Benjamin Premkumar

MMSE detection for high data rate UWB MIMO systems

In this paper, a uniform matrix representation for ultra-wideband (UWB) multiple-input multiple-output (MIMO) systems with time-hopping spread spectrum (THSS) and direct-sequence spread spectrum (DSSS) methods is formulated. The inter symbol interference (ISI) is considered for high data rate UWB transmissions in the presence of dense multipath in an indoor environment. The bit error rate (BER) performance is evaluated based on the minimum mean square error (MMSE) detection criterion. In addition, a simplified UWB MIMO channel model is assumed for performance analysis based on the IEEE 802.15.3a channel model recommendation and the MIMO channel covariance matrix representations. Results show that a UWB MIMO system with THSS method is capable of delivering promising performance up to very high symbol rate transmission for short range. On the other hand, a UWB MIMO system with DSSS method is more vulnerable to high symbol rate transmission due to the existence of severe inter chip interference (ICI) in addition to ISI.

Error concealment scheme for MPEG-AAC

MPEG-AAC is the state-of-the-art in audio compression technology. Promising CD quality at 64 kbps, AAC is the leading option for low bandwidth audio streaming applications. In spite of the support from channel coding and the error resilience features of AAC itself, handling erroneous transmission requires the decoder to be equipped with error concealment scheme to cater for missing packets. However, complexity is limited by the real time requirement in this application. We present in this paper a novel scheme of error concealment, specifically for AAC. This scheme exploits the time domain aliasing cancellation property of MDCT based codec and the block-switching feature of AAC. Preliminary simulations show significant improvement in both error performance and subjective quality testing compared to general method of concealment. The scheme does not introduce major computational load, and hence suitable for real time low power decoder

A combined MMSE receiver for high data rate UWB systems

In this paper, an ultra wideband (UWB) receiver is proposed based on optimally combining of multiple minimum mean square error (MMSE) detector outputs. These outputs are obtained by applying shifted observation windows. On one hand, each MMSE detector uses smaller number of taps and can be trained separately. This makes the combined structure suitable for adaptive filter bank implementation and for parallel processing. On the other hand, the system performance is effectively improved by diversity combining. The analysis and simulation results show that the combined MMSE receiver is capable of very high data rate transmission based on IEEE 802.15.3a channel model recommendation (J. Foerster)

Thermometer Code Based Modular Arithmetic

This paper presents a novel approach to perform modular arithmetic addition and subtraction using base-1 thermometer code data format for operands corresponding to the residues of the same modulus. Two n-bit thermometer code operands are first concatenated and logically shifted to produce a normalized 2n-bit thermometer code intermediate sum. Modulo operation is then applied to this 2n-bit intermediate sum to produce an n-bit datum corresponding to the modular sum of the two input operands. This approach greatly simplifies the modulo addition operation by eliminating the carry bit propagation during the arithmetic operation encountered when using the conventional base-2 binary code data format. It also enables practical applications of modular arithmetic for signal processing algorithms in a very efficient way. Circuit for implementing the modular arithmetic units using multiplexers and basic logic gates are also described in this paper.

Tap selection based MMSE equalization for high data rate UWB communication systems

The performance of a tap selection based minimum mean square error (MMSE) equalization technique for high data rate ultra wideband (UWB) systems is analyzed. This technique is shown to outperform significantly the conventional uniformly spaced equalizer with the same number of taps. In addition, the impact of channel estimation error on the performance of tap selection based decision feedback equalization (DFE) is analyzed. This channel estimation based DFE is shown to perform well with only a few training symbols. It can then be adapted effectively using a least mean squares (LMS) algorithm for improved performance. Numerical analysis and simulation results show that the tap selection based equalization technique is a promising approach for practical high performance UWB system implementation with reduced complexity.

A power-efficient access point operation for infrastructure basic service set in IEEE 802.11 MAC protocol

Infrastructure-based wireless LAN technology has been widely used in todays personal communication environment. Power efficiency and battery management have been the center of attention in the design of handheld devices with wireless LAN capability. In this paper, a hybrid protocol named improved PCF operation is proposed, which intelligently chooses the access point- (AP-) assisted DCF (distributed coordinator function) and enhanced PCF (point coordinator function) transmission mechanism of IEEE 802.11 protocol in an infrastructure-based wireless LAN environment. Received signal strength indicator (RSSI) is used to determine the tradeoff between direct mobile-to-mobile transmission and transmission routed by AP. Based on the estimation, mobile stations can efficiently communicate directly instead of being routed through AP if they are in the vicinity of each other. Furthermore, a smart AP protocol is proposed as extension to the improved PCF operation by utilizing the historical end-to-end delay information to decide the waking up time of mobile stations. Simulation results show that using the proposed protocol, energy consumption of mobile devices can be reduced at the cost of slightly longer end-to-end packet delay compared to traditional IEEE 802.11 PCF protocol. However, in a non-time-critical environment, this option can significantly prolong the operation time of mobile devices.