Jaiganesh Balakrishnan

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.

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

Exploiting signal and noise statistics for fixed point FFT design optimization in OFDM systems

Scaling the different stage outputs in an FFT appropriately is crucial for getting a good Signal to Quantization noise ratio (SQNR) in fixed point FFT design. Traditional designs have either handled this through Convergent block floating point technique (CBFP), which has memory, computation and latency penalties or through time consuming simulations. In this paper, we consider the special case of FFT design for OFDM transceivers. We exploit the Gaussian nature of OFDM signals to predict the bit-growth of the signal through the various stages of the FFT and propose a technique to scale the signal appropriately. Additionally, we investigate the quantization error profile and propose a technique to improve SQNR by exploiting the presence of null tones at the band edges. With the proposed techniques, the performance comes close to the CBFP design, with no increase in complexity compared to existing static designs. Simulation results illustrating the performance improvements of the proposed technique are presented.

Robust floor determination for indoor positioning

In recent years, indoor positioning system using existing Wi-Fi infrastructure and MEMS sensors have received significant attention. A key requirement for a complete 3-dimensional indoor positioning algorithm is to provide an accurate and robust user floor location estimate in a multi-floor building. In this paper, we propose novel techniques to estimate user floor location based on the received radio signal strengths from Wi-Fi access points in the building. We illustrate its performance advantages through simulations and multiple field test results across typical use case scenarios. We further discuss Wi-Fi outage scenarios during which the performance of the Wi-Fi based floor determination algorithm is limited. We then propose a method to blend the Wi-Fi access points based floor estimate with pressure sensor measurements to enhance the overall performance of the algorithm. Finally, we present performance results from field tests to validate the efficacy and robustness of our proposed techniques.

Mitigation of narrowband interference in differentially modulated communication systems

Integration of RF/Analog and digital into a single chip can result in the coupling of digital spurs in the analog front end of a communication systems and can severely degrade the receiver performance. In this paper, we propose a novel receiver architecture of a notch filter together with the feedback filter (FBF) of the decision feedback equalizer (DFE) to mitigate the impact of narrowband interference in differentially modulated communication systems without enhancing the inter-symbol interference (ISI). The proposed solution can be adapted to receiver architectures that employ either coherent or non-coherent demodulation of differentially modulated systems. Simulation results demonstrating the spur mitigation capability of the proposed solution is presented.

Parameter mismatch estimation in a parallel interleaved ADC

In this paper we propose a novel method for estimating the gain, DC offset and sample timing mismatches between component ADCs in a Parallel Interleaved ADC. We propose the use of a reference ADC, operating at a lower rate, that precludes the need for any calibration period. The reference ADC is clocked using a novel scheme to provide reference samples that are used by the estimation algorithm We propose the use of Least Squares (LS) based approach for correcting the gain and DC offset mismatches as well as a bisection search and a fixed step adaptation for estimating/correcting the sample timing offset. These algorithms ensure fast initial convergence and good steady state tracking with little implementation overhead. Simulation results are presented to illustrate the efficacy of these techniques.

Dummy tone insertion for spectral sculpting of the multi-band OFDM UWB system

As the UWB regulations have not been finalized in many parts of the world, spectral flexibility is a key requirement for an UWB system to win in the global marketplace. The unlicensed nature of the UWB spectrum makes it imperative for these devices to have the capability to coexist with existing services. The multi-band OFDM UWB communication system, standardized by ECMA for high-speed WPAN, has the ability to satisfy these requirements. This paper describes a time-domain windowing technique and a dummy tone insertion mechanism that enable spectral sculpting of a multi-band OFDM UWB communication system. The protection offered to radio astronomy bands in Japan is used as an example to illustrate the advantages of these techniques.

Scenario-Aware Dynamic Power Reduction Using Bias Addition

Typical modern communication systems operate over a wide dynamic range of signal strengths. We consider the approach of adding a bias as offset to reduce switching activity, and study the average bit toggle for signals with different distributions, as a function of signal span and the actual bias value added. From the analysis, we provide guidelines to choose an optimal bias value, based on the system scenario, to obtain the lowest power consumption. Simulation results confirm the accuracy of the theoretical analysis. Various finite-impulse response filter architectures are evaluated, and we propose suitable enhancements to them to enable improved power savings. We apply the bias addition technique and the proposed architectural enhancements to a wireless local area network digital receiver chain, and demonstrate that over 25% power savings can be achieved under different signal conditions.