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Telecommun Syst (2015) 60:179–186DOI 10.1007/s11235-014-9932-1
Analytical and comparative investigation of 60 GHz wirelesschannels
Yi Zhu · Chong Tang · Lixing Song · Shaoen Wu ·Saâd Biaz
Published online: 13 February 2015© Springer Science+Business Media New York 2015
Abstract This work analytically investigates the wirelesschannel characteristics at 60GHz in five aspects: path losson the range of a wireless communication system, multi-pathfading, spatial diversity, high peak-to-average power ratio oforthogonal frequency division multiplexing (OFDM), anddetection time of clear channel assessment. The analysisis conducted in comparison to the 2.4GHz system. Theobservations are summarized as follows: the 60GHz sys-tem experiences large path losses and can support onlyshort range communications, approximately half of that at2.4GHz. However, it suffers lightly from multi-path fad-ing due to its small average fade duration. Because of theshort wave length, the 60GHz communication can sup-port packing a large number of antennas at a transceiverto exploit spatial diversity that can significantly boost thetransmit or receive gains. One problem of 60GHz in using
Y. ZhuSchool of Computing, University of Southern Mississippi,118 College Drive, Box 5106, Hattiesburg, MS 39406, USAe-mail: [email protected]
C. TangDepartment of Computer Science, University of Virginia,85 Engineer’s Way, P.O. Box 400740, Charlottesville, VA, USAe-mail: [email protected]
L. Song · S. Wu (B)Department of Computer Science, Ball State University,RB #455, Ball State University, Muncie,IN 47306, USAe-mail: [email protected]
L. Songe-mail: [email protected]
S. BiazDepartment of Computer Science and Software Engineering,Auburn University, Auburn, AL 36849-5347, USAe-mail: [email protected]
OFDM to avoid frequency selective fading on its wide band-width is high peak-to-average power ratio that increases thecost and complexity in implementing transceivers. Due toits wide bandwidth that hints to high symbol rate, the 60GHz system can achieve a fast detection of signals in carriersense.
Keywords 60 GHz · Wireless communication · Channelcharacteristics
1 Introduction
Wireless communications at 60GHz spectrum are allo-cated of a large unlicensed bandwidth of at least 5GHzin many countries around the world. This wide band-width makes 60GHz wireless communications promisingto achieve Gbps (gigabit per second) rates to support manyheavily bandwidth consuming applications, such as high-definition video, sync-and-go file transfer and wireless dis-play. With these applications and advances in technolo-gies for low cost solutions, 60 GHz technology attractsintensive interests from academia, industry and standard-ization bodies. For example, IEEE 802.15.3c [1] aimsat supporting high rate WPAN transmission and ECMA387 [2] is developed for high-rate 60 GHz PHY andMAC for short-range unlicensed communications. Wire-lessHD [3] is a protocol developed for high definitionaudio/video streaming. WiGig [4] is dedicated to variousGbps data rate applications and IEEE 802.11-ad is underdevelopment to enable Gbps WiFi networks. Comparedwith conventional unlicensed wireless communication at2.4/5GHz, 60GHz systems face some new technical chal-lenges.
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The 60GHz wireless channel is significantly different incharacteristics from the traditional open access 2.4/5GHzspectrum. The path loss at 60GHz is at least 20dB higherthan at 2.4GHz. To mitigate the significant signal strengthattenuation due to the path loss, directional antennas andantenna array with high transmission power are proposedto achieve a high antenna gain. With the carrier wavelengthof 5mm, antenna array can be implemented intensively atboth transmitter and receiver. Besides, 60GHz regulationsallow high transmission power to tackle the severe signalattenuation problem. MIMOwith multiple antennas can alsoalleviate the effect of flat fading by exploiting the transmit-ter and/or receiver diversity. The multi-path fading prob-lem is worsened on the large bandwidth provided by the60 GHz system for very high data rates. As a result, thefrequency selective fading dominates the wireless communi-cations on the 60 GHz spectrum. Technologies that addressthe frequency-selective fading must be taken into consid-eration. OFDM is a simple and flexible solution. However,the high peak-to-average signal strength ratio presents chal-lenges, especially when a large bandwidth is split into anumber of sub-channels in OFDM. Another problem ofthe 60GHz system is the overhead of the MAC layer. Theimmense increase of the data rate requires the renovationof MAC layer protocols to deliver high utilization and effi-ciency.
