Anuj Batra

Design of a multiband OFDM system for realistic UWB channel environments

In February 2002, the Federal Communications Commission allocated 7500 MHz of spectrum for unlicensed use of commercial ultra-wideband (UWB) communication devices. This spectral allocation has initiated an extremely productive activity for industry and academia. Wireless communications experts now consider UWB as available spectrum to be utilized with a variety of techniques, and not specifically related to the generation and detection of short RF pulses as in the past. There are many differences between real-world behavior of narrow-band and UWB systems. All wireless systems must be able to deal with the challenges of operating over a multipath propagation channel, where objects in the environment can cause multiple reflections to arrive at the receiver (RX). For narrow-band systems, these reflections will not be resolvable by the RX when the narrow-band system bandwidth is less than the coherence bandwidth of the channel. The large bandwidth of UWB waveforms, instead, significantly increases the ability of the RX to resolve the different reflections in the channel. The UWB channel model developed by the IEEE 802.15.3a standard body is described in this paper. For highly dispersive channels, an orthogonal frequency-division multiplexing (OFDM) RX is more efficient at capturing multipath energy than an equivalent single-carrier system using the same total bandwidth. OFDM systems possess additional desirable properties, such as high spectral efficiency, inherent resilience to narrow-band RF interference, and spectral flexibility, which is important because the regulatory rules for UWB devices have not been finalized throughout the entire world. This paper describes the design of a UWB system optimized for very high bit-rate, low-cost, and low-power wireless networks for personal computing (PC), consumer electronics (CE), and mobile applications. The system combines OFDM modulation technique with a multibanding approach, which divides the spectrum into several sub-bands, whose bandwidth is approximately 500 MHz. The system described in this paper has been selected by several key industry organizations [Mulitband OFDM Alliance, WiMedia, Wireless Universal Serial Bus (USB)] because of its very good technical characteristics for the diverse set of high performance short-range applications that are eagerly anticipated for CE, PC, and mobile applications.

A multi-band OFDM system for UWB communication

In this paper, a multi-band OFDM system for ultra-wideband communication (UWB) is described. In this system, the UWB spectrum is divided into bands that are 528 MHz wide and the data is transmitted across the bands using a time-frequency code. Within each sub-band, an OFDM modulation scheme is used to convey the information. An overview of the multi-band OFDM system and various system design trade-offs are discussed within this paper. In addition, bit precision requirements, power consumption estimates and performance results in realistic multi-path environments are provided for the multi-band OFDM system.

Cognitive radio techniques for wide area networks

The cellular wireless market has begun the transition to data centric services including high speed Internet access, video, high quality audio, and gaming. Communications technology can meet the need for very high data link speeds, and can also improve network throughput, but dramatically more spectrum is needed to provide ubiquitous wireless data service. Cognitive radio is a new technology that allows spectrum to be dynamically shared between users. It offers the potential to dramatically change the way spectrum is used in systems and to substantially increase the amount of spectrum available for wireless communications. This paper introduces cognitive radio and explains the promise, possible operating modes, and benefits it may offer.

Multi-band OFDM: a new approach for UWB

In this paper, a multi-band OFDM (MB-OFDM) system for ultra-wideband communication is described. In this system, the UWB spectrum is divided into bands that are 528 MHz wide and the data is transmitted across the bands using a time-frequency code. Within each subbands using a time-frequency code. Within each subband, an OFDM modulation scheme is used to transmit the information. An overview of the MB-OFDM system and various systems design consideration are discussed. In addition, the performance results in realistic multi-path environments are provided for the MB-OFDM.

Multi-band OFDM: a cognitive radio for UWB

Ultra-wideband (UWB) communication systems are able to achieve high data rates by utilizing a large amount of spectrum. This signal typically overlaps with many existing and potentially future services. Therefore, for an UWB device to be successful, it will need to be able to identify and protect these victim receivers via cognitive radio techniques. Multi-band orthogonal frequency division modulation (MB-OFDM) has an inherent ability to perform the functions of a cognitive radio. With minimal changes to the technology, it is possible to identify victim services and provide the necessary protection mechanisms via spectral notches based on distance, bandwidth and other such measures. In this paper, the techniques for detecting interfering signals, estimating the distance between victim and interfering devices, and shaping the spectrum to mitigate interference into the victim receiver are described

Space-time Chase decoding

Multiple-antenna wireless systems are of interest because they provide increased capacity over single-antenna systems. Several space-time signaling schemes have been proposed to make use of this increased capacity. Space-time techniques, such as space-time block coding and spatial multiplexing, can all be viewed as signaling with a multidimensional constellation. Because of the large capacity of multiple-input multiple-output (MIMO) channels, these multidimensional constellations often have large cardinalities. For this reason, it is impractical to perform optimal maximum-likelihood (ML) decoding for space-time systems, even for a moderate number of transmit antennas. In this paper, we propose a modified version of the classic Chase decoder for multiple-antenna systems. The decoder applies successive detection to yield an initial estimate of the transmitted bit sequence, constructs a list of candidate symbol vectors using this initial estimate, and then computes bit likelihood information over this list. Three algorithms are presented for constructing the candidate vector list. This decoder can be adjusted to have a fixed or variable complexity, while maintaining performance close to that of an ML decoder.

