Next generation wireless communication networks aim to achieve higher data throughput and improved coverage. Because of the regulatory limits on frequency bands and transmit power, any future data rate increase needs to come from using other degrees of freedom.
Exploiting space as a degree of freedom by using a Multiple-Input Multiple-Output (MIMO) concept has enabled dramatic increase in the efficiency of wireless communications in the past decade. Some examples of wireless standards that successfully deployed MIMO are 801.11n for personal area network and the new LTE standard for cellular (4G).
MIMO does have potential for achieving large data rates, butit also has few drawbacks. It has been shown that antennas need to be separated by at least half the carrier wavelength to provide independent fading.
In today’s cellular systems this distance isof the order of 5-10cm and is eventually limiting the number of antennas that can be mounted on the mobile device. Another issue is the power consumption of the multi-antenna transceiver and thefundamental limit on scaling of the battery capacity.
Increase in throughputs also requires increase in the density of the deployed infrastructure (basestations and femto-cells), which increases the amount of interference.
Finally, the infrastructure becomes irregular and is being deployed ad-hoc (in the form of femto-cells). One key challenge in thefuture wireless standards is how to constructively exploit interference in a dense, irregularly deployed network, while keeping power consumption within given constraints.
Another challenge is how to use nearby idle terminals to help transmission. A number of theoretical concepts have emerged in thepast few years that show that cooperation between terminals can can signicantly increase the capacity of the whole network. Simultaneously, advances in technology are enabling implementation of the higher complexity wireless systems required for this cooperation and their deployment is planned in the next wireless cellular standards LTE Advanced (5G) and beyond.
The motivation for this work is the practical demonstration of a wireless system that exploits cooperation between terminals. The simplest configuration of such a system is a cooperative MIMO system. Compared to the standard MIMO, it has two transmit terminals with one antenna each.
One of them is the original transmitter that has certain data stream to be sent and will be referred to as the source. The second one, referred to as the relay, represents the half-duplex transceiver that listens to the sources transmission and forwards part of that stream further to the destination.
Both transmit terminals have only one antenna, which simplifies the hardware design and reduces the power consumption. In general, there may be more than one relay but the complexity of these terminals is still less than the complexity of the single multi-antenna transmitter.
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