In this paper, we analytically investigate the wireless com-munications at 60 GHz in comparison with the traditional2.4 GHz in five aspects. First, we present the large pathloss of the 60 GHz frequency band over the communica-tion distance that is determined by the cell coverage area andthe outage probability. Second, after analyzing the effects oflarge scale propagation, we then visit the small scale multi-path fading in the wireless communications. The averagefade duration is inspected. Third, to counteract multi-pathfading, we compare the performance of the spatial diversitygain that can be achieved with the antenna array. Fourth,to eliminate the frequency-selective fading, OFDM multi-carrier modulation technology can be employed. We presentthe high peak-to-average signal strength ratio problem inOFDMdue to the large bandwidth of 60GHz system.Finally,we examine the detection time of clear channel assessmentand suggest an enhancement that can lead to a significantimprovement of the MAC layer protocol efficiency at the at60 GHz.
The rest of this paper is organized as follows. We sum-marize the related work in Sect. 2. Next, Sect. 3 presentsthe large scale propagation and the path loss of the commu-nication system at 60 GHz. Then, small scale propagationand the effect of fast fading are presented in Sect. 4. Spatialdiversity to mitigate the signal attenuation due to fading at60 GHz is discussed in Sect. 5 and in Sect. 6 we illustratethe high peak-to-average power ratio problem of OFDM in
60 GHz system. Section 7 analyzes the time of clear channelassessment. The paper is concluded in Sect. 8.
2 Summary of related work
Themost attractive perspective ofwireless communication at60GHz is the huge bandwidthwhich supports high data appli-cations such as high-denition audio/video streaming trans-mission [3] overmulti-hopwireless networks [5]. In addition,multimedia applications are delay-sensitive requiring for lowlatency [6,7]. From 2.4/5 to 60 GHz carrier frequency, thereare some unprecedented challenges. Particularly, large pathloss is one of the major challenges, which makes the 60GHzband unsuitable for outdoor long distancewireless communi-cations.Most of the literaturework focused onmeasurement-based studies of the wireless channels at 60 GHz in short-distance communications and indoor environments. Xu etal. [8] presented the measurements that reveal the spatial andtemporal properties of millimeter-wave (another name of 60GHz wireless) channels. Anderson et al. [9] described themeasured data of the path loss in an office building. Moraitiset al. [10] reported the path loss measurements for both line-of-sight (LOS) and non-line-of-sight (NLOS) cases betweenfixed terminals. Geng et al. [11] showed the measurementresults of the 60 GHz radio propagation in various indoorenvironments. All these works provide the understanding ofthe radio wave propagation at millimeter wave length at 60GHz frequency. The empirical data can be useful for design-ing wireless systems working on 60 GHz channels. Though,we present an analytical study of 60 GHz channels in thiswork.
3 Large scale propagation
Large scale propagation is important for the link budget andnetwork planning. The carrier frequency spectrum of 60GHzincurs large path losses. As a result, the ranges of the commu-nication system are greatly shrunk. The large scale propaga-tion consists of two portions: path loss and shadowing. Pathloss refers to the attenuation of transmitted signal strengthover the communication distance. It is defined as the ratio ofthe transmitted signal strength to the receive signal strength.Shadowing is a result of the variation of the propagationpath. Log-normal shadowing is the most common model forshadowing. Many measurements have suggested that shad-owing on 60 GHz channels follows log-normal distribu-tions [12,13]. The received signal strength in long-distancewireless communications is primarily determined by the pathloss and shadowing. Therefore, the communication range of awireless system is designed based on the path loss and shad-owing models. The conventional simplified models can be
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Analytical and comparative investigation of 60 GHz wireless channels 181
used to capture the large scale propagation and analyze thevalid communication range, which depend on two factors:outage probability and cell coverage.
