Witold A. Krzymien

Guided scrambling: a new line coding technique for high bit rate fiber optic transmission systems

The technique introduced has relatively simple encoding and decoding procedures which can be implemented at the high bit rates used in optical fiber communication systems. Because it is similar to the established technique of self-synchronizing scrambling but is also capable of guiding the scrambling process to produce a balanced encoded bit stream, the technique is called guided scrambling, (GS). The concept of GS coding is explained, and design parameters which ensure good line code characteristics are discussed. The performance of a number of guided scrambling configurations is reported in terms of maximum consecutive like-encoded bits, encoded stream disparity, decoder error extension, and power spectral density of the encoded signal. Comparison of guided scrambling with conventional line code techniques indicates a performance which approaches that of alphabetic lookup table codes with an implementation complexity similar to that of current nonalphabetic coding techniques. >

Simplified Fair Scheduling and Antenna Selection Algorithms for Multiuser MIMO Orthogonal Space-Division Multiplexing Downlink

We consider the downlink of a multiuser multiple-input multiple-output (MIMO) system, where the base station and the mobile receivers are equipped with multiple antennas. We propose simplified algorithms for channel-aware multiuser scheduling in conjunction with receive antenna selection for two downlink multiuser orthogonal space-division multiplexing techniques: block diagonalization and successive optimization. The algorithms greedily maximize the weighted sum rate. The algorithms add the best user at a time from the set of users that are not selected yet to the set of selected users until the desired number of users has been selected. To apply the proportional fairness criterion, simplified user scheduling metrics are proposed for block diagonalization and successive optimization. Two receive antenna selection algorithms are also proposed, which further enhance the power gain of the equivalent single-user channel after orthogonal precoding by selecting a subset of the receive antennas that contributes the most toward the total power gain of the channel. A user grouping technique is used to further lower the complexity of the selection algorithms. We compare various multiuser MIMO scheduling strategies that are applied to block diagonalization and successive optimization transmission techniques through simulation. Simulation results demonstrate the effectiveness of the proposed algorithms in ensuring throughput fairness among users. Results also show that when the number of users is large, the proposed scheduling algorithms perform close to the exhaustive search algorithms and previously proposed greedy scheduling algorithms, but with much lower complexity.

Sensitivity of the MC-CDMA access scheme to carrier phase noise and frequency offset

The combination of multicarrier (MC) modulation and code-division multiple access (CDMA) is seen as a very promising technique for the development of high-capacity wireless local communications. On the other hand, this scheme is very sensitive to the signal distortion generated by the imperfect frequency down conversion at the receiver due to the local oscillator phase noise and frequency offset. In this work, we consider the MC-CDMA architecture that was independently proposed in Yee et al. (1993) and Falsafi et al. (1996), and, in particular, we evaluate the impact of both the carrier frequency offset and the phase noise on the system performance. The results are given in terms of symbol error rate (SER) obtained by means of an analytical approach for the additive white Gaussian noise (AWGN) channel. However, the procedure can be applied to the performance evaluation in the presence of a multipath fading channel as part of a semianalytical approach. This approach offers a realistic means of performance evaluation when the target error probability values are of the order of 10/sup -6/ or lower.

Effect of carrier phase noise and frequency offset on the performance of multicarrier CDMA systems

Kaiser (1995) showed that OFDM-CDMA outperforms DS-CDMA in radio fading channels in terms of spectral efficiency, under the assumption of ideal carrier recovery and chip synchronization. However, residual carrier frequency offset and phase noise degrade the system performance significantly if suitable countermeasures are not taken. In this work we consider the downlink of a cellular multicarrier CDMA system. In particular, we evaluate the impact of the carrier frequency offset and Wiener phase noise on the system performance considering typical indoor radio channels. Moreover, a frequency detector is used to reduce the impairment produced by the carrier frequency offset. Results are given in terms of BER obtained by means of simulations and analytical computations.

Rapid acquisition algorithms for synchronization of bursty transmissions in CDMA microcellular and personal wireless systems

Two rapid synchronization acquisition algorithms applicable to spread spectrum links of code division multiple access (CDMA) personal communication systems are proposed and evaluated. The algorithms operate within a self-referencing matched filter synchronizer structure, and are particularly useful in reducing synchronization overhead on links designed to carry packet-type services. The main distinguishing characteristic between the two schemes is that one uses hard-decision while the other uses soft-decision detection. The proposed schemes are especially applicable to reverse link transmissions in quasisynchronous CDMA systems in which timing at portable terminals is established via pilot and synchronization signals received on respective code-division channels from the home base. If discontinuous (bursty) transmission is used on reverse links, the acquisition process is required for each transmission burst because of the propagation time uncertainty. Analysis of the algorithms on additive white Gaussian noise (AWGN) and Rayleigh fading channels reveals that their performance depends significantly on the choice of synchronizer parameters and the average despread signal-to-interference ratio (SIR). When this choice is proper, acquisition over a single preamble of relatively short duration can be achieved with high probability. The soft-decision scheme introduces a performance advantage of between 4-9 dB depending on the length of the synchronizing preamble.

Performance Effects of the Uplink Asynchronism in a Spread Spectrum Multi‐Carrier Multiple Access System

In this paper we investigate the effects of the asynchronism of user signals in the uplink of a spread spectrum multi-carrier multiple access (SS-MC-MA) system. Different propagation delays due to different distances between the mobile transmitters and their base station cause asynchronous arrivals of the uplink signals in addition to multipath propagation. The proposed uplink SS-MC-MA scheme uses for synchronization only the frame structure received on the synchronous downlink and requires no additional synchronization measures. A guard interval that is smaller than the sum of the maximum time offset between the users and the maximum excess delay of the frequency selective multipath channel is used. It minimizes the loss in bandwidth efficiency by allowing residual interference. The residual interference is minimized by proper positioning of the detection interval in the receiver. It is shown that with this approach the guard interval can be reduced by about 50%. The performance of the proposed uplink scheme is compared to an uplink SS-MC-MA scheme with perfect synchronization.

