In wireless communication networks, direct transmission between the source and destination requires high transmitted power which is very costly and it makes the network life shorter i.e. battery drain becomes fast and interference increases. Fading also occurs which reduces the signal strength going through the channel and the signal received at the destination differs from original signal transmission. MIMO uses diversity technique to offer significant increase in data throughput and link range without additional bandwidth or transmit power. Diversity technique reduces the fading by transmitting the data or information over multiple paths or multiple independent fading channels. These multiple copies of the same transmitted signal are combined at the receiver using any of the diversity technique i.e. maximum ratio combining or equal gain combining diversity etc. Cooperative communication provides transmit diversity to single antenna users.
Cooperative communication is a technique to minimize the effects of fading. In this technique, multiple faded versions of a signal are transmitted from source to destination. The communication provided by cooperation is reliable. As compared to wired communication medium, there are so many factors in wireless communication medium which negatively affect the quality of the original transmitted signal, and the received signal at the destination is not the same as transmitted signal from the source. Fading is one of those factors which are responsible for changes in the quality of received signal as compared to original transmitted signal. MIMO has become an important part of wireless communication due to its innumerable advantages. Although transmit diversity, which is inherent feature of MIMO, is advantageous but it may not be practical for all the scenarios. Especially due to cost, size or hardware limitations, a wireless agent, for example: a mobile phone may not be able to support multiple transmit antennas,.
In cooperative communication, more than one, cooperative users get the transmitted signal copies and retransmit those copies of the signal toward the destination. These users are called relays. Relays may be virtual antenna elements which have wide separation and wireless communication link between them. In relayed transmission, the signal or data from source follows individual transmission paths over shorter distances in wireless communication with low power.
The signals from all transmitting relays are combined at the destination as multiple transmissions of same signal to reduce the performance degradation due to signal fading. Spatial diversity combining technique can be used as antenna sharing between relaying terminals, creating a virtual array to combat fading. Diversity technique can be used to combat fading phenomenon. The users basically share their information and transmit with cooperation,. Relayed transmission extends the coverage using spatial diversity to avoid fading in cooperative wireless communication system,,. Relaying scheme and cooperative diversity form a virtual antenna array based on multiple antenna systems,,,,.
Cooperative communication allows single antenna mobiles to reap some benefits of MIMO system. So, we can summarize that cooperative communication is a technique where three or more active nodes, operating in a common wireless network, share their resources to jointly transmit messages while improving system performance through their inherent spatial diversity. The main concept behind co-operative-communication is sharing. Mobiles do have single antenna. These mobiles in a common wireless network share their antennas in a manner that will create virtual MIMO system. There are various theoretical and simulation studies examining the potential benefits of co-operative-communication. In this technique, wireless agent who is termed as user or active node increases the effective quality of service via co-operation.
Hence, cooperative communication is a technique to minimize the harmful effects of fading in which some neighbouring nodes relay the multiple copies of the same transmitted signal. This technique is possible because of broadcast nature of wireless medium, to have nodes adjacent to the source of message. As a result, nodes in the network act not only as end users but also as relays for other user messages. The multiple retransmissions of messages inherent to cooperative communication creates a spatial diversity that allows for increased throughput of reliability,.
Thus, this technique overcomes the severe impact on the system performance which arises because of fading. As the number of end users increases, it becomes extremely inefficient to use all available relays. To avoid this situation and improve the performance of cooperative relaying significant research has been done into the selection of a best relay among the multiple relays to participate in any given communication.