In wireless communications, outage probability and cellcoverage are two critical metrics in performance analysis[14]. The outage probability, as in Formula 1, is the prob-ability that the received signal strength at a given distancefalls below the minimum required signal strength.
p(P(d) ≤ Pmin) = 1 − Q
(Pmin − (Pt − PL(d))
σ
), (1)
where Pmin is the required minimum received power; σ isthe standard variation of shadowing fading; and Q(x) is theprobability that aGaussian randomvariable X ,with themeanof 0 and the variance of 1, is greater than x .
The cell coverage, as in Formula 2, is the expected per-centage of the area within a cell where the received signalstrength is above a given minimum:
C ≈ Q(α) + exp
(2 − 2αβ
β2
)· Q
(2 − αβ
β
)
α = Pmin − Pr (D)
σ, β = 10n · log10(e)
σ, (2)
where Pr (D) is the received power at the edge of the cell; σand Q(x) are similarly defined as in Formula 1. The size ofthe cell should be so determined that the minimal receivedSNR is guaranteed to maintain the quality of service.
The cell coverage and the outage probability are crucialfor system cost analysis and network planning. In designingthe range of a wireless communication system, the outageprobability must be kept below a certain level, while keepingthe cellular coverage above a specific level in order to main-tain an acceptable performance in most areas of the cell.
In the following, we analyze the cell size of 60 GHz wire-less communication and compare it to that of 2.4 GHz tech-nology. The analysis is focused on the indoor non-line-of-sight (NLOS) case where there is no direct path between asender and a receiver. We employ the path loss model as inFormula 3proposedby the IEEE802.11n [15,16] for 2.4GHzwireless channels and the model as in Formula 4 proposedby the IEEE 802.11ad [17] for 60 GHz wireless channels.
PL(d) = PLFS(d) d ≤ dBP
PL(d) = PLFS(dBP ) + 35log10(d/dBP ) d > dBP (3)
In the IEEE 802.11n channel model for 2.4 GHz, the pathlossmodel consists of the free space loss PLFS up to a break-point distance and a slope of 3.5 after the breakpoint distance.In our analysis, dBP is set to 5m; and the standard variationof the shadowing is 4 dB.
PL(d) = K + 20 · log10( fc) + 10n · log10(d) (4)
5 10 15 20 25 30 35 40 45 500
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1
Distance (m)
Cel
l Cov
erag
e A
rea
Cell Coverage Area
2.4GHz60GHz
Fig. 1 Analysis of cell size (coverage)
In the IEEE 802.11ad channel model for 60 GHz, K andn are constants that depend on the scenarios. fc is the carrierfrequency which is 60 GHz. In our analysis, we consider theliving room NLOS scenario, where K is 44.7 and n is 1.5.The standard variation of the shadowing is 3.4 dB.
In this analysis, the transmitted signal strengths take oneof the values recommended by the regulations: 35 dBm for60 GHz and 25 dBm for 2.4 GHz. The receiver minimuminput sensitivity is assumed to be the typical value of -60dBm in calculating the coverage and the outage probability.Figure 1 depicts the coverage variation on the y-axis versusthe communication distance on the x-axis. From the figure,to provide 99.5% coverage of a cell, the 60 GHz commu-nication can only support a cell of 5m in radius due to itsfast path loss upon distance, while the 2.4 GHz communica-tion can support up to about 11m. Figure 2 shows the outageprobability analysis with the probability on the y-axis andthe distance on the x-axis. Similar to the coverage, to limitthe outage probability to no greater than 1%, which approx-imately corresponds to the 99.5% coverage, the communi-cation in the 60 GHz system must remain within 5m whilethe 2.4 GHz system doubles the range to 11m. From thesetwo figures, even with higher suggested transmission power,the range of the communication system at 60 GHz is signifi-cantly smaller than that at 2.4 GHz. Some possible solutionsto compensate the high path loss are to take advantage ofdirectional antenna technologies or antenna array to achieveantenna gains or transmitter/receiver diversity.
