Cooperative Wireless Communication
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The mobile wireless channel suffers from fading, in other words, the signal attenuation can vary significantly over the course of a given transmission. When transmitting independent copies of the signal, it generates diversity and can eff3ectively combat the deleterious effects of fading. In particular, spatial diversity is generated by transmitting signals from different locations, thus allowing independently faded versions of the signal at the receiver. Cooperative communication generates this diversity in a new and interesting way.
Consider two mobile agents communicating with the same destination. Each mobile has one antenna and cannot individually generate spatial diversity. However, it may be possible for one mobile to receive the other, in which case it can forward some version of overheard information along with its own data. Because the fading paths from two mobiles are statistically independent, this generates spatial diversity.
Cooperative communications exploit the spatial diversity inherent in multiuser systems by allowing users with diverse channel qualities to cooperate and relay each other’s information to the destination. Each transmitted message is passed through multiple independent relay paths and thus, the probability that the message fails to reach the destination is significantly reduced. Without having the knowledge of the channel conditions or even the amount of resources available, each user is given a fair opportunity of utilizing the cooperative relaying channel. However, if the channel state information is available to the users, one can redistribute the resources usage or traffic load to improve the communication efficiency.
The basic ideas behind cooperative communication can be traced back to the ground breaking work of cover and EL Gamal on the information theoretic properties of the relay channel. This work analysed the capacity of the three-node network consisting of a source, a destination, and a relay. It was assumed that all nodes operate in the same band, so the system can be decomposed into a broadcast channel from the view point of the source and a multiple access channel from the view point of the destination.
Many ideas that appeared later in the cooperation literature was first exposited in. However, in many respects the cooperative communication that we consider is different from the relay channel. First, recent developments are motivated by the concept of diversity in a fading channel, while cover and EL Gamal mostly analyse capacity in an additive white Gaussian noise (AWGN) channel. Second, in the relay channel, the relay’s sole purpose is to help the main channel, whereas in cooperative communication the total system resources are fixed, and users act both as information sources as well as relays. Therefore, although the historical importance is indisputable, recent work in cooperative has taken a somewhat different emphasis.
BASIC CONCEPTS OF COOPERATIVE COMMUNICATIONS
Cooperative communication typically refers to a system where the user share and coordinate their resources to enhance the transmission quality. This idea is particularly attractive in wireless environment due to the diverse channel quality and the limited energy and bandwidth resources. With cooperation, users that experience a deep fade in their link towards the destination can utilize quality channels provided by their partners to achieve the desired quality of services. This is also known as the spatial diversity gain, which is similarly achieved in multiple-input-multiple-output (MIMO) wireless systems.
In cooperative wireless communication, people are concerned with a wireless system for the cellular or ad hoc variety, where the wireless agents, who are identified as call users, can enhance their effectual quality of service- which can be calculated at the physical layer through rates of bit error, rates of block error, or outage possibility-through cooperation. In the system of cooperative communication, every wireless user is supposed to shift information with the act as a cooperative agent for different user
Cooperative methods apply the broadcast style for wireless signals through assessing that a source signal aimed for an important place can be overheard at neighbouring areas node. These nodes, known as partners, relays, or supporters, process the signals they hear and shift towards the place.
There are two features that differentiate cooperative transmission schemes from conventional non-cooperative schemes:
- The use of multiple users’ resource to transmit data of a single source.
- A proper combination of signals from multiple cooperating users at the destination.
This is evidenced where we have two users transmitting their local messages to the destination over independent fading channels. Suppose that the transmission fails when it enters a deep fade, this is to mean when the signal-to-noise ratio (SNR) of the received signal falls below a certain threshold. If the two users cooperate by relaying each other’s messages and the inter user channel is sufficiently reliable, the communication outage only occurs when both the users experience poor channels at the same time.
The main advantages of the cooperative communications are:
- Higher spatial diversity
- Higher throughput/lower delay
- Reduced interference/lower transmitted power
- Adaptability to network conditions
Detect and Forward Method
This method is close to the idea of a traditional relay. In this method a user attempts to detect the partner’s bits and then retransmits the detected bits. The partners may be assigned mutually by the base station, or via some other technique. The most important factor is that each user has a partner that provides a second data path. The easiest way to visualize this is via pairs, but it is also possible to achieve the same effect via other partnership topologies that remove the strict constraints of pairing. This method of signal has the advantage of simplicity and adaptability to channel conditions. This method has the limitation that the base station needs to know the error characteristics of the inter user channel for optimal decoding.
