Walid K. M. Ahmed

Random coding error exponents for two-dimensional flat fading channels with complete channel state information

In this correspondence, we consider the random coding error exponent for nondispersive two-dimensional (2-D) (quadrature) fading channels for which the channel state information (CSI) is perfectly known at the receiver and the input to the transmitter is constrained in its average power. Also, the effect of space diversity on improving the performance is demonstrated. The results obtained in this correspondence shed light on the effect of fading on communications reliability as well as the amount of coding complexity required to achieve a certain decoding error rate. A treatment for the random coding error exponent for time-correlated flat fading channels with Rayleigh fading and Bessel function correlation (the Jakes model) is also provided.

Random coding error exponents for flat fading channels with realistic channel estimation

There has been a considerable interest in determining the limits to communications over multipath fading channels. However, most studies have assumed that the channel is perfectly known at the receiver. In this paper, the random coding error exponent for flat fading channels with realistic channel state information (CSI) is studied. It is assumed that the CSI is obtained via some practical technique which utilizes a linear estimation scheme. Two commonly used techniques for channel estimation are considered in this paper, namely pilot tone extraction and pilot symbol transmission. The degradation in the achievable performance due to partial CSI is assessed and comparison of the different channel estimation methods is made. The focus of this paper is on the Jakes mobile Rayleigh flat fading model. Although Jakes model does not have a Markov property, such as that found in the commonly used exponential correlation model, which is usually attractive from the mathematical tractability point of view, Jakes model has a physical basis. Also, this model is considered herein from the standpoint of the random coding exponent. The results in this paper shed light on the amount of degradation in the achievable performance that is expected when the receiver has partial CSI. Finally, the sensitivity of the loss in achievable performance for the various channel estimation techniques with respect to channel parameters, such as Doppler spread and signal-to-noise ratio (SNR), is studied.

Simulation and performance evaluation of IS-95 handoff algorithms

In this paper, we investigate the performance of handoff algorithms for the IS-95A and IS-95B standards. We compare the performance of the algorithms specified in the standards with enhanced base station processing that we refer to as the base-station assisted handoff (BAHO) algorithm. Since soft handoff performance in code division multiple access (CDMA) networks is a function of the systems operating load on the air interface, we study the performance of these handoff algorithms using system-level simulations. At the same carried load, the handoff algorithms and the parameter settings determine the rate (and capacity) of handoff activity (for example, leg drops, adds) and the soft handoff overhead (for example, messaging processing, bandwidth) in terms of number of legs and network infrastructure processing. Based on our observations of the aforementioned performance metrics, we provide recommendations on handoff algorithms and parameter values. These recommendations would enable a CDMA system to retain quality while achieving handoff efficiency that improves cell-site capacity. The results from our simulations provide significant insight into the handoff management for IS-95 systems, as well as for future third-generation (3G) systems.

Uncoded symbol error rate estimation: methods and analysis

In wireless communications systems, channel quality estimates are often used to provide a measure of the quality of service or to enable resource allocation techniques that improve system capacity and/or user quality. The uncoded bit or symbol error rate (SER) is specified as a channel quality metric in second and third generation cellular systems (e.g., general packet radio service (GPRS), enhanced general packet radio service (EGPRS), and universal mobile telecommunications system (UMTS). Nonetheless, techniques to estimate the uncoded SER are typically outside the scope of these wireless standards and are not specified. In this paper, we analyze the performance of a number of uncoded SER estimation techniques, including a novel technique in which we use the soft information of the received symbols to obtain a fast and accurate estimate of the uncoded SER. The technique we introduce has been found to outperform, in terms of accuracy and required estimation interval, conventional hard-decision based techniques that use test-patterns, or use a decode/re-encode/compare approach . Our technique also outperforms the brute-force technique, which is to send a known test-pattern, demodulate it at the receiver, and count the observed discrepancies.

Achievable performance for coded modulation over fading channels

We compute the random coding error exponent for memoryless flat fading channels with ideal channel state information (CSI) and inputs drawn from a discrete alphabet. The signal sets we consider here are standard signal constellations commonly used in digital coded modulation. In addition, estimates of the code lengths required to achieve a certain error probability are determined and compared to those required for the AWGN channel. Finally, the effect of receiver diversity combining on improving communications reliability over the fading channel is considered. The results we obtain here shed light on the achievable performance over the fading channel with coded modulation.

Achievable performance over fading channels with antenna diversity

There has been much interest in wireless communications and thus it is appropriate to determine the Shannon limit for such systems. There have been a number of papers that consider channel capacity and also the random coding error exponent for various scenarios, In this paper, we consider Gallagers random coding error exponent for flat fading channels with antenna (or space) diversity employed at the receiver. Receiver antenna diversity is well known to be a fundamental technique for combating fading in wireless communications. The random coding error exponent is also well known to be a key quantity in assessing the ultimate, or achievable, performance over communications channels. We derive the random coding error exponent for various fading scenarios for which diversity combining is used. The cases we consider include memoryless fading channels with complete channel state information (CSI) at the receiver as well as time-correlated fading channels with complete CSI at the receiver. We also present results for time-correlated channels with partial CSI at the receiver via some practical channel estimation technique. The results we present in this paper shed light on the achievable performance over fading channels with receiver diversity combining and provide insight regarding the amount of coding complexity required for reliable communications over such channels.

Random coding error exponents for flat fading channels

In this paper, the random coding reliability function, or error exponent, has been derived for various fading channels that model current wireless and mobile communication channels. A short summary of the results is presented.

Upper and lower bounds to estimate the random coding exponent for a peak-power-limited channel

In this paper, we consider the estimation of Gallagers (1968) random coding exponent (RCE) for a peak power constraint at the transmitter for the two-dimensional fading channel with additive white Gaussian noise (AWGN) and with perfect channel estimation at the receiver. Despite the fact that many wireless channels are peak-power-limited, the RCE for such channels has not been considered previously in the literature. Such a problem has only been partially solved for the AWGN case due to the difficulty in finding an input signal distribution that yields the RCE. In this paper, we adopt a different approach in which we develop upper and lower bounds to the RCE with the hope of trapping it in a narrow region. Our quest is successful as we can estimate the RCE to an error of only 0.72 bit per modulation symbol. Furthermore, we find that the RCE for the peak-power-limited channel does not represent a severe degradation relative to that attained for the average-power-limited channel. Thus, good modulation and coding schemes at reasonable complexity should exist for a peak-power-constrained fading channel.

Error bounds for the amplitude limited flat fading channel

Peak transmitted power is a key issue in wireless systems. We consider upper and lower bounds to Gallagers random coding error exponent (1968) for the two dimensional (or quadrature) memoryless flat fading channel with perfect channel state information (CSI) at the receiver and when a peak power constraint is imposed at the transmitter.

Information theoretic considerations for coded modulation over fading channels

We compute the random coding error exponent for coded modulation transmitted over a flat, memoryless, Rayleigh fading channel. In addition, estimates of code lengths required to achieve a certain error probability are determined and compared to those required for the additive white Gaussian noise channel. Finally, the effect of receiver antenna diversity is also considered as a method to compensate for fading and is shown to have a significant, positive impact on the error exponent. The results we obtain represent an information theoretic view that complements the existing literature on the performance of coded modulation over fading channels with receiver diversity.