Dana Porrat

Channel Uncertainty in Ultra-Wideband Communication Systems

Channel uncertainty limits the achievable data rates of certain ultra-wideband systems due to the need to estimate the channel. The use of bursty duty-cycled transmission reduces the channel uncertainty because the receiver has to estimate the channel only when transmission takes place, but the maximum amount of burstiness and hence the possible reduction of channel uncertainty both depend on the spectral efficiency of the modulation scheme used. This general principle is demonstrated by comparing the channel conditions that allow duty-cycled direct-sequence spread spectrum (DSSS) and pulse position modulation (PPM) to achieve the additive white Gaussian noise (AWGN) channel capacity in the wideband limit. We show that duty-cycled DSSS systems achieve the wideband capacity as long as the number of independently faded resolvable paths increases sublinearly with the bandwidth, while duty-cycled PPM systems can achieve the wideband capacity only if the number of paths increases sublogarithmically. The difference is due to the fact that DSSS is spectrally more efficient than PPM and hence allows more bursty transmission

Prediction and Modeling for the Time-Evolving Ultra-Wideband Channel

We conduct a feasibility study of ultra-wideband (UWB) channel prediction to answer the following two questions: Is the UWB channel predictable? Is UWB channel prediction useful? We setup the problem in the following way: A receiver travels along a linear trajectory at a constant velocity. The transmitter and environment are stationary. Using past channel measurements, the receiver predicts future measurements of the channel, assuming its direction of movement and velocity remain constant. Our approach is to decompose the time evolution of the channel, which is jointly correlated in time and delay, in terms of the time evolution of individual paths, which are independent across delay. A measurement campaign was conducted in the Berkeley Wireless Research Center, where measurements were taken with line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. We develop a channel prediction algorithm, and evaluate results in terms of the matched filter output energy (MFOE). Iterating through the six strongest paths, our prediction algorithm achieves more than 70% (40%) of the possible MFOE over a prediction distance of 34 cm for the LOS (NLOS) conditions. These results are good since the coherence distance, being the distance for which the channel is approximately constant, is less than 1 cm.

Non-Peaky Signals in Wideband Fading Channels: Achievable Bit Rates and Optimal Bandwidth

In the context of fading channels it is well established that, with a constrained transmit power, the bit rates achievable by signals that are not peaky vanish as the bandwidth grows without bound. Stepping back from the limit, we characterize the highest bit rate achievable by such non-peaky signals and the approximate bandwidth where that apex occurs. As it turns out, the gap between the highest rate achievable without peakedness and the infinite-bandwidth capacity (with unconstrained peakedness) is small for virtually all settings of interest to wireless communications. Thus, although strictly achieving capacity in wideband fading channels does require signal peakedness, bit rates not far from capacity can be achieved with conventional signaling formats that do not exhibit the serious practical drawbacks associated with peakedness. In addition, we show that the asymptotic decay of bit rate in the absence of peakedness usually takes hold at bandwidths so large that wideband fading models are called into question. Rather, ultrawideband models ought to be used.

Noncontact Tremor Characterization Using Low-Power Wideband Radar Technology

Continuous monitoring and analysis of tremor is important for the diagnosis and establishment of treatments in many neurological disorders. This paper describes noncontact assessment of tremor characteristics obtained by an experimental new ultrawideband (UWB) system. The system is based on transmission of a wideband electromagnetic signal with extremely low power, and analysis of the received signal, which is composed of many propagation paths reflected from the patient and its surroundings. A description of the physical principles behind the technology, a criterion, and efficient algorithms to assess tremor characteristics from the bulk UWB measurements are given. A feasibility test for the technology was conducted using a UWB system prototype, an arm model that mimics tremor, and a reference video system. The set of UWB system frequencies and amplitudes estimations were highly correlated with the video system estimations with an average error in the scale of 0.1 Hz and 1 mm for the frequency and amplitude estimations, respectively. The new UWB-based system does not require attaching active markers or inertial sensors to the body, can give displacement information and kinematic features from multiple body parts, is not limited by the range captured by the optical lens, has high indoor volume coverage as it can penetrate through walls, and does not require calibration to obtain amplitude estimations.

