Abstract This paper presents unified formulas of the outage probability and the bit error rate (BER) for multiuser diversity with different multiple‐input multiple‐output (MIMO) schemes, including (1) transmit selective/receive selective (SC/SC); (2) transmit selective/receive maximum ratio combining (SC/MRC); and (3) space–time block codes (STBC) on Nakagami‐mfading channels. The exact closed‐form expressions can be derived for the outage probability with arbitrarymand for the BER with integerm. For a non‐integer fading parameter, the BER is expressible as a single infinite series of Gamma function. We also achieve simplified calculations for the performance at high SNR to gain insights regarding the factors determining the performance in fading. It is shown that the multiuser MIMO systems can achieve a full diversity order. The achieved results provide an analytical framework for the assessment of multiuser MIMO systems. All the analytical results are verified by Monte Carlo simulations. Copyright © 2010 John Wiley & Sons, Ltd.

The automatic gain control (AGC) loops are usually used in communication systems to stabilize the performance. In this paper, the combined performance of digital delay-locked loop (DDLL) and AGC loop for direct-sequence spread-spectrum systems is analyzed. Especially, the inherent coupling effects between DDLL and AGC loop are developed. The numerical results show that the DDLL with AGC loop can offer a reliable and satisfactory performance.

The performance of first- and second-order non-coherent digital delay lock loops (DDLL) for direct-sequence spread-spectrum (DS-SS) signals is investigated in the mobile radio environment. The mobile radio channel is first characterized by Rayleigh fading and Doppler shift. A closed-form expression for the timing error transition probability density function of the Chapman-Kolmogorov (C-K) equation is proposed. The probability density function of the steady-state timing error for the first- and second-order DDLL is obtained by solving the C-K equation numerically, and the results are confirmed by computer simulations. Furthermore, the mean time to lose lock (MTLL) of the first-order loop is evaluated, and some numerical results and simulation results are reported. Finally, the steady-state timing error and MTLL of the first-order loop for DS-SS signals in the log-normal fading environment are also presented, and the results are compared with those of Rayleigh and AWGN channels.

This paper considers discrete time analyses of digital delay lock loops (DDLL), and presents the results of an investigation concerning the performance degradation due to Doppler. The performance measures evaluated include the steady-state timing error probability density function (PDF) and the mean time to lose lock. The linear approximations are also provided and compared to the numerical results and simulations. The delay lock loop is a well-known technique to track the pseudo-noise (PN) codes for spread spectrum systems. Under a severe Doppler environment, such as in the low Earth orbit (LEO) satellite communication, it will be difficult to track the PN code for the first-order loop. Since the digital systems are more compact, lower cost, and more stable than their analog counterparts, we focus our attention on the digital first and second order code tracking loops that suffer from severe Doppler.