In order to avoid the error of propagation, Lanemanet al proposed a hybrid decode and forward method where at times when the fading channel has high instantaneous signal to noise ration, users detect and forward their partners data, but in case the channel has low SNR, users revert to a non cooperative mode.
Amplify and Forward Method
Another simple cooperative signal method is the amplify-and-forward method. In this method, receives a noisy version of the signal transmitted by its partner. The user then amplifies and retransmits this noisy version. The base station combines the information sent by the user and the partner, and makes a final decision on the transmitted bit.
In amplify and forward method, it is assumed that the base station knows the inter user channel coefficients to do optimal decoding, so some mechanism of exchanging or estimating this information must be incorporated into any implementation. Another potential challenge is that sampling, amplifying, and retransmitting analogue values is technologically nontrivial.
Coded Cooperation Method
Coded cooperation is a method that integrates cooperation into channel coding. Coded cooperation works by sending different portions of each user’s code word via two independent fading paths. The basic idea is that each user tries to transmit incremental redundancy to its partner. Whenever that is not possible, the users automatically revert to a non cooperative mode. The key to the efficiency of coded cooperation is that all this is managed automatically through code design with no feedback between the users.
The users divide their source data into blocks that are augmented with cyclic redundancy check (CRC) code. In coded cooperation, each of the users’ data is encoded into a codeword that is partitioned into two segments, containing N1 bits and N2 bits, respectively
In general, various channel coding methods can be used within this coded cooperation framework. For example, the overall code may be a block or convolution code, or a combination of both. The code bits for the two frames may be selected through puncturing, product codes, or other forms of concatenation.
The users act independently in the second frame, with no knowledge of whether their own first frame was correctly decoded. As a result, there are four possible cooperative cases for the transmission of the second frame: neither user cooperates. Analysis of the effects of these four cases is beyond the scope of this article, and we refer the reader to the literature for more comprehensive treatment. We only note that the performance curves shown in this article include all the effects of the inter user channel.
The figure above give some examples of the performance of cooperative communication using the three classes of signalling described in the previous section.
For comparisons one must take note that, unlike amplify-and-forward and detect-and-forward methods, coded cooperation is inherently integrated into channel coding. In order to present equitable comparisons, we consider a coded baseline system with the same overall rate of ¼ for all cases: non cooperative, amplify-and-forward, detect-and-forward, and coded cooperation.
For both hybrid decode-and-forward and amplify-and-forward, the users initially transmit a RCPC code word punctured to rate 1/2. This code word is subsequently repeated by the relay, resulting in an overall rate of ¼.
For coded cooperation, a cooperation level of 25 percent is used. The two users transmit a code word punctured to rate 1/3 in the first frame. In the second frame, the relay transmits the bits punctured from the first frame such that the total bits received for each user form a rate 1/4 code word.
The plot above illustrates a case in which the mean uplink SNR for user 1 is 10 dB higher than that of user 2, while the inter user mean SNR is equal to that of the uplink channel for user 2. Two significant results of cooperation can be noted. First, user 2, as one might expect, improves significantly by cooperating with a user that has a better quality uplink channel.
More interestingly, however, user 1 also improves significantly, despite cooperating with a user having a poorer quality uplink channel.
This result illustrates that even a user with a good uplink channel has strong motivation to cooperate. Second, we note that the difference in performance between users 1 and 2 is significantly reduced by the cooperation methods. This shows that cooperation inherently reallocates the system resources in a more effective manner.
In comparing the three cooperative transmission schemes, we see that both amplify-and-forward and hybrid decode-and-forward are not very effective at low SNR. This is due to the fact that their signalling is equivalent to repetition coding, which is relatively inefficient at low SNR. Coded cooperation, however, has graceful degradation and performs better than or as well as a comparative non cooperative system at all SNRs. In addition, coded cooperation generally performs better than other cooperative methods for moderate to high SNR.
This tutorial describes wireless cooperative communication, a technique that allows single antenna mobiles to share their antennas and thus enjoy some of the benefits of multiple antenna systems. Several signalling schemes for cooperative communication are presented. Practical implications and requirements on system design are discussed, as well as extensions to the basic idea. Results to date are indicative of a promising future for cooperative communication.
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