Masoud Sajadieh

Modulation-assisted unequal error protection over the fading channel

Unequal error protection (UEP) is essential when different portions of the source data do not contribute evenly to the overall duality of the decoded information. Conventional techniques achieve UEP by independent coding of the bit streams or by adopting conventional unequal error-protection codes. In this paper, we propose a practical alternative to the QPSK-based transmission systems with explicit UEP, such as GSM or IS-54 wireless transmission standards. In this scheme, the task of providing unequal protection is divided between the channel encoder and a nonuniform signal set, which discriminates in favor of the more important bits. The new approach allows for a simpler convolutional encoder and, hence, a less complex decoding procedure. Specifically, a reduction by more than half in the number of encoder states can easily be achieved using our scheme. Countering the degradation of the less important bits, we propose to adopt a high-rate punctured convolutional code to minimize the incurred transmission rate penalty. We also discuss a pilot sequence transmission scheme which realizes a coherent reception. Decentralizing the bit protection culminates in an extra degree of freedom which, in turn, introduces more flexibility into the system design.

A block memory model for correlated Rayleigh fading channels

Channels with memory offer higher capacities provided that the system design can take advantage of this memory. In this paper, we study and characterize the dynamics of a correlated Rayleigh fading channel with known phase. The model follows a continuous-time Markov double-chain for the random fading process with the Markov states corresponding to the quantized amplitude levels. The analytical expression for the distribution of the state dwell times is derived. We also calculate the mean dwell times of the Markov states in terms of the fading properties and the quantization level. Observations from simulating a typical land-mobile channel verify that the Markovian dynamics are best represented through a finer quantization of the lower level fading amplitudes.

Analysis of two-layered adaptive transmission systems

A major challenge for personal communication systems is to meet the growing demand for various services when the frequency resources are increasingly scarce. Given the time varying nature of the radio channel, an efficient resource utilization calls for channel-adaptive transmission schemes. This paper integrates the two concepts of channels with side information and broadcast channels and introduces a model for flat Rayleigh fading channels with perfect interleaving and a single bit channel state information at the receiver. A true-level modulation constellation is deployed to send information to a two-state channel, where the state of the channel is determined by monitoring the received signal to noise ratio. The limited adaptability of the system helps gear up to a higher data rate as channel conditions improve, without any adjustment at the transmitter. A two-stage receiver, driven by the channel state estimation device, demodulates at full rate only if the channel signal to noise ratio is above a pre-set threshold. The average mutual information of the model as well as the bit probability expressions are derived for a non-uniform 8 PSK signal set. It is shown that the achievable rate is higher than that obtained using a uniform QPSK with only a negligible loss in error rate.

A reduced complexity decoding scheme for wireless applications

Cellular transmission standards such as GSM and IS-54 produce data streams with varying degrees of significance at the source encoder level. We propose an unequal error protection scheme based on a non-uniform signal set which provides the more important data with a preferential Euclidean distance. The unequal error protection is partly accomplished by the modulator in contrast to the conventional systems where the channel encoder is solely responsible. The coding gain resulting from the asymmetric modulation can be translated into a reduction in the complexity of the channel encoder; specifically a reduction by more than half in the number of encoder states can be expected. We study the interaction between the code complexity and the modulation asymmetry in quantitative terms. For differentially coherent systems, a /spl pi//6-shifted differential embedded QPSK is proposed. Decentralizing the bit protection culminates in an extra degree of freedom which in turn introduces more flexibility into the system design.

Layered transmission of audio/video signals

In a broadcast channel, a single transmitter communicates with a number of receivers having different channel capacities available to them. A typical example of such a channel is the over the air TV broadcast. For the two-receiver case, we examine the performance of some common orthogonal transmission methods in terms of their achievable rates. Layered transmission, whereby the information intended for the better channel is superimposed on the portion of information common to both receivers, can provide an optimal solution. On this premise, we construct a bi-rate transmission model for a Gaussian broadcast channel, in which receivers are distributed according to an exponential pdf. A basic grade of service is thus maintained throughout the entire coverage area in addition to a higher quality video offered to the receivers with better reception conditions. The performance evaluation indicates that this model offers a higher per capita data rate comparing to the conventional single rate transmission systems. The multirate paradigm exhibits a stepwise degradation, mitigating the sharp cutoff threshold of the current digital broadcast systems. Application of this multirate broadcast proves very promising in the area of multiresolution transmission of digital HDTV.