Composite multipath fading/shadowing environments are frequently encountered in different realistic scenarios. These channels are generally modeled as a mixture of Nakagami-m multipath fading and log-normal shadowing. The resulting composite probability density function (pdf) is not available in closed form, thereby making the performance evaluation of communication links in these channels cumbersome. In this paper, we propose to model composite channels by the G-distribution. This pdf arises when the log-normal shadowing is substituted by the inverse-Gaussian one. This substitution will prove to be very accurate for several shadowing conditions. In this paper we conduct a performance evaluation of single-user communication systems operating in a composite channel. Our study starts by deriving an analytical expression for the outage probability. Then, we derive the moment generating function of the G-distribution, hence facilitating the calculation of average bit error probabilities. We also derive analytical expressions for the channel capacity for three adaptive transmission techniques, namely, i) optimal rate adaptation with constant power, ii) optimal power and rate adaptation, and iii) channel inversion with fixed rate.

On the performance analysis of composite multipath/shadowing channels using the G-distribution

An assessment of spectrum efficiency for a microcellular land mobile radio system is presented by considering the desired signal as (fast) Rician fading with (slow) log-normal shadowing and cochannel interfering signals as uncorrelated (fast) Rayleigh fading superimposed over (slow) log-normal shadowing. Spectrum efficiency is defined in terms of reuse distance, i.e., cluster size, traffic intensity, bandwidth of the system, and area of a cell by considering cochannel interference probability. The expression for cochannel interference probability is derived using appropriate path-loss law for microcells for four different cases: Rician plus log-normal desired signal and Rayleigh plus log-normal interfering signals; Rician desired signal and Rayleigh fading plus log-normal shadowing interfering signals; Rician desired signal and Rayleigh interfering signals; and both desired and interfering signals as Rician fading. The performance of a microcellular system is compared with that of a conventional macrocellular system. >

/pdf/effects-of-rician-faded-and-log-normal-shadowed-signals-on-zo0ecjnfzf.pdf

Effects of Rician faded and log-normal shadowed signals on spectrum efficiency in microcellular radio

The level crossing rate (LCR) of a random process conveys useful information about the underlying process, and is of interest in diverse engineering fields. In wireless communications, it is related to the system characteristics such as handoff, outage probability, fading rate, average duration of fades, velocity (or maximum Doppler shift) of the mobile, and the effect of diversity on fading. The LCR formula was originally derived by Rice in terms of the joint probability density function (pdf) of the underlying process and its time derivative. In this letter, we express the LCR in terms of the joint characteristic function (cf). This new formula is useful for many cases where the joint cf is simpler to derive than the associated joint pdf. As an application and for a direct-sequence code-division multiple-access system, the fading rate at the output of a RAKE receiver with either maximal ratio combiner or postdetection equal gain combiner, operating over a frequency-selective fading channel with different path statistics, is easily calculated using the new cf-based LCR formula.

/pdf/level-crossing-rate-in-terms-of-the-characteristic-function-okoa1b28wm.pdf

Level crossing rate in terms of the characteristic function: a new approach for calculating the fading rate in diversity systems

The throughput and delay characteristics of a land-mobile satellite channel are analyzed for both slotted ALOHA. And slotted direct-sequence CDMA (code division multiple access), using binary phase shift keying (BPSK) modulation and forward error correction coding (FEC). In the case of CDMA, the application of path diversity techniques-maximal ratio combining and selection diversity-is also taken into account. Packet success probabilities are derived for both slow and fast fading, in order to evaluate the throughput and delay. Numerical results are presented for arbitrary code lengths and for specific values of the number of resolvable paths. It is shown that CDMA can offer a substantial improvement over slotted ALOHA, especially when the chip time is less than the delay spread. >

/pdf/slotted-aloha-and-code-division-multiple-access-techniques-500qvuo5oe.pdf

Slotted ALOHA and code division multiple access techniques for land-mobile satellite personal communications

The design of low-complexity, -size, and -cost modems for Code-Division Multiple Access (CDMA) radio links like those envisaged in the 21st-century Personal Communications Network puts forth challenging issues as far as signal detection for best power efficiency is concerned. As is often the case in modem design, the “ancillary” functions of signal synchronization may reveal fundamental in such an optimum design problem. Spread-Spectrum CDMA signals reveal particularly demanding in this respect, so that the basic issues in carrier, code and chip/data timing should be carefully re-considered before tackling light-heartedly the design of a Digital Signal Processing (DSP)-based CDMA modem. It is the aim of this paper to present an overview of those code acquisition/tracking and carrier frequency/phase recovery techniques for a Direct-Sequence Spread-Spectrum signal, that appear particularly suited to an efficient VLSI implementation.

Signal synchronization for direct‐sequence code‐division multiple access radio modems

The performance of a direct-sequence spread-spectrum land-mobile satellite transmission system, using binary phase-shift keying (BPSK) modulation, is analysed. The satellite channel is modeled as having Rician fading characteristics. The bit-error probability is evaluated, considering both the envelope and the phase variation. Assuming a Gaussian approximation for the interference, numerical results are obtained for both spread-spectrum and narrowband land-mobile satellite communication systems with BPSK modulation. A comparison of the two systems is made for light, average, and heavy shadowing. >

/pdf/direct-sequence-spread-spectrum-in-a-shadowed-rician-fading-1ojc6h2ybq.pdf

H.S. Misser

Papers

Direct-sequence spread spectrum in a shadowed Rician fading and land-mobile satellite channel