This article studies the closed-form expressions of outage performance for opportunistic relay under aggregate power constraint in decode-and-forward （DF） relay networks over Rayleigh fading channels, assuming that multiple antennas are available at the relay node. According to whether instantaneous signal-to-noise ratio （SNR） or average SNR can be utilized for relay selection, two opportunistic relay schemes, opportunistic multi-antenna relay selection （OMRS） and average best relay selection （ABRS） are proposed. The performances of both two schemes are evaluated by means of theoretical analysis and simulation, it is observed that OMRS is outage-optimal among multi-antenna relay selection schemes and closely approaches the beamforming （BF） scheme known as theoretical outage-optimal. Compared with previous single-antenna opportunistic relaying （OR） scheme, OMRS brings remarkable performance improvement, which is obtained from maximum ratio combining （MRC） and beamforming techniques. It is also shown that the performance of ABRS in asymmetric channels is close to OMRS in the low and median SNR range.
In this paper, the feedback load reduction problem in wireless systems based on orthogonal frequency division multiplexing （OFDM） is investigated and an opportunistic feedback scheme （OFS） is proposed. The key idea behind OFS is that only the key channel gains which can significantly affect the system throughput are fed back to the BS. Firstly, the key channel gains are proved to belong to a channel gain interval. Secondly, a statistical method is proposed to estimate the channel gain interval. Thirdly, the opportunistic feedback scheme is formulated and the feedback load of OFS is analyzed. The advantage of OFS is threefold： the first is OFS can work in both OFDM-based multicast system and OFDM-based unicast system. The second is the channel fading type of the BS-user link is not required, which is more realistic. The third is OFS can get better feedback load performance compared with other schemes, while achieving almost the same throughput performance compared with that of full feedback scheme.
In this paper, the outage performance of opportunistic relay （OR） and equal gain transmissionbeamfomaing （EGT-BF） are studied in decode-and-ibrward （DF） relay networks over Nakagami-m fading channels, assuming multiple relays are available in the networks. We derive the moment generation function （MGF） of the received signal-to-noise ratio （SNR） at the destination, then obtain the exact closedform solution of OR and the approximate closed form expression of EGT-BF in terms of outage probability respectively through numerical inversion of Laplace transform. Simulation results are given to verify the correctness of theoretical analysis. We also compare the outage performance of OR and EGT-BF with oth- er transmission strategies and find that when decoding results of relays are available at the destination, OR achieves high performance and is the optimal scheme to achieve the tradeoff between performance and overheads. When relay decoding information is unachievable, the performance of OR degrades significant- ly, and the EGT-BF is a more preferred scheme.
