Fred Daneshgaran

Saturation throughput analysis of IEEE 802.11 in the presence of non ideal transmission channel and capture effects

In this paper, we provide a throughput analysis of the IEEE 802.11 protocol at the data link layer in non-saturated traffic conditions taking into account the impact of both transmission channel and capture effects in Rayleigh fading environment. The impact of both non-ideal channel and capture become important in terms of the actual observed throughput in typical network conditions whereby traffic is mainly unsaturated, especially in an environment of high interference. We extend the multi-dimensional Markovian state transition model characterizing the behavior at the MAC layer by including transmission states that account for packet transmission failures due to errors caused by propagation through the channel, along with a state characterizing the system when there are no packets to be transmitted in the buffer of a station. Finally, we derive a linear model of the throughput along with its interval of validity. Simulation results closely match the theoretical derivations confirming the effectiveness of the proposed model.

Design of interleavers for turbo codes: iterative interleaver growth algorithms of polynomial complexity

This paper addresses the problem of designing interleavers for parallel concatenated convolutional codes (PCCCs) tailored to specific constituent codes. We start by establishing the role of the interleaver in the PCCC and the various parameters that influence the performance of the PCCC with a given interleaver. Subsequently, we define a canonical form of the interleaving engine denoted as the finite-state permuter (FSP) and demonstrate the minimal delay property of this canonical form. For any given permutation, we present a procedure for deriving the canonical FSP engine. We address the issue of implementation of the FSP and propose a very simple structure for the FSP. Next, using the structural property of the FSP engine, we develop a systematic iterative technique for construction of interleavers with a complexity that is polynomial in the interleaver size. Subsequently, we develop a cost function that, coupled with the iterative interleaver growth procedure, can be used to design optimized interleavers for PCCCs. We provide examples of application of the interleaver design technique, and compare the designed interleavers with some of the interleavers of comparable size found in the literature.

A PC-based software receiver using a novel front-end technology

Since the software radio concept was introduced, much progress has been made in the past few years in making it a reality. Many software radio based systems have been designed through the development efforts of both commercial and noncommercial organizations. While the term software radio has meant many things, the ultimate goal in software radio has been the realization of an agile radio that can transmit and receive signals at any carrier frequency using any protocol, all of which can be reprogrammed virtually instantaneously. Such a system places great demands on the limits of data converter and processor technologies since it requires real-time disposition of gigasamples of data produced by direct conversion of wireless signals into digital data. From a processing standpoint, the challenge in software radio is to exploit the three basic processor types-fixed architecture processors, FPGAs, and programmable DSPs/RISCs/CISCs-in such a way as to optimize the three-way trade-offs between speed, power dissipation, and programmability. With respect to the latter characteristic, the issues of high-level language interfaces, portability, and reprogramming speed must be considered. This article describes the architecture and operation of a PC-based software radio receiver. The development environment is a real-time PC-based platform that allows testing to be done in a simple manner using the main software functionality of a PC. The front-end of the receiver implemented in hardware represents a novel wideband design (bandwidth of up to 100 MHz centered at a carrier frequency of up to 2 GHz) that functionally converts wireless signals directly into a gigasample digital data stream in the receiver (and vice versa in the transmitter). This direct conversion approach shows the greatest promise in realizing the main goal of software radio.

On the throughput performance of multirate IEEE 802.11 networks with variable-loaded stations: analysis, modeling, and a novel proportional fairness criterion

This paper focuses on multirate IEEE 802.11 Wireless LAN employing the mandatory Distributed Coordination Function (DCF) option. Its aim is threefold. Upon starting from the multi-dimensional Markovian state transition model proposed by Malone et.al. for characterizing the behavior of the IEEE 802.11 protocol at the Medium Access Control layer, it presents an extension accounting for packet transmission failures due to channel errors. Second, it establishes the conditions under which a network constituted by N stations, each station transmitting with its own bit rate, Rd(s), and packet rate, λs, can be assumed loaded. Finally, it proposes a modified Proportional Fairness (PF) criterion, suitable for mitigating the rate anomaly problem of multirate loaded IEEE 802.11 Wireless LANs, employing the mandatory DCF option. Compared to the widely adopted assumption of saturated network, the proposed fairness criterion can be applied to general loaded networks. The throughput allocation resulting from the proposed algorithm is able to greatly increase the aggregate throughput of the DCF, while ensuring fairness levels among the stations of the same order as the ones guaranteed by the classical PF criterion. Simulation results are presented for some sample scenarios, confirming the effectiveness of the proposed criterion for optimized throughput allocation.

An extensive search for good punctured rate-k/(k+1) recursive convolutional codes for serially concatenated convolutional codes

In many practical applications requiring variable-rate coding and/or high-rate coding for spectral efficiency, there is a need to employ high-rate convolutional codes (CC), either by themselves or in a parallel or serially concatenated scheme. For such applications, in order to keep the trellis complexity of the code constant and to permit the use of a simplified decoder that can accommodate multiple rates, a mother CC is punctured to obtain codes with a variety of rates. This correspondence presents the results of extensive search for optimal puncturing patterns for recursive convolutional codes leading to codes of rate k/(k+1) (k an integer) to be used in serially concatenated convolutional codes (SCCC). The code optimization is in the sense of minimizing the required signal-to-noise ratio (SNR) for two target bit-error rate (BER) and two target frame-error rate (FER) values. We provide extensive sample simulation results for rate-k/(k+1) SCCC codes employing our optimized punctured CC.

