## 2 Related Works  
A large amount of research evaluating the performance of co-operative wireless communications has been published, using either simulation, theory, or testbeds. In this section, we first review and evaluate some proposals, based on their obtained performance. Next, we present some co-operative test benches which have been developed and analyzed their performances. After that, we compare these results with those acquired by the platform developed and presented in this article.  
Some Co-operative Communication Systems are based on the Amplify-&-Forward scheme. For example, in [34], an A&F RNCS was studied and a comprehensive set of experiments were conducted, in which it was shown that the A&F Co-operative Systemcan provide better BER performance than a Non-Co-operative System. Similarly, in [31], an A&F protocol based on multiple antennas and a MMSE pre-equalization scheme was presented, which was compared with the ZF pre-equalization technique. In this context, the observed benefit of the pre-equalization schemes were that, at the destination, there were noticeably fewer errors when the A&F pre-equalization Co-operative System was employed. However, it was demonstrated that A&F with MMSE pre-equalization had better performance, compared with A&F with ZF pre-equalization.  
On the other hand, some Co-operative Systems are based on Decode-&-Forward Relay Nodes. In [35], simulation results showed that a D&F Co-operative System can improve the average BER performance, in comparison with a Non-Co-operative system. Furthermore, in [36], the performances of the A&F and D&F have been compared, where it was observed that the Symbol Error Rate (SER) performance of the D&F was gradually enhanced, in comparison to the A&F Relay Node.  
Several research works have been published which were focused on the development of test-benches for the study of co-operative communications using relay nodes in wireless networks. In [17; 37], the Wireless Open-Access Research Platform (WARP) and an OFDM-MIMO A&F Relay Node were respectively presented. The WARP is a scalable and extensibly programmable wireless testbed. However, the main drawback is its limitation to a single relay, which is due to it not having an external clock interface allowing for the synchronization of multiple relays. Field Programmable Gate Arrays (FPGAs) provide another testbed platform, which has been employed in [16] to evaluate the performance of a Co-operative System. In [16], the A&F and D&F protocols were implemented, where the throughput of the proposed platforms had guaranteed high performance; nevertheless, the associated implementation and development costs hinders its usage by research communities.  
On the other hand, an D&F RN has been designed, in [38], using USRP and GNU Radio, one of the most widely used SDR platforms. However, with the introduction of the Matlab(tm) software, another line to design and evaluate wireless systems has opened. In this sense, our investigation team presented, in [18], a D&F implementation using USRP and Matlab(tm), as the first step towards designing a Co-operative System with major benefit in the future. Under this background, in [19; 39], the testbed was enhanced through the insertion of a new USRP with higher performance in the backhaul link (eNB-RN link), which improved the capacity of the system, given that is capable of exploiting the 2 \(\times\) 2 MIMO scheme. In this paper, the results associated with an evaluation of our developed test bench measurement bank are presented, based on the Matlab(tm) software and USRP platform. The developed platform supports both single- and multi-relay co-operation. The results show that co-operative transmission can achieve significant performance enhancement, in terms of link reliability and end-to-end throughput.