In this paper, we study the energy efficiency of small-cell on/off control schemes in which small-cells turn on when the number of active users reaches a certain threshold value called on-threshold. In particular, we derive analytical formulas for various performance metrics such as expected small-cell power consumption, average consumed energy per transmitted bit, expected energy saving, and energy saving probability. Moreover, we show that the expected small-cell power consumption strictly decreases with increased on-threshold value and derive a mathematical expression for the optimal on-threshold value that minimizes the average consumed energy per transmitted bit. With simulation results, we not only verify the validity of the analytical results but also discuss how the theoretical results can be used in system performance evaluation and optimization. We believe that many of the results in this paper can be used in practical small-cell network design and operation.

We propose employing receiver cooperation in centralized topology control to improve energy efficiency as well as network connectivity. The idea of transmitter cooperation has been widely considered in topology control to improve network connectivity or energy efficiency. However, receiver cooperation has not previously been considered in topology control. In particular, we show that we can improve both connectivity and energy efficiency if we employ receiver cooperation in addition to transmitter cooperation. Consequently, we conclude that a system based both on transmitter and receiver cooperation is generally superior to one based only on transmitter cooperation. We also show that the increase in network connectivity caused by employing transmitter cooperation in addition to receiver cooperation is at the expense of significantly increased energy consumption. Consequently, system designers may opt for receiver-only cooperation in cases for which energy efficiency is of the highest priority or when connectivity increase is no longer a serious concern.

In this paper, we present analytical bit error probability results for M-ary modulation concatenated with Reed Solomon (RS) codes. The analysis of bit error probability is nontrivial as the number of bits per symbol for the RS codes may not be an integer multiple of the number of bits per symbol for a modulation symbol. We propose a Markov chain technique which allows analytical evaluation of the bit error probability for such cases. The performance of RS coding with coherent biorthogonal, coherent/non-coherent orthogonal modulation over an additive white Gaussian noise (AWGN) channel is evaluated. Simulation of the bit error probability of RS code concatenated with a Nordstrom Robinson (NR) code as an inner code is performed and compared with the case of biorthogonal modulation. From the results, we notice that a stronger inner code gives better bit error probability. In addition, the throughput of the coded system with biorthogonal modulation over an AWGN channel is discussed. For a Rayleigh flat fading and block fading channel, we analyze the bit error probability of RS codes concatenated with biorthogonal modulation. From the result, we notice that a stronger outer code gives a better bit error probability for the case of Rayleigh flat fading channel.

In this letter, we investigate the fundamental relationship between the performance of a maximum ratio diversity combining scheme and the normalized mean square covariance (NMSV) over wide-sense stationary uncorrelated scattering channels. In particular, we show that the reliability achieved by diversity combining is closely related to the value of NMSV of the channel under consideration.

A set of parameters collectively called recentered normalized mean square correlation (RC-NMSR) are introduced to characterize the quality of multipath fading channels with nonzero specular components. The consideration of nonzero specular components requires nontrivial generalization of the parameters previously introduced for wide-sense stationary uncorrelated scattering (WSSUS) channels. The new parameters are obtained by studying the principles of diversity combining. We illustrate the effectiveness of these parameters by theoretical and empirical study of a simple diversity combining scheme and frequency hopping spread spectrum systems.

A set of second-order statistics collectively called normalized mean square covariance (NMSV) are defined to characterize the frequency and/or the time selectivity of wide-sense stationary uncorrelated scattering (WSSUS) channels. Normalized frequency mean square covariance (NFMSV) quantifies the frequency selectivity, while normalized time mean square covariance (NTMSV) characterizes the time selectivity. Normalized frequency-time mean square covariance is defined to characterize the combined effect of frequency and time selectivities. The NMSVs of a WSSUS channel can be computed easily from the scattering function. We show that there is a very close relationship between the NMSV of a WSSUS channel and the performance of various diversity combing schemes. Also we discuss, with practical system design problems, how useful the parameters are for efficient system design.

This paper examines the asymptotic (M/spl rarr//spl infin/) performance of M-ary frequency-shift keying (M-FSK) in multi-channels, or multiple frequency-nonselective, slowly fading channels, with coding, side information, and diversity reception. In particular, Reed-Solomon (RS) coding is considered in conjunction with the ratio-threshold test (RTT), which generates side information regarding the reliability of received symbols. The asymptotic performance of orthogonal signaling in multichannels with maximal ratio combining (MRC), postdetection equal gain combining (EGC), hybrid selection combining (H-SC), and selection combining (SC) is derived for an arbitrary statistical fading model and diversity order. The derivations reveal that coherent and noncoherent implementations of diversity combining schemes yield the same performance asymptotically. In addition, the asymptotic results are evaluated assuming a Nakagami-m fading model, and the effect of fading severity, diversity order, code rate, and side information upon the performance of the various diversity combiners is investigated. The minimum signal-to-noise ratio (SNR) required to achieve arbitrarily reliable or error-free communication, as well as the associated optimal RS code rate, are determined for various cases.

This paper examines the asymptotic performance of M-ary orthogonal modulation with noncoherent square-law detection and combining in Rayleigh fading. The analysis takes into consideration Reed-Solomon coding and evaluates the gain provided by the ratio-threshold test (RTT) which generates side information regarding the reliability of received symbols. The minimum received signal-to-noise ratio (SNR) required for error-free communication and the optimal code rate are derived for arbitrary orders of diversity. These asymptotic results are then extended to a direct-sequence code division multiple access (DS-CDMA) system with frequency-selective fading.

A trellis-based multiuser detection algorithm called bias compensated matched filter decision feedback sequence estimation (BCMFDFSE) is considered for DS-CDMA systems under frequency-selective fading environments. The development of the wideband DS-CDMA standard demands the consideration of frequency selectivity in designing systems. In this paper, we propose BCMFDFSE as a possible receiver structure that has nice performance/complexity tradeoffs. We compare BCMFDFSE with other algorithms such as successive interference cancellation (SIC) and parallel interference cancellation (PIC).