Measured Channel Capacity Of SIMO-UWB For Intra-Vehicle .

EuCAP 2011 - Convened PapersMeasured Channel Capacity of SIMO-UWB forIntra-Vehicle CommunicationsFengzhong Qu1 , Jia Li2 , Liuqing Yang3, and Timothy Talty4of Ocean Science and Engineering, Zhejiang University, Hangzhou, China,Email: jimqufz@gmail.com2 Department of Electrical and Computer Engineering, Oakland University, Rochester, MI 48309,Email: li4@oakland.edu, Tel: 1 248 370 2661, Fax: 1 248 370 46333 Department of Electrical and Computer Engineering, Colorado State University,1373 Campus Delivery, Fort Collins, CO 80523-1373,Email: lqyang@engr.colostate.edu, Tel: 1 970 491 6215, Fax: 1 970 491 22494 ECI Laboratory, General Motors Research and Development Center, Warren, MI 48090, Email: t.j.talty@ieee.org1 DepartmentAbstract— The rapid progresses made in the area of intelligenttransportation systems (ITS) call for high rate intra-vehicle wireless communications. Ultra wideband (UWB) and multi-antennaare both promising technologies providing high data rate. Thispaper evaluates the channel capacity of intra-vehicle singleinput multiple-output (SIMO)-UWB using the measured signalsin the experiment. The channel measurement is carried out intwo vehicles, a sedan Ford Taurus and an SUV GM Escalade,with different transceiver placements, beneath the chassis andinside the engine compartment. Both channel state informationavailable to the transmitter (CSIT) and channel state informationavailable to the receiver (CSIR) cases are involved in our results.The results reveal that the channel capacity remain unchangedalthough experiment settings changes, including vehicle type,engine status, and transceiver location; and that water fillingfails to demonstrate its advantage as the number of receiverantenna increases.I. I NTRODUCTIONIn recent years, intelligent transportation systems (ITS)attract more and more interests by improving the currenttransportation systems in all aspects. Intelligent transportationspaces (ITSp) integrate multiple ITS modules, as well as theparticipants and devices in transportation into spaces withdistributed and pervasive intelligence [1]. Those participantsand devices include various intra-vehicle ones, such as sensorsfor the vehicles and passengers, driving assistance devices,multimedia devices, etc. Since the pervasive intelligence inITSp requires computational data exchange, high rate intravehicle wireless communications become a key enabler forITSp.In 2002, FCC authorized the unlicensed use of ultrawideband (UWB) on the band from 3.1GHz to 10.6GHzwith the emission limit as low as 41.3dBm/Hz that isthe same limit applies to unintentional emitters. This hugebandwidth supports high data rate communications up to 480Mbps over a short distance of 10 15m at very low powerlevels. The extremely wide transmission bandwidth of UWBprovides fine time resolution, which is an enabler of multipathdiversity collection. The wide bandwith with low transmissionpower also provides resistance to narrowband interference.Furthermore, UWB radio have several unique advantages, including enhanced capability to penetrate obstacles, localizationprecision down to the centimeter level, very high data rates andhigh user capacity, small low latency, and potentially smalldevice size and processing power [2].The advantages of UWB open a door for high data rateintra-vehicle wireless communications and intra-vehicle UWBbecomes a hot area. Since 2006, research on intra-vehicleUWB channel measurement, experiment, statistics, and otherrelated results have been published consecutively. Ref. [3]compared the measured intra-vehicle channel in experimentswith the channel model described in IEEE 802.15.3a. Ref.[3] plotted the root mean squared (RMS) delay distributionsof the measured intra-vehicle channels and those of modelsgiven by IEEE 802.15.3a, and concluded that indoor modelsare not suitable for the UWB channels within commercialvehicles. Some channel statistics, such as maximum excessdelay, RMS delay and the number of multipath componentsare theoretically derived [4]. The results in [5] show that thereceived signals within the vehicle are stable and high datarate UWB system can be implemented intra-vehicle. In [6],we reported our work in measuring and modeling the UWBpropagation channel in commercial vehicles with differenttransceiver locations, beneath the chassis and inside the enginecompartment. It is observed that paths arrive in clusters in thelatter environment but such clustering phenomenon does notexist in the former case.In indoor environments, multi-input multi-output (MIMO)schemes have long been used to provide improved capacityand accordingly enhanced data rates, such as IEEE 802.11n.In recent years, motivated by the increasing requirements ofdata rate and reliability of wireless transmissions, MIMOUWB appeals growing interests [7]. Ref. [7] gives an overviewof MIMO-UWB systems. The channel models and measuredchannel capacity are reported in [8]–[11]. However, most ofthem are for indoor environments except [8] in a rectangular3056

