Ultra Wide Band Communication System

Term paper

on

“Ultra Wide Band Communication System”

Ramjee Prasad

RH6802B54

10804900

B.Tech(LEET)

6th sem

Overview:

Ultra-Wideband (UWB) is a technology for transmitting information spread over a large bandwidth (>500 MHz) that should, in theory and under the right circumstances, be able to share spectrum with other users. Regulatory settings of FCC are intended to provide an efficient use of scarce radio bandwidth while enabling both high data rate "personal area network" (PAN) wireless connectivity and longer-range, low data rate applications as well as radar and imaging systems.

Ultra Wideband was traditionally accepted as pulse radio, but the FCC and ITU-R now define UWB in terms of a transmission from an antenna for which the emitted signal bandwidth exceeds the lesser of 500 MHz or 20% of the center frequency. Thus, pulse-based systems—wherein each transmitted pulse instantaneously occupies the UWB bandwidth, or an aggregation of at least 500 MHz worth of narrow band carriers, for example in orthogonal frequency-division multiplexing (OFDM) fashion—can gain access to the UWB spectrum under the rules. Pulse repetition rates may be either low or very high. Pulse-based UWB radars and imaging systems tend to use low repetition rates, typically in the range of 1 to 100 mega pulses per second. On the other hand, communications systems favor high repetition rates, typically in the range of 1 to 2 giga-pulses per second, thus enabling short-range gigabit-per-second communications systems. Each pulse in a pulse-based UWB system occupies the entire UWB bandwidth, thus reaping the benefits of relative immunity to multipath fading (but not to intersymbol interference), unlike carrier-based systems that are subject to both deep fades and intersymbol interference

Introduction:
The definitive, end-to-end guide to high-performance UWB system designWith the FCC's approval of new ultra wideband standards, UWB is poised to drive breakthroughs in both commercial and military communications. However, UWB system design is radically different from conventional communications system design, and traditional design guides are insufficient-or even misleading. Now, for the first time, there's an authoritative and comprehensive guide to the latest best practices in UWB system design.Authored by leading-edge experts and researchers, Introduction to Ultra Wideband Communication Systems systematically addresses every major issue engineers will face in designing, implementing, testing, and deploying successful systems. The authors cover propagation, antennas, receiver and transmitter implementation, standards and regulations, interference, simulation, modulation and multiple access, networking, applications, and more.Topics include UWB fundamentals: technology, system components, standards/regulations, and the controversies surrounding UWB Never-before-published techniques for addressing propagation, modelling, and channel simulation UWB antenna design-including surprising differences between UWB and narrowband systems Effective transmitter design reflecting UWB's modulation principles Receiver design for impulse UWB systems-including key differences from conventional carrier-based systems In-depth coverage of interference, focusing on "peaceful coexistence" between UWB and narrowband radio systems Practical simulation techniques that avoid unacceptably long simulation times How UWB's physical layer capabilities impact the performance and design of upper layers Real-world UWB applications and case studies of existing systems. This topic will concern with the every professional involved with UWB: RF engineering concerned with UWB's impact on radio design; experienced computer and DSP engineering moving into radio communications; systems engineers who must master UWB radio link design; and technical managers who must clearly understand the challenges they'll face in delivering successful systems.
Research and development:
The amount of research in the field of Ultra Wideband (UWB) communication hasincreased since the FCC approval of Ultra Wide Band for commercial applications.The FCC limits transmission using UWB to a centre frequency between 3.6 to 10.6GHz. The maximum power spectral density is limited to -4ldBm/MHz and the bandwidthmust be at least 500 MHz. Theincrease in research efforts around UWB as a communication protocol is likely the result of a wireless technology boastinghigh potential bandwidth and low power consumption.Physical access to UWB communication channels allow analysis of transmitteddata. Without access to an actual UWB communication channel, it is difficult todevelop the signal processing algorithms required for retrieving data from UWB signals.
As popularity in UWB research grows, it would be beneficial to allow otherresearchers access to transmit and analyze data across an actual UWB communicationchannel so that they may also develop signal processing algorithms for recoveringdata from UWB signals.After the initial FCC approval of UWB for commercial applications, two UWBCommunication systems were constructed by the MIT UWB researchers. These systemsserved as a starting point to demonstrate the proof of concept and were concernedprimarily with transmission of data using UWB and did not measurepacket detection rate, packet loss, or overall bandwidth. The first system, constructedentirely using discrete components, demonstrated transmission and reception only in13baseband. The second system utilized discrete off the shell components to demonstratedata transmission with a centre frequency of 5.355 GHz. Baseband signals wereconverted up to 5.355 GHz, transmitted, and converted back down to baseband foranalysis.Now, as the next iteration of UWB communication systems are under development, tools should be available for determining more than single packet reception.Since it has already been demonstrated that UWB communication systems can transmitdata, the next iteration of UWB communication systems should be developed andcharacterized for performance. Furthermore, the next UWB communication systemshould be developed maximizing bandwidth and packet detection while minimizingpacket loss. To fine tune and evaluate the next iteration of UWB communicationsystems, a development platform dedicated to testing and debugging UWB systemsis required. With a dedicated UWB development platform, new systems can be finetuned and more compelling end-to-end scenarios can be demonstrated.

