In this letter, we present exact statistical expressions for opportunistic amplify-and-forward (AF) relaying under a tight power constraint, where a best relaying node is selected based on partial channel information (PCI) or full channel information (FCI), respectively. Based on the statistical analysis, we compare PCI with FCI strategy in terms of outage and average capacity. Numerical investigation shows that, as the number of candidate relays increases, the performances with FCI are continuously improved while those improvements with PCI are bounded. However, for the small number of the relays at a low SNR region, PCI and FCI achieve very similar performances. The numerical results also show that the relaxed power constraint that ignores the noise power at the relay leads to significant deviations from the tight constraint, which is the more reasonable one, especially in computing the outage with FCI.

We present a study on the outage probability of multihop wireless communication systems with multiple-input multiple-output (MIMO) link based on the transmit antenna selection and the maximal-ratio combining (MRC) at the receiver. A nonregenerative system (NS) is investigated with an ideal amplifying gain. MIMO channels are assumed in uncorrelated Rayleigh fading.We derive a moment generating function (MGF) of the reciprocal of the end-to-end signal-to-noise ratio (SNR) and obtain a closed-form approximation on the outage probability through the numerical inversion of a Laplace transform. Numerical results show that the presented outage is exactly matched with the outage probability when assuming the ideal relay gain. For more practical gains, the result is shown to be a lowerbound that gets tight at high average SNR as well as for a small number of hops and/or of antennas. We also compare the outage probabilities of nonregenerative MIMO relaying with a regenerative counterpart for multiple hops.

Wireless dual-hop relaying is envisioned to provide better coverage as well as signal reliability for existing single-hop wireless communication systems. The simplest feature of the dual-hop relay has attracted increasing interest from research and commercial communities. Emerging transmit diversity techniques using space-time transmission with multiple antennas could further enhance the dual-hop relaying performance. In this paper, we examine the coverage of dual-hop space-time relaying and allocation of limited power to the source transmitter as well as the relay. We finally compare different coverage-extension strategies, namely, increasing Tx power, employing multiple Tx antennas to achieve transmit diversity, and adopting either a nonregenerative relay or a regenerative one to enforce signal reliability at the destination. It is numerically shown that the space-time transmission that is used in the dual-hop relaying can further extend the coverage, although its performance depends on various parameters, namely, relaying methods, location of the relay, and power allocation.

Combining a dual-hop relaying with multi-input multi-output (MIMO) transmission is a natural extension to overcome the channel impairments. Transmit beamforming (TBF) and maximal ratio combining (MRC) are widely accepted ones, which maximize the signal-to-noise ratio (SNR) at the receiver when channel state information is available. With these methods, there are four possible combinations in constructing dual-hop transmission: TBF-TBF, MRC-MRC, MRC-TBF, and TBF-MRC, respectively. We provide optimal amplify-and-forward (AF) weights at a relay, which maximize the end-to-end SNR for the four systems, respectively, and show the equivalence of the four systems in terms of the SNR. Using relaxed AF weights from the optimal ones, we provide a probability density function (PDF) and a moment generating function (MGF) for the end-to- end SNR per bit with an assumption of an equal number of diversity branch for each hop, which is used to obtain the BER performance for M-ary QAM and PSK constellations, respectively. Numerical results show that the BERs with the relaxed AF weights provide tight lower bounds for those with optimal AF weights. We also compare the BER performance of above AF relaying with that of dual-hop decode-and-forward (DF) relaying.

Greedy transmission scheduling achieves great capacity by maximally exploiting independent time-varying channels across different mobile users. The improvement in capacity, however, depends on the degree of completeness of the channel quality information (CQI) fed back from the receiver to the transmitter. To be motivated by an insight that too many CQI feedbacks may rather impair the capacity gain, due to causing congestion in feedback link, this letter proposes a quality-based CQI reporting (QBR) scheme where the CQIs are fed back to the transmitter only for receivers whose signal quality is above a predefined threshold. The capacity is provided in terms of the threshold and feedback-error rate. The results show that QBR achieves outstanding performance when the feedback error is present. In addition, it quickly approaches an unimpaired ideal capacity, as the number of users increases if the error is not assumed