4 The effect of multi-path fading
In wireless communication on the land, the signal normallypropagates through multiple paths to reach a receiver dueto reflection, diffraction, and scattering from the surround-
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0 5 10 15 20 25 30 35 40 45 500
0.1
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1Outage Probability
Distance (m)
Out
age
Pro
babi
lity
2.4GHz60GHz
Fig. 2 Analysis of outage probability
ing environment. The multi-path phenomenon is particu-larly popular in urban and indoor environments. The signalstrength of the received signalwidely varies in both space andtime due tomulti-path fading. For some low latency and high-quality wireless applications, such as real-time wireless dis-play and gaming, small scale multi-path fading highly affectsthe performance of these applications. The low latency ser-vice requires stable data communications, that says, it expectsthe duration inwhich the received signal strength drops belowthe minimum level due to multi-path fading is as short aspossible. The average fade duration delineates how fast themulti-path fading is and how much it affects the commu-nication performance. It is defined as the average time thatthe received signal strength falls below the minimum levelrequired to maintain acceptable performance. The averagefade duration tells the impact of multi-path fading on the per-formance of wireless communications. A large average fadeduration is likely to incur Inter-Symbol-Interference (ISI)because the prior signal coming from the longest path mayarrive when the current signal is being received. The thumbrule is that the shorter the average fade duration is, the lessthe transmitted signal is affected.
We analyze the average multi-path fading duration at both60 and 2.4 GHz. In our analysis, Rayleigh fading [18] is usedas the channel model because it is widely accepted for urbanand indoor wireless communications. The average fade dura-tion on Rayleigh fading channels [14] can be calculated as:
T̄ = eρ2 − 1
ρ fD√2π
, (5)
where fD is themaximumDoppler shift, andρ =√Pmin/P̄r .
Pmin is the required minimal received power; and P̄r is theaverage received power.
Fig. 3 The variance of received signal amplitude over time due to fastfading
For Rayleigh fading, the received signal strength variesdue to the constructive and destructive addition of multi-pathsignal components. Our calculation takes the typical value0.1 for ρ. The average fade duration decreases with the max-imum Doppler shift fD . In the indoor case with pedestrianspeed (about 1.5m/s), the maximum Doppler shift is about300 Hz for the 60 GHz system and 12 Hz for the 2.4 GHzsystem. According to Formula 5, the average fade durationsare 11.1 and 0.44ms respectively for 2.4 and 60 GHz respec-tively. Therefore, 60 GHz shows small average fade durationthat leads to light fast fading impact. This is validated by thefollowing illustration. We plot the signal strength variationsover the time in Fig. 3. The communications at 2.4 GHz areenabled at 100 Mbps (a typical bit rate in the IEEE 801.11n)and that at 60 GHz is enabled at 1 Gbps. It is obvious fromthe figure that the signal strength of the 60 GHz communi-cations varies much faster than that of 2.4 GHz communica-tions. Namely, the average fade duration of 60 GHz is muchshorter. Therefore, the 60 GHz is robust to multi-path fadingin short distance communications. Even though the data rateis increased to 1 Gbps at 60 GHz, it is still less likely forthe information bits to be damaged by fast fading within theaverage fading duration than in the 2.4 GHz system.
5 Spatial diversity for fading mitigation
Fading over wireless channels leads to signal degradation,especially over deep fading channels. To mitigate the effectsof fading, one possible solution is to constructively combineat a receiver the signals from multiple independent paths bypositively exploiting the multi-path propagation popular inland wireless communications. Because it is mostly unlikelythat all transmissions on these paths experience deep fading
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Analytical and comparative investigation of 60 GHz wireless channels 183
Fig. 4 Bit error rate in spatial diversity
at the same time, the combined signal at the receiver normallyreduces the error probability. The fact that each propagationpath results in different fading in wireless communicationsis called spatial diversity. One approach to exploit the spa-tial diversity is to equip multiple antennas so that the signaltransmitted or received from each antenna propagates overdifferent paths. The benefit of multiple antennas at a senderis called transmitter diversity and that at a receiver is calledreceiver diversity.