Channel Uncertainty in Ultra WidebandCommunicationSystems:

Wide band systems operating over multipath channels may spread their power over an infinitely widebandwidth if they use duty cycle. At the limit of infinite bandwidth, direct sequence spread spectrumand pulse position modulation systems with duty cycle achieve the channel capacity, if the increase ofthe number of channel paths with the bandwidth is not too rapid. The higher spectral efficiency of thespread spectrum modulation lets it achieve the channel capacity in the limit, in environments wherepulse position modulation with non-vanishing symbol time cannot be used because of the large numberof channel paths. Channel uncertainty limits the achievable data rates of power constrained wide bandsystems; Duty cycle transmission reduces the channel uncertainty because the receiver has to estimatethe channel only when transmission takes place. The optimal choice of the fraction of time used fortransmission depends on the spectral efficiency of the signal modulation.

This data rates of systems with very wide bandwidths. Consideringcommunication with an average power constraint, the capacity of the multipath channel inthe limit of infinite bandwidth is identical to the capacity of the additive white Gaussian noise(AWGN) channel CAWGN = P/N0 log e, where P is the average received power and N0 is thereceived noise spectral density. Kennedy and Gallager proved this for fading channels using FSK signals with duty cycle transmission; Telatar and Tseextended the proof formultipath channels with any number of paths. The AWGN capacity is achievable on multipathchannels also by dividing the spectrum into many narrow bands and transmitting bursty signalsseparately on each band.When using spreading modulations, M`edard and Gallager show that direct sequence spreadspectrum signals, when transmitted continuously (no duty cycle) over fading channels (that havea very large number of channel paths), approach zero data rate in the limit of infinite bandwidth.

A similar result was shown by Subramanian and Hajek. Telatar and Tse show that overmultipath channels, the data rate in the limit of infinite bandwidth is inversely proportional tothe number of channel paths.This work is motivated by a recent surge in interest in ultra wide band systems, where spreadingsignals are often desired. It shows that under suitable conditions, spreading signals can achieveAWGN capacity on multipath channels in the limit of infinite bandwidth, if they are used with duty cycle. In other words, peakiness in time is sufficient to achieve AWGN capacity, and thetransmitted signal does not have to be peaky in frequency as well. We analyze direct sequencespread spectrum (DSSS) and pulse position modulation (PPM) signals, and show that when thescaling of the number of channel paths is not too rapid, these signals achieve the capacity inthe limit as the bandwidth grows large.Our results can be seen as a middle ground between two previous results: 1. FSK with dutycycle achieves AWGN capacity for any number of channel paths and 2. direct sequence spreadspectrum signals with continuous transmission (no duty cycle) have zero throughput in the limit,if the number of channel paths increases with the bandwidth.Our main results are as follows.

