For wideband MIMO-OFDM systems, scheduling and link adaptation are key techniques to improve the throughput performance. However, in systems without reciprocity between the uplink and the downlink channels, these techniques require a high feedback overhead of the channel quality indication (CQI) information. In this paper, we propose a novel CQI feedback reduction method, which is based on the conventional compression techniques exploiting the discrete cosine transformation (DCT). The basic idea is to adaptively permute the CQI sequences of different MIMO streams according to one of the possible patterns before the DCT compression so that the amount of feedback bits is minimized. The possible patterns used are carefully designed according to our analysis of the two types of correlations (the inter-stream correlation and the inter-subband correlation) that exist in MIMO-OFDM transmission, as well as their impact on the compression efficiency. Simulation results verify that the proposed method can effectively reduce the CQI feedback overhead under varying channel conditions.

In this paper, we study the problem of joint resource allocation in the two-way relay system, where a pair of multi-antenna users wish to exchange information via multi-antenna amplify-and-forward relay under orthogonal frequency-division multiplexing (OFDM) modulation. We formulate a sum-rate maximization problem subject to a limited power constraint for each user and relay. Our resource allocation strategy aims at finding the best pairing scheme and optimal power allocation over subchannels in frequency and space domains. This turns out to be a mixed integer programming problem. We then derive an asymptotically optimal solution though the Lagrange dual decomposition approach. Finally, simulation results are provided to demonstrate the performance gain of the proposed algorithms.

A scheme that jointly estimates carrier frequency offset and channel is proposed for the orthogonal frequency division multiplexing (OFDM) system. In the proposed scheme, the carrier frequency offset (CFO) and the channel state information (CSI) are first estimated by an minimum mean square error (MMSE) estimator and an maximum likelihood (ML) estimator, respectively. By exchanging the estimation information between these two estimators, the final estimation of CFO and CSI is then obtained by an iterative method. In the iterative process, the effect of imperfect CSI is considered. It can improve the estimation precision for a shorter preamble and accelerate the iterative convergence rate. To reduce the complexity of the proposed scheme, a procedure is adopted to eliminate the inverse operation of covariance matrix that is recalculated at each iteration. In addition, a sufficient condition for the convergence of the proposed method is deduced. The numerical simulation results show that the BER performance of our scheme is better than that of joint MLE for a shorter preamble and is comparable to that of joint MLE for a longer preamble. Furthermore, the average iterative time of our method is reduced by half as compared to the MLE methods without considering the effect of imperfect CSI.

A multiple-antenna receiving and combining scheme is proposed for high-data-rate transmitted-reference (TR) Ultra-Wideband (UWB) systems. The nonlinearity of the inter-symbol interference (ISI) model is alleviated via simple antenna combining. Under the simplified ISI model, frequency domain equalization (FDE) is adopted and greatly reduces the complexity of the equalizer. A simple estimation algorithm for the simplified ISI model is presented. Simulation results demonstrate that compared to the single receive antenna scheme, the proposed method can obtain a significant diversity gain and eliminate the BER floor effect. Moreover, compared to the complex second-order time domain equalizer, FDE showed better performance robustness in the case of imperfect model estimation.

A simplified equalization method based on the band structure of the frequency domain channel matrix is proposed for the single carrier systems employing cyclic prefix (SC-CP) over time-varying wireless channels. Using both theoretical analysis and computer simulation, it is shown that the complexity of this method is proportional to the number of symbols in one SC-CP block and is less than that of traditional block equalization methods. We also show that they have similar performance.

Prior studies have shown that the performance of amplify-and-forward (AF) relay systems can be considerably improved by using multiple antennas and low complexity linear processing at the relay nodes. However, there is still a lack of performance analysis for the cases where the processing is based on limited feedback (LFB). Motivated by this, we derive the closed-form expression of the outage probability of AF relay systems with LFB beamforming in this letter. Simulation results are also provided to confirm the analytical studies.

In this letter, a novel power allocation scheme is proposed to improve the outage performance of an amplify-and-forward (AF) cooperative communication network with multiple potential relays under the assumption that only mean channel gains are available at the transmitters. In this scheme, power allocation is studied jointly with a relay selection algorithm which has low computational complexity. Simulation results demonstrate the performance improvement of the proposed scheme in terms of outage behavior.

In this letter, power allocation methods are devised for Amplify-and-Forward (AF) opportunistic relaying systems aiming at minimizing the outage probability. First, we extend the result on outage probability in and develop an approximate expression to simplify the power allocation problem. A corresponding optimization problem is constructed and proved to be convex. Then an iterative numerical method is proposed to find the optimal power allocation factor. We also propose a near-optimal method which can directly calculate the power allocation factor to reduce computational complexity. Numerical results show that the proposed methods have a similar performance with the ideal one, and outperform equal power allocation significantly with little overhead.

This paper studies the optimization of the effective channel capacity of wideband code division multiple access (WCDMA) systems under Rayleigh fading environments. Firstly, the results for Shannon capacity of fading channels with channel side information are reviewed, where the capacity is achieved by using an optimal power control scheme. Secondly, an optimal interference threshold is set for a given system outage probability Pout to minimize total interference. Finally, the effective channel capacity of WCDMA is defined and a target SIR level γ* is derived with the Lagrangian multiplier method to maximize the effective channel capacity. It is shown that is dependent on the power control interference ratio (PCIR) ρ, the number of diversity paths identified by the receiver M, and the outage probability of the system. Simulation results are provided to validate the theoretical deductions. We conclude that the total effective channel capacity will be maximized as long as M4, and ρ0.5 for a proper value of .