LDPC-based channel coding of correlated sources with iterative joint decoding

This letter considers low-density parity-check (LDPC) coding of correlated binary sources and a novel iterative joint channel decoding without communication of any side information. We demonstrate that depending on the extent of the source correlation, additional coding gains can be obtained. Two stages of iterative decoding are employed. During global iterations, updated estimates of the source correlation are obtained and passed on to the sum-product decoder that performs local iterations with a predefined stopping criterion and/or a maximum number of local decoding iterations. Simulation results indicate that very few global iterations (2-5) are sufficient to reap significant benefits from implicit knowledge of source correlation. Finally, we provide analytical performance bounds for our iterative joint decoder and comparisons with sample simulation results.

A Novel Class of Decimation Filters for Sigma-Delta A/D Converters

A major bottleneck to the design of a truly wideband reconfigurable Software Radio (SR) transceiver is the front end of the system. It is desirable to push the analog-to-digital boundary of the SR transceiver as close to the antenna as possible. This places great demands on analog-to-digital converters that must produce high-resolution samples of the incoming signal centered around a carrier frequency in the gigahertz range. A very promising architecture for the design of such converters is the ΣΔ architecture. A critical component of the ΣΔ converter is the rate-conversion filter that is responsible for rejection of the out-of-band noise. Cascaded-Integrator-Comb (CIC) filters are efficient antialiasing rate-conversion filter structures realized by cascading integrator and comb cells separated by a decimation block. High-order structures, attempting to increase the rejection of the out-of-band noise that folds into the useful signal bandwidth because of the decimation, have the drawback of inserting multiple zeroes in the same positions throughout the stop-band and to increase the edge-band attenuation. In this paper, we propose a class of decimation filter architectures composed of a cascade of modified CIC filters, which have higher attenuation of the quantization noise produced by a ΣΔ modulator around the folding bands and lower passband drop than classic CIC structures. The design criteria of the proposed filters are described, with the goal of maximizing the denoising effect and minimizing the passband drop, and the problem of the practical realization of the proposed decimation scheme is addressed. Simulations confirm the effectiveness of the proposed decimation filter architectures. Copyright

Iterative joint channel decoding of correlated sources employing serially concatenated convolutional codes

This correspondence looks at the problem of joint decoding of serially concatenated convolutional codes (SCCCs) used for channel coding of multiple correlated sources. We assume a simple model whereby two correlated sources transmit SCCC encoded data to a single destination receiver. We do not assume the existence of, nor do we use channel side information at the receiver. In particular, we present a novel iterative joint channel decoding algorithm for correlated sources by using the empirical cross-correlation measurements at successive decoding iterations to provide extrinsic information to the outer codes of the SCCC configuration. Two levels of soft metric iterative decoding are used at the receiver: 1) iterative maximum a posteriori probability (MAP) decoding is used for efficient decoding of individual SCCC codes (local iterations) and 2) iterative extrinsic information feedback generated from the estimates of the empirical cross correlation in partial decoding steps is used to pass soft information to the outer decoders of the global joint SCCC decoder (global iterations). We provide analytical results followed by simulation studies confirming the robustness of the cross-correlation estimates to channel-induced errors, justifying the use of such estimates in iterative decoding. Experimental results suggest that relatively few global iterations (two to five) during which multiple local iterations are conducted are sufficient to reap significant gains using this approach specially when the sources are highly correlated.

Interleaver design for serially concatenated convolutional codes: theory and application

This paper addresses the problem of interleaver design for serially concatenated convolutional codes (SCCCs) tailored to the constituent codes of the SCCC configuration. We present a theoretical framework for interleaver optimization based on a cost function closely tied to the asymptotic bit-error rate (BER) of the block code C/sub s/ resulting from proper termination of the constituent codes in the SCCC code. We define a canonical form of the interleaving engine denoted as the finite state permuter (FSP) and using its structural property, develop a systematic iterative technique for construction of interleavers. The core theoretical results focus on the asymptotic behavior of a class of cost functions and their martingale property, which is then used to develop an order recursive interleaver optimization algorithm. We address the issue of the complexity of the interleaver growth algorithm presented in the paper and demonstrate that it has polynomial complexity. Subsequently, we provide details about the application of the proposed technique and present a modification of the algorithm that employs error pattern feedback for improved performance at a reduced complexity. Sample experimental results are provided for an SCCC code of rate 1/3 and information block length 320 that achieves a minimum distance of d/sub min/=44.

On the Linear Behaviour of the Throughput of IEEE 802.11 DCF in Non-Saturated Conditions

We propose a linear model of the throughput of the IEEE 802.11 distributed coordination function (DCF) protocol at the data link layer in non-saturated traffic conditions. We show that the throughput is a linear function of the packet arrival rate (PAR) lambda with a slope depending on both the number of contending stations and the average payload length. We also derive the interval of validity of the proposed model by showing the presence of a critical lambda, above which the station begins operating in saturated traffic conditions. The analysis is based on the multi-dimensional Markovian state transition model proposed by Liaw et al. with the aim of describing the behaviour of the MAC layer in unsaturated traffic conditions. Simulation results closely match the theoretical derivations, confirming the effectiveness of the proposed linear model.