In the limit of infinite bandwidth, DSSS systems where thereceiver knows the path delays achieve AWGN capacity if the number of channel path is sub–linear in the bandwidth, formally if L/W →0 where L is the number of independently fading

channel paths and W is the bandwidth, and the system uses an appropriate duty cycle. PPM

systems too can achieve AWGN capacity in the limit of infinite bandwidth, but this is possiblefor smaller number of channel paths. A PPM system with a receiver that knows the path delaysachieves AWGN capacity ifL/logW→0. PPM systems with lower bounded

symbol time havezero throughput if LlogW →infinity . In systems where the receiver does not know the path gainsor delays, we show that DSSS systems can achieve AWGN capacity ifL/(W/ logW)→ 0 as thebandwidth increases. Measurements of the number of channel paths vs. bandwidth in Figure 1show an increase of the number of channel paths that appears to be sub–linear.

The effect of duty cycle can be understood in terms of the channel uncertainty a communicationsystem faces. The data rate is penalized when the receiver has to estimate the channel, soinfrequent usage of the channel leads to a small channel uncertainty and a small penalty. Thespectral efficiency of the modulation scheme plays an important role in determining the channeluncertainty a system handles. A system with a low spectral efficiency can pack a small numberof bits into each transmission period, and in order to maintain a high data rate it must transmitoften. Thus, low spectral efficiency forces the communication system to estimate the channeloften, and suffer from a large penalty on its data rate.A useful intuition is given by the ratio

SNRest =(P/N0)×(Tc/ᶱL) .

This ratio compares the channel uncertainty per unit time θL /Tcto the data rate in the limit of infinitebandwidth that is proportional to P/N0. L is the number of independent channel components, Tc isthe coherence time and µ is the duty cycle parameter or the fraction of time used for transmission.The ratio SNRest can also be interpreted as the SNR over a coherence period per uncertaintybranch of the channel. A communication system can achieve the channel capacity in the limitof infinite bandwidth only if the channel uncertainty per unit time becomes insignificant relativeto the capacity orSNRest →infinity.

In systems with bounded average received power, the duty cycle parameter must diminish in

order to balance the increase in the number of channel components L, and let the overall channeluncertainty diminish. Spectrally efficient modulation schemes permit infrequent transmission (orsmall µ), thus reducing the channel uncertainty per unit time. In contrast, low spectral efficiencyforces frequent transmission, and the duty cycle parameter must stay high.Performance of Ultra-Wideband CommunicationsWith Suboptimal Receivers in Multipath ChannelsThe performance of a single-user ultra-wideband(UWB) communication system employing binary block-coded pulse-position modulation (PPM) and suboptimal receivers in multipath channels is considered. The receivers examined include a rake receiver with various diversity combining schemes and an autocorrelation receiver, which is used in conjunction with transmitted reference (TR) signaling. A general framework is provided for deriving the performance of these receivers in multipath channels corrupted by additive white Gaussian noise (AWGN). By employing previous measurements of indoor UWB channels, we obtain numerical results for several cases which illustrate the tradeoff between performance and receiver complexity.

Analysis:

ULTRA-WIDEBAND(UWB) communications involves the transmission of short pulses with a relatively large fractional bandwidth. More specifically, these pulses possess a 10-dB bandwidth which exceeds 500 MHz or 20% of their center frequency and is typically on the order of one to several gigahertz. The large bandwidth occupancy of UWB signals primarily accounts for both the advantages and disadvantages associated with UWB communication systems. For instance, the large bandwidth of UWB signals inconjunction with appropriate spreading techniques provides robustness to jamming, as well as a low probability of intercept and detection. These favorable characteristics are offset by the fact that UWB communication systems must coexist with narrowband and wideband systems already operating in dedicated frequency bands. In order to minimize interference to these systems, UWB systems must follow strict regulationswhich limit the achievable data rates, transmission range, and implementation of power control. The presence of multiple interfering signals also necessitates additional receiver complexity, even with the potentially large processing gains of UWB spread-spectrum (SS) systems. This duality regarding UWB signaling also manifests itself in the effects of the channel. Because UWB signals possess such a large bandwidth, the channel is extremely frequency-selective and the received signal contains a significant number of resolvable multipath components. The fine time resolution of UWB signals reduces the fading caused by several multipath components from different propagation paths overlappingin time and adding destructively. However, each multipath component (or more appropriately, pulse) associated with a particular path collectively exhibits distortion after reflections,diffractions, and scattering and does not resemble the ideal received signal corresponding to the line-of-sight (LOS) path. This heightened sensitivity of UWB signals to different scatterers makes them particularly well-suited for radar applications, while making it more difficult for practical communications receivers to fully exploit the multipath diversity inherent in the received signal.

