half-duplex communication mode are simpler to implement than the nodes of the full-duplex communication mode, the corresponding cooperative coding scheme is easier to implement, and the complexity is lower. The industrial system has a higher system capacity [4].

In full-duplex communication and half-duplex communication modes, the problems faced when designing a cooperative coding scheme are different.

For time-division half-duplex communication, the data processing steps of the source node, the cooperative (relay) node and the target node in cooperative coding can generally be designed as follows: In the first time slot, the source node sends data while the relay node and The target node receives the data, and the relay node decodes the received data. In the second time slot, the source node does not send data, but the relay node re-encodes the decoded data and forwards it to the destination node. At the same time, the destination node can manage to recover the original data from the source node based on the encoded data received from the source node in the last time slot and the encoded data forwarded by the relay node in the current time slot. The design of cooperation and cooperation agreements among the entire users is relatively easy. Recent research shows that in order to further improve the performance of cooperative coding in half-duplex communication, the source node should also send additional coding redundancy information to the target node in the second time slot. In this case, the target node needs to use overlapping symbols The interference detection technology distinguishes the encoded data from the source node and the forwarded encoded data from the relay node.

In the full-duplex communication mode, the source node will send data to the relay node and the target node in turn in each time slot; while the relay node receives the new encoded data from the source node, it will place it in the previous time slot Receive the encoded data from the source node for decoding and recovery, and then re-encoded and forwarded to the destination node; in any time slot, the destination node will simultaneously receive two pieces of encoded data corresponding to different data from the source node and the relay node, On the basis of using overlapping symbol interference detection technology to distinguish the encoded data from the current time slot source node and the forward time slot from the relay node, the combined time slot receives the encoded data from the source node, and restores each time in turn Information data sent by the slot source node. Obviously, the cooperative coding and decoding design in the full-duplex communication mode is more complicated than that in the half-duplex communication mode, but the cooperative coding in the full-duplex communication mode often has better performance [5].

3 Cooperative coding scheme in the relay communication model

Since there are a large number of channel coding and decoding technology achievements that can be directly applied, in recent years, around the classic relay communication model, various types of channel coding and decoding schemes have been actively researched on the application schemes of cooperative coding techniques.

In 2002, Todd E Hunter et al. Earlier proposed a cooperative coding scheme suitable for rate-based erasure convolutional code (RCPC) under the relay communication model. The advantage of the RCPC code-based cooperative coding scheme is that the relay node can flexibly select the RCPC coding scheme according to the changes in the cooperative communication environment and conditions, but the disadvantage is that due to the limited error correction performance of the convolutional code itself, the cooperation based on the RCPC code The coding gain brought by the coding scheme is limited.

Due to its excellent error control performance, the cooperative coding technology based on Turbo codes and low-density sparse check matrix codes (LDPC) has become a research hotspot of cooperative coding technology application schemes.

In 2003, ZHAO Bin [6] and others earlier studied the distributed Turbo coding and decoding scheme suitable for the relay communication model in the half-duplex communication mode: the data of the source node is encoded and sent by a recursive system convolutional encoder; After receiving the data from the source node, the relay node first performs Viterbi decoding. If the decoding result is correct through the cyclic redundancy check (CRC), the recovered information from the source node is interleaved and sent to the recursive system The convolutional encoder re-encodes and sends; the target node will receive the two component recursive system convolutional codewords from the source node and the cooperative (relay) node in two different time slots, so iterative translation can be used The code algorithm more reliably recovers the information from the source node from the received complete Turbo code word.

ZHANG Zheng et al. [4-5] proposed an improved cooperative Turbo coding and decoding scheme that is suitable for approaching the capacity limit of relay communication systems in full-duplex communication and time-division half-duplex communication modes. The difference is that in order to improve the performance of cooperative coding, in the improved cooperative Turbo coding and decoding scheme, the source node uses a complete Turbo encoder and the relay node uses a Turbo decoder. In addition, the improved cooperative Turbo coding and decoding scheme adopts the strategy and mechanism for the source node to transmit data as much as possible to improve the information transmission capability of the cooperative Turbo coding and decoding system. In summary, the following three schemes can be used for cooperative coding based on Turbo encoding and decoding, that is, the source node and the relay node use the Turbo encoding scheme of Turbo encoding and decoding, and the source node and the relay node use the distributed Turbo component encoding and decoding. Turbo encoding scheme. Research shows that the cooperative turbo coding scheme based on punctured turbo codes has better performance. In addition, in the cooperative Turbo coding scheme, the relay node only needs to forward the check digit data to the target node without repeatedly sending the data information. Similarly, LI Yonghui et al. [7] proposed a distributed Turbo coding and decoding scheme (DTC-SIR) based on soft information relay. In the DTC-SIR scheme, the relay node calculates the posterior probability soft information of the check symbol according to the posterior probability of the information bit, and forwards it to the target node. The distributed Turbo coding and decoding scheme based on soft information relay can effectively alleviate the adverse effect of relay decoding errors on cooperative coding to a large extent.