Chase decoding for space-time codes

Multiple antenna wireless systems are known to provide a higher capacity than traditional single antenna systems. Over the past few years, space-time signaling schemes that make use of this increased capacity have been studied. Because of the large capacity of multiple-input multiple-output channels, the multidimensional constellations used by these space-time techniques are large in size, making it impractical to perform optimal maximum likelihood decoding even for a moderate number of transmit antennas. In this paper, we propose a space-time version of the binary Chase decoder. The decoder generates an initial estimate of the transmitted bit sequence from successive detection and then uses this bit estimate to generate a reduced search space (or list) to perform minimum distance decoding. Three algorithms for constructing the space-time reduced search space are overviewed.

On the Coexistence of Overlapping BSSs in WLANs

In this paper, we investigate the issue of 20/40 MHz coexistence in next generation wireless local area networks (WLAN). To that end, we present simulation results of overlapping basic service sets (BSSs) an 802.11n BSS operating in 20/40 MHz mode and a legacy BSS operating in 20 MHz channel. Our results show that if clear channel assessment (CCA) is not used in the overlapping channel; i.e., the 20 MHz channel used by both BSSs, the throughput in legacy BSS falls to zero, while the throughput of 802.11n BSS decreases dramatically. If CCA is used in the overlapping channel, the throughput of both, legacy and 802.11n BSS increases compared to the earlier case. When devices operating in 20/40 MHz BSS are able to dynamically switch between 20 and 40 MHz transmissions and in addition, CCA is used in the overlapping channel, the throughput of both BSSs further increases. Reducing transmission opportunity (TXOP) interval for 40 MHz transmissions also improves fairness and throughput for legacy BSSs.

Robust transceiver to combat periodic impulsive noise in narrowband powerline communications

Non-Gaussian noise/interference severely limits communication performance of narrowband powerline communication (PLC) systems. Such noise/interference is dominated by periodic impulsive noise whose statistics varies with the AC cycle. The periodic impulsive noise statistics deviate significantly from that of additive white Gaussian noise, thereby causing dramatic performance degradation in conventional narrowband PLC systems. In this paper, we propose a robust transmission scheme and corresponding receiver methods to combat periodic impulsive noise in OFDM-based narrowband PLC. Towards that end, we propose (1) a time-frequency modulation diversity scheme at the transmitter and a diversity demodulator at the receiver to improve communication reliability without decreasing data rates; and (2) a semi-online algorithm that exploits the sparsity of the noise in the frequency domain to estimate the noise power spectrum for reliable decoding at the diversity demodulator. In the simulations, compared with a narrowband PLC system using Reed-Solomon and convolutional coding, whole-packet interleaving and DBPSK/BPSK modulation, our proposed transceiver methods achieve up to 8 dB gains in Eb/N0 with convolutional coding and a smaller-sized interleaver/deinterleaver.

Time-Frequency Modulation Diversity to Combat Periodic Impulsive Noise in Narrowband Powerline Communications

Non-Gaussian noise/interference severely limits communication performance of narrowband powerline communication (PLC) systems. Such noise/interference is dominated by periodic impulsive noise whose statistics varies with the AC cycle. The periodic impulsive noise statistics deviates significantly from that of additive white Gaussian noise, thereby causing dramatic performance degradation in conventional narrowband PLC systems. In this paper, we propose a robust transmission scheme and corresponding receiver methods to combat periodic impulsive noise in OFDM-based narrowband PLC. Towards that end, we propose (1) a time-frequency modulation diversity scheme at the transmitter and a diversity demodulator at the receiver to improve communication reliability without decreasing data rates; and (2) a semi-online algorithm that exploits the sparsity of the noise in the frequency domain to estimate the noise power spectrum for reliable decoding at the diversity demodulator. In the simulations, compared with a narrowband PLC system using Reed-Solomon and convolutional coding, whole-packet interleaving and DBPSK/BPSK modulation, our proposed transceiver methods achieve up to 8 dB gains in Eb/N0 with convolutional coding and a smaller-sized interleaver/deinterleaver.