In an Orthogonal Frequency Division Multiplexing (OFDM) systems, the Peak to Average power Ratio (PAR) is high. The clipping signal scheme is a useful and simple method to reduce the PAR. However, it introduces additional noise that degrades the systems performance. We propose an oversampling scheme to deal with the received signal in order to reduce the clipping noise by using finite impulse response (FIR) filter. Coefficients of the filter are obtained by correlation function of the received signal and the oversampling information at receiver. The performance of the proposed technique is evaluated for frequency selective channel. Results show that the proposed scheme can mitigate the clipping noise significantly for OFDM systems and in order to maintain the system's capacity, the clipping ratio should be larger than 2.5.

In this letter, we analyze the pairwise error probability (PEP) behaviour of distributed space-time code (DSTC) with amplify-and-forward relaying over Nakagami-m multipath channels. An upper bound of PEP for DSTC is derived. From our analysis, it is seen that of the paths from the source to relays and from relays to the destination, those with smaller diversity order result in an overall system performance bottleneck. Numerical examples are provided to corroborate our theoretical analysis.

Based on the Khatri-Rao matrix product, we propose a novel unitary space-time modulation design called KR-USTM in this paper. Different from existing USTM schemes, such as the systematic approach and space-time frequency-shift keying (ST-FSK), KR-USTM does not require any computer search and can be applied to any number of transmit antennas. Moreover, the special structure of KR-USTM also makes it a high-rate scheme and achieve full antenna diversity as well as lower decoding complexity. Simulation results show that the proposed KR-USTM constellation achieves error performance comparable to existing USTM designs at low rates, while it outperforms them at high rates.

Time variations of wireless multipath channels can lead to severe intercarrier interference (ICI) in orthogonal frequency division multiplex (OFDM) systems, whereas large Doppler frequency spread can provide us with time diversity gain. In order to take advantage of the time diversity and to suppress the interference and noise enhancement at the same time, the receiver normally detects the data successively. In this letter, we propose an improved detection ordering based on the log-likelihood ratio (LLR) rather than the signal-to-noise ratio (SNR) for the successive detector. Using both theoretical analysis and computer simulation, it is shown that this scheme outperforms the traditional successive detection methods.

Recently, it has been shown in the literature that in a relaying network utilizing multiple relay precoding techniques, the signal-to-noise ratio (SNR) at each destination node will scale linearly with the number of relays K, which is referred to as the distributed array gain (DAG) K. In this paper, we focus on the performance of multiple relay precoding based on limited channel state information (CSI) feedback, which is different from the prior studies that assume perfect CSI at each of the relay nodes. Our analysis shows that the conventional limited feedback scheme fails to obtain the DAG K, which is a consequence of the phase ambiguity introduced by the channel quantization function. Based on the theoretical analysis, we propose a novel feedback and precoding procedure, and prove that the proposed procedure can obtain the DAG K with only one additional feedback bit for quantizing each relay-destination channel compared with the conventional scheme. Simulation results verify that with the proposed procedure, the SNR performance is effectively improved when the number of relays K is small, and scales linearly with K in relatively large K regime.

An improved iterative minimum mean-squared error (MMSE) channel estimation method is proposed for joint coding and precoding OFDM systems. Compared with the traditional simplified estimator, the proposed scheme provides higher estimation quality with slight complexity increment at low signal-to-noise ratio (SNR) values. The performance of the iterative receiver including the proposed estimator approaches that of the perfect MMSE estimator without any simplification.

A delay-oriented packet scheduling scheme is proposed for downlink OFDMA networks with heterogeneous delay requirements. Using a novel packet utility concept, the proposed algorithm can exploit diversity from traffic characteristics and requirements to improve delay performance for delay sensitive traffics. Besides, the proposal also shows good ability in balancing fairness and efficiency. Simulation results show that our proposal outperforms existing delay-oriented scheduling schemes in terms of both delay performance and spectrum efficiency.

In this letter, we derive a lower bound on the diversity multiplexing tradeoff (DMT) in multiple-input multiple-output (MIMO) nonorthogonal amplify-and-forward (NAF) cooperative channels with resolution-constrained channel state feedback. It is shown that power control based on the feedback improves the DMT performance significantly in contrast to the no-feedback case. For instance, the maximum diversity increase is exponential in K with K-level feedback.

In this letter we investigate the relaying strategies for multihop transmission in wireless networks over Rayleigh fading channels. Theoretical analysis reveals that equally allocating power among all transmitters and placing relays equidistantly on the line between source and destination are optimal in terms of outage capacity. Then equal time duration for the transmission of each hop is also proved to be optimal. Furthermore, the optimum number of hops is also derived and shown to be inversely proportional to the signal-to-noise ratio (SNR). Numerical simulations agree well with the reported theoretical results.

An improved soft-input soft-output (SISO) minimum mean-squared error (MMSE) detection method is proposed for joint coding and precoding OFDM systems under imperfect channel estimation. Compared with the traditional mismatched detection which uses the channel estimate as its exact value, the signal model of the proposed detector is more accurate and the influence of channel estimation error (CEE) can be effectively mitigated. Simulations indicate that the proposed scheme can improve the bit error rate (BER) performance with fewer pilot symbols.

Multi-cell resource allocation under minimum rate request for each user in OFDMA networks is addressed in this paper. Based on Lagrange dual decomposition theory, the joint multi-cell resource allocation problem is decomposed and modeled as a limited-cooperative game, and a distributed multi-cell resource allocation algorithm is thus proposed. Analysis and simulation results show that, compared with non-cooperative iterative water-filling algorithm, the proposed algorithm can remarkably reduce the ICI level and improve overall system performances.