The number of antennas that can be equipped is con-strained by the wavelength of the carrier frequency: the dis-tance between two antennas is required to be at least 0.4 ofthe wavelength. Because of its short wavelength, the 60 GHzsystem allows packing more antennas on a chip than the 2.4GHz system.Thewavelength at 60GHz is 5mm.On the otherhand, thewavelength at 2.4GHz is 125mm,which is 25 timesof that at 60 GHz. The separation distances between anten-nas required for independent fading paths are 50 and 2mmfor systems at 2.4 and 60 GHz respectively. As a result, in anarea of 30× 30mm2, we can only install one antenna for the2.4GHz system, but four antennas or even more to providediversity gain for the 60GHz system.
With multiple antennas, the 60GHz system can signifi-cantly mitigate the impact of the multi-path fading. In addi-tion, the gain of antenna array enables high level modulationthat leads to high bit rate, while maintaining low bite-error-rate (BER). In Fig. 4, we plot the BER for the wireless chan-nels at both 60 GHz with four antennas and 2.4 GHz but withonly one antenna with the assumption of the antenna area of30×30 mm2. The y-axis refers to the BER and the x-axisstands for the signal-to-noise-ratio (SNR). The figure showsthe significant improvement of the BER with the antennaarray exploiting the spatial diversity. Therefore, although thepath loss in the 60 GHz system is large and fast, the signalstrength degradation can be compensated to some degree by
exploring the spatial diversity with multiple antennas, whichare facilitated due to the short wavelength of the 60 GHzspectrum.
6 Peak-to-average signal strength ratio of OFDM
In 60 GHz wireless communications system, a wide band-width is provided for high data rate transmissions. However,the wide bandwidth also leads to frequency-selective fadingthat incurs unstable wireless channels and worsens bit errors.To achieve a high data ratewhilemaintaining a high quality ofservice, technologies that overcome the frequency-selectivefading must be employed.
Orthogonal frequency division multiplexing (OFDM) iswidely used in wireless networks to mitigate frequency-selective fading. In OFDM, a wide bandwidth is split intoa number of small sub-channels (subcarriers). Each sub-channel has a narrow bandwidth so that the channel expe-riences flat fading. Although the bit rate of each sub-channelis lower, the aggregated bit rates of all sub-channels is simi-lar to that of the whole wide bandwidth. The information bitswill be scheduled to transmit across sub-channels in paral-lel. OFDM exhibits excellent performance in wireless com-munications that suffer from frequency-selective fading. Inaddition, the cyclic prefix of OFDM avoids the inter-symbolinterference.
The main problem of OFDM is the high peak-to-averagesignal strength ratio (PAR). PAR is defined as the ratio ofthe maximum instant signal strength to the average signalstrength. OFDM multi-carrier signals normally experiencea much larger variance of PAR than single-carrier signalsbecause it sums up the signals of all subcarriers. A highPAR requires that the transmitted signal strength amplifierhas a large linear region to avoid signal distortion and spec-tral growth. Besides, the receiver A/D converter must supportlarge dynamic range of the signal. These requirements addcomplexity and cost of the implementation of transceivers.
We analyze the PAR performance of OFDM at 60 GHz.Because the maximum instant signal strength rarely occurs,the cumulative distribution of the PAR above a given thresh-old is normally used to evaluate the PAR performance. Ouranalysis considers indoor environments that typically havethe delay spread (Tds) of 50 ns. In the 2.4 GHz system, thebandwidth is taken as 20 MHz as suggested by the IEEE802.11 standard. In the 60 GHz system, the bandwidth isconsidered of 200 MHz for communication. The number ofapproximately flat-fading subcarriers (N ) generated in eachspectrum must be large enough to guarantee that each sub-carrier bandwidth (B/N ) is much smaller than the coher-ent bandwidth Bc, namely, B/N � Bc or, in practice,B/N ≈ 0.1×Bc, where B refers to the total bandwidth.Withthe relation: Bc ≈ 1
Tdsand the practical fast fourier trans-
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1 2 3 4 5 6 7 8 9 100
0.1
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0.8
0.9
1
PAR Threshold
PA
R O
utag
e P
roba
bilit
y
2.4GHz OFDM system60GHz OFDM system
Fig. 5 PAR outage probability
formation (FFT) implementation, accordingly, the requiredminimum numbers of subcarriers are 16 and 128 for 2.4 GHzand 60 GHz respectively. The probability that PAR exceedsa threshold (P0) is
p(PAR ≥ P0) = 1 − (1 − e−P0)N , (6)
where N is the number of sub-channels [14,19]. Figure 5shows the PAR outage probability for OFDM at 60 GHz of128 subcarriers and 2.4 GHz of 16 subcarriers.