metal cavity. As a result, very limited MIMO-UWB researchhas been done for intra-vehicle environments, which are verydifferent from the indoor ones. The intra-vehicle channel facesparticularly harsh multipath and shadowing constraints. Theclosed or semi-closed metallic intra-vehicle structure makesthe compartments reverberation chambers, but with someregions shielded from other regions [12]. Moreover, the placement of antennas is highly constrained for the limited intravehicle space. All these pose challenges for high rate intravehicle MIMO communications. Ref. [12] evaluates MIMOperformance for intra-vehicle communications in aircraft andcars with a focus on low data rates.This paper presents our subsequent work of [6]. In thispaper, measured results in the experiments in [6] are usedto evaluate the channel capacity of intra-vehicle single-inputmultiple-output (SIMO)-UWB systems. The SIMO results inthis paper can be regarded as preliminary work of MIMOsystems. Our results cover channel state information availableto the transmitter (CSIT) and channel state information available to the receiver (CSIR) cases. In the CSIT case, waterfilling is done at the transmitter for energy allocation on thefrequency band while in the CSIR case, the transmit energycan only be allocated evenly on the entire band. The resultsreveal that although different settings, such as vehicle type,engine status, and transceiver locations, play important roles inchannel characteristics, they hardly affect the channel capacity.In addition, water filling does not show its advantage as thenumber of receiver antenna increases. In most cases in ourexperiments, CSIT and CSIR have very close channel capacitywhen the number of receiver antennas is 3 or more.The content of this paper is organized as follows: the nextsection introduces the system model. Section III presents theintra-vehicle UWB experiment settings. Section IV presentsmeasured SIMO channel capacity. Finally, Section V givesconcluding remarks.II. S YSTEM M ODELA. Channel ModelB. Channel CapacityLetHn (f ) L αnl e j2πf (l 1)τnl(2)l 1be the spectrum of hn (t) and the vector form H : [H1 (f ), . . . , HN (f )]. Let S(f ) be the power spectrum density(PSD) function of the transmitted signal X(t) with the powerconstraint S(f )df S,(3)Bwhere B is the signal pass band.Since UWB systems are wideband, the capacity is obtainedby the integration in the frequency domain. The capacity withgiven H(f ) is S(f )H(f )H H (f )log2 1 df, (4)C maxN0S(f )df S BBwhere N0 is the noise PSD and (·)H denotes the matrixHermitian.In the CSIT case, the channel information is available atthe transmitter so that the optimum S(f ) is achieved by waterfilling as [11] N0,(5)S(f ) Θ H(f )H H (f ) where [·] means only taking the value that is greater than orequal to 0 and Θ is a constant that satisfies N0df S(6)Θ H(f )H H (f )f FΘ Bwith FΘ the range of f in which S(f ) 0.In the CSIR case where the channel information is onlyavailable at the receiver, the only thing the transmitter can dois to equally distribute the power throughout the band. Hence,the capacity of CSIR is accordingly log2 1 ρH(f )H H (f ) df,(7)C BIn [6], we used different UWB channel models for the caseswhen the transceivers are beneath the chassis and inside theengine compartment because the channel statistics vary. Sincethis paper focuses on the channel capacity, for simplicity, theSIMO UWB channels with N receive antennas are modeledas an extension of the single-input single-output (SISO) casein [7]L αnl δ(t τnl ),(1)hn (t) l 1where hn (t) is the impulse response of the physical channel,δ the Dirac delta function, n 1, 2 . . . N the index of thereceive antennas, l, L, and τnl the index, number, and theaccording delay of multipath, and αnl the amplitude. Sincethe real impulse is transmitted in UWB systems, αnl is a realnumber.where ρ is the signal-to-noise ratio (SNR).III. E XPERIMENT S ETTINGSThe measurement is performed in time domain by soundingthe channel with narrow pulses and recording their responseswith a digital oscilloscope. The block diagram in Fig. 1illustrates the connections of the measurement apparatus. Atthe transmitter, a Wavetek sweeper along with an impulsegenerator from picosecond works to produce narrow pulsesof width 100 picoseconds, as shown in Fig. 2. These pulsesare fed into a scissors-type antenna, as shown in Fig. 3. Atthe receiving side, a digital oscilloscope of 15GHz bandwidthfrom Tektronix is connected to the receive antenna to recordthe received signals. The channel measurement was carriedout in two vehicles, a sedan Ford Taurus and an SUV GMEscalade, with different transceiver placements, beneath thechassis and in the engine compartment.3057