In 2007, HU Jun [8] and others gave a cooperative LDPC coding and decoding scheme suitable for approaching the system capacity limit in full-duplex communication and time-division half-duplex communication mode. In the full-duplex communication mode, the source node and the relay node can use the same LDPC coding, or different LDPC coding schemes. The data of the source node is encoded and sent by an LDPC encoder; the source node can use the puncturing technology to adjust the coding rate of the LDPC code. In order to better help the relay node decode the data of the source node, it is recommended to retain the system information bits in the puncturing as much as possible ; After receiving the data from the source node, the relay node decodes it first, and re-adopts LDPC encoding on the basis of the decoded data, and forwards the encoded codeword to the target node; the target node will be based on the received source The two parts of the LDPC codeword of the node and the cooperative (relay) node complete the decoding and restore the source node data. Similar to the cooperative Turbo encoding and decoding scheme approaching the system capacity limit, in order to improve the information transmission capability of the cooperative Turbo encoding and decoding system, the cooperative LDPC encoding and decoding scheme also adopts a strategy and mechanism for the source node to transmit as much data as possible. This requires that the target node also needs to use the interference symbol detection technology before the LDPC decoding process. The related research results show that when the number of iterations of LDPC decoder iteration decoding and the number of joint iterations of interference symbol detection and LDPC decoding are reasonably set, the cooperative LDPC coding scheme based on irregular LDPC codes uses different LDPC coding schemes at the source node and the target node Under the conditions, the performance of the improved cooperative Turbo coding scheme can be obtained.

In a wireless communication system, multiple users share antennas and other resources with each other to form a virtual antenna array, thereby obtaining spatial diversity gain. In 2004, Janani et al. Studied the space-time transmission scheme and decoding mechanism of cooperative space-time RCPC codes and Turbo codes in conjunction with the idea of ​​space-time coding. The research results show that: by adjusting the proportion of cooperative transmission time, it is convenient to achieve collaboration Flexible adjustment of the degree; under the condition that the cooperation degree is not 1, the space-time transmission scheme of the cooperative coding under the condition of slow fading channel can achieve full set, while the order of diversity under the condition of fast fading channel depends on the minimum of the relevant cooperative coding The sum of Hamming distances [9].

The research progress in recent years shows that the complete space-time code has the best performance of full-rate and full-range diversity at the same time, and it has become an effective technical means to achieve the best compromise between spatial diversity and spatial multiplexing under the condition of MIMO system. However, since the complete space-time code is a new type of space-time coding construction method developed in recent years, in view of its excellent performance and easy-to-implement features, the cooperative space-time coding technology based on the complete space-time code is undoubtedly a future cooperation An important direction of space-time coding research.

4 Cooperative coding scheme in complex network communication model

In addition to the research on the cooperative coding and decoding scheme that is suitable for the classic relay communication model, for other complex network communication models, the research on the corresponding cooperative coding technology scheme is also the focus of the cooperative coding technology research.

From the perspective of information transmission, Ameesh Pandya and Greg PotTIe systematically analyzed the cooperative coding problem applicable to wireless network communication conditions including two cooperative transmitting nodes and two cooperative receiving nodes [10]. Related research shows that cooperative relay is a cooperative communication model with low complexity and should be given priority in cooperative coding. Secondly, the cooperation of the transmitting end is significantly better than that of the receiving end. The performance gain is small, so under distributed network communication conditions, the scenario of multiple cooperative receiving nodes can be simplified to a single receiving node model when possible. Other network communication scenarios also include: a cooperative coding scheme in the scenario of a single transmitting node and two cooperative receiving nodes, a single transmitting node, two cooperative relay nodes, a cooperative coding scheme in a single receiving node scenario, etc. [11].

In addition to the cooperative coding research under the above complex network communication model, how to meet the cooperative coding communication needs of any multi-source multi-sink wireless communication network, and how to apply network coding to the wireless communication network to further improve the capacity of the network system have caused people Attention. Essentially, the goal of collaborative communication and network coding is to enhance the processing capabilities of relay nodes and improve network performance. Network coding collaboration that organically integrates the two is a reasonable technical choice. In the network coding collaboration technology, the relay node can use network coding to process data from multiple sources and then forward it to the corresponding multiple sinks. Recent performance analysis shows that network coding collaboration can better achieve such a scheme of diversity gain and multiplexing gain, and requires less bandwidth and has similar performance as cooperative coding [12].

In addition, adaptive cooperative coding research has also begun to attract people's attention. The choice of collaboration area and collaboration partners in coding collaboration has a direct impact on collaboration performance. For collaborative coding, there is a collaboration area determined by the position of the source node and the destination node, with the source node as the reference position; in order to obtain better user collaboration gain, in the case of multiple collaboration nodes, the distance should be optimized The node closer to the source node. In 2004, Michael R Souryal et al. Studied the adaptive cooperative Turbo coding and decoding scheme based on the link channel state information CSI under time-varying Rayleigh fading channels. Related research shows that cooperative coding under time-varying fading conditions requires time diversity and space. Reasonable compromise between diversity; under slow time-varying conditions, reasonable choice of relay nodes and determination of cooperative coding scheme can greatly improve the performance of cooperative Turbo coding and decoding; under fast time-varying conditions, the The spatial diversity gain gradually decreases; according to the change of channel conditions, the adaptive cooperative coding of the relay node dynamically selected between the source node and the target node can further improve the cooperative coding performance [13]. For the problem of cooperative coding in complex network environments, BAO Xingkai et al. Proposed a class of adaptive network coding cooperation schemes based on LDGM codes and LT-LDPC codes. The code graph matches the changes in network topology to achieve adaptive cooperative coding At the same time, due to the constraint relationship that depends on the code graph, there is no need for strict synchronization between cooperative nodes in the cooperative encoding process [14].

5 Conclusion

As a coding technology in a distributed wireless communication network environment, more effective cooperative communication can be achieved through coding. In the future broadband wireless communication network environment, the existing channel coding, space-time coding and network coding fields can be fully utilized A large number of research results, research on cooperative coding technology is in the ascendant. From a technical implementation point of view, most of the existing cooperative coding technical solutions require strict transmission and reception synchronization between cooperative nodes. In order to realize cooperative coding, in addition to controlling the terminal, it is also necessary to specially design control signaling for the relay. In the future, there are a large number of theoretical and technical issues that need to be further deepened in the future adaptive cooperative coding technology and network coding cooperation technology.

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