The 60 GHz system is allocated a larger bandwidth than2.4 GHz. Therefore, it can support much higher bit rates, butrequires more subcarriers in order to mitigate the frequency-selective fading at the cost of higher PAR and outage prob-ability. Therefore, it calls for innovative schemes to reducethe PAR with low signal distortion at 60 GHz [20,21].
7 The detection time of clear channel assessment
Wireless communications in the 60GHz spectrum providemuch higher data rates than at 2.4GHz. However, the con-siderable overhead of the MAC layer incurred by the high bitrates significantly hurts the efficiency and utilization of thechannel and system. Recent research work [22] revealed thatthe overhead reaches 45% at 54Mbps and 82% at 300Mbpsand rises up to 91% at 600Mbps. This inefficiency can beeasily understood from an illustration. Let us define utiliza-tion as λ = t
T+t where t represents the time in transmitting aframe and T refers to the time of protocol overhead. In cur-rent IEEE 802.11 standard, T consists mainly of the backoffprocess, physical layer preamble, and channel piloting. InT , the backoff process is based on a number of time slotsand the physical preamble is always sent at the base rateof 6 Mbps. The time slot has a constant duration regardlessof the bit rate. Therefore, T does not vary much because it
0 2 4 6 8 10 12 14 160.4
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0.9
1
The Number of Symbols
Det
ectio
n P
roba
bilit
y
SNR=10dBSNR=15dB SNR=20dB
Fig. 6 CCA accuracy upon SNR and the number of symbols
is dominated by the preamble transmission and the backoffprocedure (constant time slot size) while t decreases whenthe frame transmission rate increases. For example, if thebit rate increases from 100Mbps to1 Gbps, then the chan-nel utilization at 1 Gbps becomes λ = 0.1t
T+0.1t = t10T+t ,
much worse than tT+t in 100Mbps networks. It is clear that
the channel utilization is significantly hurt at high bit rates,because the time spent on transmitting data frames drasti-cally decreases with high bit rates while the time in proto-col overhead remains almost constant (regardless of framebit rate). Therefore, as the bit rate increase, the inefficiencyworsens as well.
One possible solution to address the problem mentionedabove is to redesign the time to spendon carrier sense and thusshorten the time slot duration that dependson the carrier sensetime. Clear Channel Assessment (CCA) is the core of car-rier sense in themedium access control protocols for wirelessnetworks, which is implemented at the PHY layer. Therefore,it is of most interest for us to analyze the CCA at 60 GHz.The energy detection technique employed in the analysis is asimple and energy-efficient method, called Neyman-Pearsondetector [23], that is widely used in the wireless channelsensing. The performance is analyzed with Neyman-Pearsontheorem [23]. Figure 6 plots the detection probability (detec-tion accuracy) on the y-axis upon SNR and the number ofsymbols monitored in the detection on the x-axis.
From the figure, the CCA accuracy varies upon the num-ber of symbols involved in the detection. Therefore, theCCA detection is normally conducted by detecting the signalstrength of a designated number of symbols. The probabilityof detection is subject to a given probability of false alarm.Assume that the required probability of false alarm is 0.01and the required probability of detection is greater than 0.9.The typical minimum receive SNR sensitivity is 10 dB. FromFig. 6, eight symbols are required for accurate CCA.
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Analytical and comparative investigation of 60 GHz wireless channels 185
After the required number of symbols for theCCA is deter-mined, theCCA time relies on the bit rate at which these sym-bols are transmitted.With a specificCCAdetection techniquethat works on the same number of symbols, because the 60GHz system provides much higher bit rates than the 2.4 GHzsystem, the CCA detection time can be greatly shortened.Specifically, for BPSK modulation, the basic bit rate at 2.4GHz is 2 Mbps, but it is 200 Mbps at 60 GHz with a band-width of 200 MHz. As a result, the detection time at 60 GHzcan be as small as 0.04µs, which is significantly shorter thanthat at 2.4 GHz.