Among the main claimed advantages of UWB communication systems is the availability of technology to implement low-cost transceivers which can operate over such large bandwidths. In general, current embodiments of UWB receivers sacrifice performance for low-complexityoperation and a large discrepancy in performance exists between these implementations and the theoretically optimal receiver for most indoor and outdoor environments. The most common receiver implementations cited in UWBliterature include threshold detectors, correlation or rake receivers and autocorrelation receivers. The relative performance of these receivers in multipath and jammingchannels and the inherent tradeoff between performance andcomplexity have not been fully examined.The majority of the performance analyses of UWB communicationsystems assumes the use of correlation receivers or rake receivers. Because ofthe large number of resolvable multipath components presentin the received signal, practical UWB rake receivers mustselect, process, and combine only a small subset of thesecomponents (hence, employing hybrid selection combining (H-SC) or suboptimal variations). The energy captureof UWB rake receivers can be relatively low for a moderate number of fingers and is highly dependent upon the choiceof tap delays. This selection takes on increased significancebecause UWB receivers generally do not employ a localoscillator nor do they explicitly perform phase estimationand compensation in the demodulation process for a given tapdelay.

An alternative approach to exploiting the multipath diversityis the use of an autocorrelation receiver which correlates thereceived signal with a previously received signal. This receiver can signal energy for slowly varying channels without requiringchannel estimation. The primary drawback of autocorrelationreceivers, however, is the performance degradation associatedwith employing noisy received signals as reference signalsin demodulation. Autocorrelation receivers have been typically used as suboptimal differential detectors fordifferential phase-shift keying (DPSK) in narrowband systems. The application of autocorrelation receivers toUWB systems was proposed for radar target detection purposesin. The signaling and detection scheme described in thiswork falls under the classification of a transmitted reference(TR) system. TR communication systems operateby transmitting a pair of unmodulated and modulated signalsand employing the former to demodulate the latter.

Similar to differential modulation and pilot symbol-assistedmodulation schemes, TR systems in essence transmit referencesignals to generate side information regarding the channel.

A TR-SS communication system employing UWB pulses ornoise for signaling and a hybrid of time-hopping (TH) anddirect-sequence (DS) spreading techniques has been recentlyproposed in.

This paper examines the performance of rake and autocorrelationreceivers with varying degrees of complexity. Weconsider these receivers in the context of a single-user UWBsystem employing binary block-coded pulse-position modulation(PPM) in multipath channels corrupted by additive whiteGaussian noise (AWGN). We extend previous work involvingblock-coded PPM and rake reception by providing a more general analytical framework andconsidering various suboptimal diversity combining schemes.These schemes select a subset of the received multipathcomponents, either optimally or sub optimally, and combinethem with maximal ratio combining (MRC) or square-lawcombining (SLC). Furthermore, we examine a TR systememploying block-coded PPM and an autocorrelation receiverwhich averages previously received reference pulses as ameans of noise suppression. It is noted that although UWBsystems encounter narrowband and wideband jamming, theeffect of such interference upon the system performance is nottaken into consideration for analytical simplicity. In order toobtain numerical results, the performance of both the rake andautocorrelation receivers is evaluated with measured indoorchannel data.