We developed a cross-layer design which combines adaptive modulation and coding at the physical layer with a truncated automatic repeat request protocol at the data link layer, in order to maximize spectral efficiency under prescribed delay and error performance constraints. We derive the achieved spectral efficiency in closed-form for transmissions over Nakagami-block fading channels. Numerical results reveal that retransmissions at the data link layer relieve stringent error control requirements at the physical layer, and thereby enable considerable spectral efficiency gain. This gain is comparable with that offered by diversity, provided that the maximum number of transmissions per packet equals the diversity order. Diminishing returns on spectral efficiency, that result when increasing the maximum number of retransmissions, suggest that a small number of retransmissions offers a desirable delay-throughput tradeoff, in practice. Index Terms-Adaptive modulation and coding (AMC), automatic repeat request (ARQ) protocol, cross-layer design, quality of service (QoS), wireless networks. His research interests include the areas of com-munications, signal processing, and networking, with emphasis on cross-layer analysis and design, quality of service support for multimedia applications over wired-wireless networks, and resource allocation.Shengli Zhou (M'03) received the B.S. and M.Sc. degrees in electrical engineering and information science from the University

In this paper, we introduce a new paradigm for the design of transmitter space-time coding that we refer to as linear precoding. It leads to simple closed-form solutions for transmission over frequency-selective multiple-input multiple-output (MIMO) channels, which are scalable with respect to the number of antennas, size of the coding block, and transmit average/peak power. The scheme operates as a block transmission system in which vectors of symbols are encoded and modulated through a linear mapping operating jointly in the space and time dimension. The specific designs target minimization of the symbol mean square error and the approximate maximization of the minimum distance between symbol hypotheses, under average and peak power constraints. The solutions are shown to convert the MIMO channel with memory into a set of parallel flat fading subchannels, regardless of the design criterion, while appropriate power/bits loading on the subchannels is the specific signature of the different designs. The proposed designs are compared in terms of various performance measures such as information rate, BER, and symbol mean square error

Presence of kth-order cyclostationarity is defined in terms of nonvanishing cyclic-cumulants or polyspectr a. Relying upon the asymptotic normality and consistency of kth-order cyclic statistics, asymptotically optimal x2 tests are developed to detect presence of cycles in the kth-order cyclic cumulants or polyspectra, without assuming any specific distribu ion on the data. Constant false alarm rate tests are derived in both time-and frequency-domain and yield consistent estimates of pos! iible cycles present in the kth-order cyclic statistics. Explicit algorithms for k 5 4 are discussed. Existing approaches are rather empirical and deal only with k 5 2 case. Simulation results are presented to confirm the performance of the given tests. I. INTRODUCTION HE concept of (almost) cyclostationarity an((almost) T periodically time-varying ensemble statistics I 141, [ 181, [21], [22] has gained a lot of interest in the engineering literature lately. A discrete-time zero-mean (almost) cyclctstationary process, z (i) , is characterized by the property tha; its timevarying cowiance ~2~(t ; 7

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Zero padding (ZP) of multicarrier transmissions has recently been proposed as an appealing alternative to the traditional cyclic prefix (CP) orthogonal frequency-division multiplexing (OFDM) to ensure symbol recovery regardless of the channel zero locations. In this paper, both systems are studied to delineate their relative merits in wireless systems where channel knowledge is not available at the transmitter. Two novel equalizers are developed for ZP-OFDM to tradeoff performance with implementation complexity. Both CP-OFDM and ZP-OFDM are then compared in terms of transmitter nonlinearities and required power backoff. Next, both systems are tested in terms of channel estimation and tracking capabilities. Simulations tailored to the realistic context of the standard for wireless local area network HIPERLAN/2 illustrate the pertinent tradeoffs.