Indoor Channel Spectral Statistics, K-Factor and Reverberation Distance

The frequency (spectral) statistics of the line of sight channel is adequately described by the Rice distribution. The Ricean K-factor is extracted from a large body of measurements; it is given against terminal separation and carrier frequency in the range 2-18 GHz. A model based on a power law of the terminal separation and the reverberation distance is suggested. The electromagnetic reverberation distance is characterized for two different rooms, it is bigger for the larger room and tends to increase with frequency. The non line of sight spectral and spatial statistics are similar to each other and characterized by the Rayleigh distribution.

Tremor Acquisition System Based on UWB Wireless Sensor Network

This work suggests to quantify and analyze tremorusing an Ultra Wide-Band (UWB) Wireless Sensor Network(WSN). WSN based on UWB technology provides a new technology for non contact tremor assessment with extremely low radiation and penetration through walls. Tremor is the target symptom in the treatment of many neurological disorders such as Parkinson’s disease (PD), midbrain tremor, essential tremor (ET) and epilepsy. The common instrumental approaches for the assessment of tremor are motion capture devices and video tracking systems. The new tremor acquisition system is based on transmission of a wideband electromagnetic signal from multiple sensor nodes placed indifferent locations in a home and analysis of the received signal in each sensor node. The sensors can exchange information between each other with a coded transmission or send the raw data to a UWB hub for further analysis. The data can be then sent by an internet gateway to a health care center for monitoring and for medical care. This paper describes the basic system and gives a fundamental detection technique. For a feasibility test we built an UWB based sensor node prototype and examined its performance with an arm model that fluctuated in the range of clinical tremor frequencies (3-12 Hz). A future development of this work can lead to a low cost monitoring system installed at any home, hospital or school to continuously assess and report tremor conditions during daily life activities.

A two-tier frequency reuse scheme

The race toward higher throughputs for cellular network users is getting more difficult everyday. On the one hand cellular network operators wish to increase benefits by offering new services to users, but on the other hand spare radio resource are shrinking away. The spreading of WiFi-3G dual mode devices is making this fight even harder. The new ”Femtocell” technology is expected to be the rescuer of cellular network operators. This ”mini” cellular base station will provide high indoor coverage and throughput to indoor users relying on regular home access connections to the internet. However the big challenge remaining is to efficiently allocate spectrum to this technology. As spectrum licenses are very expensive most operators do not hold enough to completely separate the femtocell and macrocell layers. Coexisting femtocells and macrocells lead to severe interference scenarios. We propose here a double frequency reuse scheme, which allows a femtocell to reuse the frequency already in use by adjacent sectors of the overlaying macrocell. We present three solutions: full, partial or mixed frequency reuse. Each has advantages and drawbacks, and may be more suitable than the others in some specific configurations. Thanks to our scheme we preserve the radio resource management efficiency without affecting the system performance.

Sub-band analysis of NLoS indoor channel responses

Key factors of indoor NLoS radio channels are examined, namely the number of channel taps, the mean delay, the RMS delay spread and the channel gain. We analyze wideband (2-11 GHz) results in bands of 528 MHz and examine the mean behavior of the parameters vs. transmitter-receiver separation, with separations of up to 30 meters, as well as the parameterspsila correlation across sub-carrier frequency. The mean dependence of the parameters on terminal separation is approximated by a simple linear fit. The measurements were taken in three office buildings in the Givat Ram campus of the Hebrew University with the terminals in the same and in adjacent floors.

On synchronization of wideband impulsive systems in multipath

A preponderance of ultrawideband radio communication systems under current study employ the use of pulse position modulation (PPM). However, there are significant issues related to the synchronization of PPM systems in multipath. If the multipath scattering is rich i.e. the number of reflections (paths) increases without bound as the bandwidth increases, then synchronization is impaired. In particular, it is shown that a threshold-type detector fails in synchronizing, for any possible threshold, in the limit of large bandwidth. The maximum likelihood detector can synchronize under some severely constrained scenarios which do not appear to reflect reality in light of recent propagation measurements

Waveguide phenomena in wideband indoor radio channel

An analysis of the radio channel in a symmetric hallway is presented with an emphasis on spatial parameters of the power distribution. Gain and modal features are shown in channel responses measured across two hallways and compared to waveguide simulations. Field patterns across hallways show faster dynamics at high frequencies, in agreement with waveguide theory. Doors appear to have a significant effect on channel in hallways.