Density Evolution for Nonbinary LDPC Codes Under Gaussian Approximation

This paper extends the work on density evolution for binary low-density parity-check (LDPC) codes with Gaussian approximation to LDPC codes over GF(q) . We first generalize the definition of channel symmetry for nonbinary inputs to include q-ary phase-shift keying (PSK) modulated channels for prime q and binary-modulated channels for q that is a power of 2. For the well-defined q-ary-input symmetric-output channel, we prove that under the Gaussian assumption, the density distribution for messages undergoing decoding is fully characterized by (q-1) quantities. Assuming uniform edge weights, we further show that the density of messages computed by the check node decoder (CND) is fully defined by a single number. We then present the approximate density evolution for regular and irregular LDPC codes, and show that the (q-1) -dimensional integration involved can be simplified using a dimensionality reduction algorithm for the important case of q=2p. Through application of approximate density evolution and linear programming, we optimize the degree distribution of LDPC codes over GF(3) and GF(4). The optimized irregular LDPC codes demonstrate performance close to the Shannon capacity for long codewords. We also design GF(q) codes for high-order modulation by using the idea of a channel adapter. We find that codes designed in this fashion outperform those optimized specifically for the binary additive white Gaussian noise (AWGN) channel for a short codewords and a spectral efficiency of 2 bits per channel use (b/cu).

Downlink Scheduling via Genetic Algorithms for Multiuser Single-Carrier and Multicarrier MIMO Systems With Dirty Paper Coding

Multiple-input-multiple-output (MIMO) systems are of interest for meeting the expected demand for higher data rates and lower delays in future wireless packet data systems. In such systems, it is optimal to simultaneously transmit to multiple users compared with a single user in a single-input-single-output system. In addition, multicarrier systems are of interest to combat frequency-selective fading that is experienced over the larger bandwidth that these future broadband systems will use. The use of dirty paper coding further complicates the matter, because the order in which the users are encoded will affect the rates that they can achieve. A well-designed cross-layer scheduling algorithm must take into account the multiple dimensions of this resource-allocation problem and other quality-of-service (QoS) parameters to fully exploit the communications channel. The scheduling problem is often expressed in terms of optimizing some utility function. Unfortunately, the search space for this optimization problem is extremely large, which prohibits an optimal exhaustive search. To this end, we investigate the use of genetic algorithms to reduce the complexity of the scheduling. This paper builds upon prior work that implements scheduling via genetic algorithms in the context of single-carrier systems using zero-forcing beamforming (ZFB). In this paper, we investigate how the genetic algorithm can be adapted to account for the effect of encoding order on the scheduling and how the scheduling can be extended to a multicarrier system. In particular, we investigate the maximum throughput and proportionally fair scheduling criteria. We demonstrate that the performance of the genetic algorithm is near optimal compared with an exhaustive search at a greatly reduced computational complexity. Furthermore, in the case of a multicarrier orthogonal frequency-division multiplexing (OFDM) system, an increase in capacity is shown relative to the single-carrier case.

Scheduling algorithms for the cdma2000 packet data evolution

The paper presents a performance comparison of several scheduling algorithms for the forward link of the high data rate (HDR) system. This system has been adopted as a possible evolutionary path for the North American 3/sup rd/ generation cdma2000 cellular system, and is designed to enable high bit rate packet data access. On the HDR forward link, only one user receives packet data in a given time slot. Hence, a scheduling algorithm is necessary to determine which user among those requesting service should receive data. In our performance analysis, we assume data packets are always available for transmission to every user. The scheduler performance is evaluated in terms of the average throughput per sector as a function of the number of users in the sector and the bit rate distributions. The throughput distributions over a set of users, the average delay per packet and per user, and the distributions of packet delays and allocated slots per user are also determined as a measure of fairness of the scheduling algorithms.

A Novel Nonlinear Joint Transmitter-Receiver Processing Algorithm for the Downlink of Multiuser MIMO Systems

This paper focuses on signal processing algorithms for the downlink of multiuser multiple-input multiple-output (MIMO) systems with multiple-antenna mobiles. A novel nonlinear joint transmitter-receiver processing algorithm is proposed based on the zero-forcing (ZF) criterion. In this algorithm, nonlinear Tomlinson-Harashima precoding (THP) is applied at the base station, whereas linear receiver processing and modulo operation are applied at each mobile. It is first shown that the proposed algorithm effectively decomposes the multiuser MIMO channel into parallel independent single-user MIMO channels, and then, the performance of each mobile can be separately optimized. Subsequently, closed-form expressions for the transmitter and receiver processing matrices are derived to optimize the asymptotic bit error rate (BER) of each mobile. When used on the downlink of multiuser MIMO systems with multiple-antenna mobiles, this algorithm achieves significantly better performance than the ZFcriterion-based nonlinear preprocessing algorithm designed for the multiuser MIMO systems with single-antenna mobiles, because it effectively utilizes the processing capabilities of the mobiles. Moreover, the proposed algorithm achieves a much higher sum capacity at a high signal-to-noise ratio (SNR) than the known block diagonalization technique due to the effective application of the nonlinear preprocessing at the transmitter. When the proposed algorithm is applied, it is found that better system performance can be achieved by suitably ordering the channel matrices of different mobiles, and a combined optimal diversity and best-first (CODBF) ordering method is proposed to perform the ordering. Simulation is used to show the advantages of the proposed algorithm and the CODBF ordering method.