8 Conclusion
This paper analytically investigates the wireless channelcharacteristics at 60GHz band by comparing themwith thoseat 2.4 GHz in five aspects: path loss on the range of a wirelesscommunication system, multi-path fading, spatial diversity,high peak-to-average power ratio of OFDM and detectiontime of clear channel assessment.
Due to fast and large path loss, the 60GHz system can sup-port only a communication range approximately half of therange at 2.4 GHz with 1% outage probability and 99.9%cell coverage. For the small-scale multi-path fading, theDoppler shift increases when the frequency shifts from 2.4to 60GHz. As a result, the average fading duration becomesshorter at 60 GHz. Therefore, the 60 GHz system suffers lessfrom multi-path fading than the 2.4 GHz system. The spa-tial diversity is a feasible approach to overcome the multi-path fading. With a 5mm wave length, the 60 GHz systemallows to pack more antennas in a tiny area at a transceiverthan the 2.4 GHz system. Therefore, the spatial diversitycan be exploited to support Gbps data rates with low biterror rate. Wireless communications at 60 GHz carrier fre-quency have a wide bandwidth. This leads to the problemof frequency-selective fading. OFDM is an effective way toalleviate this problem. However, a large bandwidth requiresa large number of sub-channels in OFDM. As a result, thehigh peak-to-average power ratio incurs high implementa-tion cost for transceivers. With high bit rates from the widebandwidth, the 60 GHz system can achieve a fast detectionof signals in Clear Channel Assessment in performing carriersense.
Acknowledgments This work is supported by the U.S. National Sci-ence Foundation through the Award OCI#1041292.
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Yi Zhu is a currently master stu-dent in the School of Comput-ing at the University of South-ern Mississippi. He received hisbachelor degree at Beijing Uni-versity of Technology, China.His research is centered oncooperative communication andMIMO.
Chong Tang is currently a Ph.D.student in Department of Com-puter Science at University ofVirginia. He works on the areaof wireless sensor networks,focusing on MAC layer proto-col design and applications inenergy conservation. He earnedhis bachelor’s degree from Xi’anInstitute of Post and Telecommu-nications and MS degree fromthe University of Southern Mis-sissippi, where he focused onRate Adaptation and MediumAccess.
Lixing Song is a M.S. stu-dent in the Department of Com-puter Science at Ball State Uni-versity. He received his B.E.from Wuhan University, Chinaat 2011. His research inter-ests include wireless networking(rate adaptation, cross-layer, pro-tocol design), wireless security,cloud computing.
Shaoen Wu (M’09) received aPh.D. in Computer Science in2008 from Auburn University.He is presently an Assistant Pro-fessor of Computer Science atBall State University. He hasheld an Assistant Professor posi-tion in the School of Comput-ing at the University of South-ern Mississippi, a researcher sci-entist position at ADTRAN Inc.and a senior software engineerposition at Bell Laboratories. Hiscurrent research is in the areas ofwireless and mobile networking,
cyber security, cyber-physical systems and cloud computing. He is arecipient of Best Paper Award of IEEE ISCC 2008 and ANSS 2011. Hehas served on the chairs, the committees and the editorial boards of sev-eral international conferences journals. More information is availableat http://www.cs.bsu.edu/~swu.
Saâd Biaz (M’98) received aPh.D. in Computer Science in1999 from Texas A&M Uni-versity and a Ph.D. in Electri-cal Engineering in 1989 fromthe University Henri Poincaré inNancy (France). He is presentlyan Associate Professsor of Com-puter Science andSoftwareEngi-neering at Auburn University. Hehas held faculty positions at theEcole Supérieure de Technologiede Fès and Al Akhawayn Uni-versity in Ifrane (Morocco). Hiscurrent research is in the areas of
distributed systems, wireless networking, and autonomous unmannedaerial systems (UASs). His research is funded by the National ScienceFoundation. Saad Biaz is a recipient in 1995 of the Excellence FulbrightScholarship. Saad has served on the committees of several conferencesand as editor for several journals. For more information, please visithttp://www.eng.auburn.edu/users/sbiaz.
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