As we have seen in fig.3 and concluded from the previous sections that there is more than one incoming transmission with the same burst of data, the incoming signals have to be combined and the original signal which was transmitted have to be retrieved. The possible strategies by which we combine these signals are as follows:
1 ERC (Equal Ratio Combining).This is self explanatory, all the incoming signals are combined or rather they are just
added up.This is done when computational time is crucial. This is the easiest way as no special computation is involved, but the performance is not satisfactory.
2 FRC (Fixed Ratio Combining) Instead of just adding up incoming signals, they are weighted with a constant rate. They will not change during communication.
3 SNRC (Signal to Noise Ratio Combining) Signals are weighted in an in an intelligent way.A very important parameter SNR is used to weight the incoming signal.
4 MRC (Maximal Ratio Combining)This technique is also termed as ratio squared combining.It is most commonly implemented and is complex to realize.
COOPERATIVE RELAYING STRATEGIES
On the fundamentals of signal processing and the manner in which forwarding is achieved at the relays, cooperative relaying strategies can be broadly classified in two types:
1.Transparent relaying technique
2. Regenerative relaying technique
Transparent Cooperative Relaying technique scaling/amplification or phase rotation. We can conclude that transparent relaying technique performs linear transformation of a signal at relay node example Amplify & Forward where as regenerative relaying include modification of a single waveform at the relay node example decode and forward, coded cooperation etc.
Cooperative relaying strategies are used to relay the multiple copies of the transmitted signal from the source node toward the destination node for minimizing the effects of fading to get the reliable communication. Vander Maulen, proposed the classical relay channel as a class of three terminal communication channels. Cover and EI Gamal determine channel capacity for discrete memoryless and additive white Guassian noise channels. Lower bounds on capacity were determined by them using cooperation in which the source message is fully decoded by relay and relayed message is transmitted to the destination jointly with source. When source to relay channel quality is very high, then cooperation gives highest achievable rate. Schein and Gallager, showed different extensions to the case of multiple relays in their work. A multiple access channel is considered by Kramer and Wijngaarden. In this channel, different sources communicate to a single destination and only a single relay is shared. Multiple access channel are examined by King Carleial, and Willems et al with generalised feedback. In generalised feedback, sources are allowed to act as relays for one another. Sendanaris et al. called the approaches of model of, the user cooperation diversity,, in the presence of multipath fading. Full diversity is achieved by cooperative protocols in which probability of outage reduces in proportion to 1/SNR2 while without cooperation it reduces in proportion to 1/SNR. Cooperative diversity becomes effective due to some level of synchronization. The way in which the information is transmitted from the source terminal to the relay terminal, and the way it is processed at the relay terminal the existing cooperative protocols can be divided into three types such as amplify and forward, detect and forward and coded cooperation.
Amplify and Forward: In this cooperative relaying strategy, when a signal from source to destination is transmitted directly, then the copies of the same signal are transmitted to the relaying nodes (i.e. neighbouring nodes) and these relaying nodes amplify the received signal and forward it toward the destination. The relay terminals simply re-transmit a scaled version of the signal that they receive from the source terminal to the destination terminal. Depending on the scaling factor, the AF relaying scheme can be further divided into two types which are called fixed gain AF system and variable gain amplify and forward system.
Detect and Forward: In this method, a user attempts to detect the partner’s bits and then retransmits the detected bits. The partners are decided by a certain protocol. Each user should have a partner that is the important thing. This partner will provide diversity path. In the first and second intervals, each user transmits its own bits. Each user then detects the other user’s second bit. In the third interval, both users transmit a linear combination of their own second bit and the partner’s second bit. The transmit power for the first, second and third intervals are variable and by optimizing the relative transmit power according to the conditions of the uplink and enter user channels, this method provides adaptability to channel conditions. This signalling has the advantage of simplicity and adaptability to channel conditions.
Coded Cooperation: In coded cooperation, each user attempts to transmit an incremental redundancy for its partner. Todd E. Hunter and Aria Nostratinia proposed coded cooperation technique. In this method cooperative signalling is
integrated with channel coding. It works by sending different portions of each user’s code via two independent fading paths. Each user, instead of repeating the received bits, tries to transmit incremental redundancy for its partner. Whenever this is not possible, the user automatically reverts to no cooperative mode. All this is managed through code design, without any feedback between the users. The users segment their source data into blocks which are encoded with an error detection code e.g. CRC code. Each block is then encoded with FEC ( forward error correcting code) so that for an overall rate R, we have N total coded symbols allocated for each source block, then two users cooperate by dividing the transmission of their coded source blocks into two successive time segments which we call frame. For the first frame each user transmits at rate R^R code word with Ni=K/Ri bits. This higher rate code can be obtained, for example by puncturing the original code word. Each user receives and decodes his partner’s first frame. If the user successfully decodes the partner’s rate R1 code word, then the user computes and transmits N2 additional parity bits for the partner’s data in the second frame (N1+N2=N) for example, if the first frame was obtained via puncturing, these N2 bits could be the puncture bits left out of the first frame. Whenever a user is unable to successfully decode his partner’s message, the user will revert to a non- cooperative mode by calculating its own N2 bits and transmitting-them.
In coded cooperation, each user always transmits a total of N bits per source block over the two frames, and the users only transmit in their own multiple access channels. We define the level of cooperation as N2/N which is the percentage of total bits per each source block that the user transmits for his partners.
Cooperative communication systems are much flexible than conventional communication systems.