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Improving the Performance of OFDM-Based Vehicular Systems Through Diversity Coding

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Improving the Performance of OFDM-Based Vehicular Systems Through Diversity Coding 132 JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 15, NO. 2, APRIL 2013 Improving the Performance of OFDM-Based Vehicular Systems through Diversity Coding Gabriel E. Arrobo and Richard D. Gitlin Abstract: In this paper, we present diversity coded orthogonal fre- is divided into many narrow sub-channels, which are transmit- quency division multiplexing (DC-OFDM), an approach to maxi- ted in parallel, such that the fading each channel experiences is mize the probability of successful reception and increase the relia- flat. bility of OFDM-based systems through diversity coding. We focus Many applications need to be communicated in a timely man- on the application of DC-OFDM to vehicular networks based on ner, so reliability of these networks is a concern. To overcome IEEE 802.11p technology and analyze the performance improve- these issues, the novel idea of employing diversity coding (DC) ment using this new technology. [2] across OFDM subchannels is proposed. DC-OFDM based It is shown that DC-OFDM significantly improves the perfor- mance of vehicular ad hoc networks in terms of throughput and systems are capable of achieving better spectrum efficiency with the expected number of correctly received symbols. excellent transmission rates, improved throughput, perform bet- ter during multipath fading, and retrieve lost information easily Index Terms: Diversity coding (DC), orthogonal frequency division without the need of retransmission or feedback from the trans- multiplexing (OFDM), performance, probability of success, relia- mitter when compared to other similar schemes employed in bility, symbol error probability. vehicular ad hoc networks (VANETs). Using DC, reliable in- formation can be transmitted especially for time-critical appli- cations even when a reliable infrastructure is available. I. INTRODUCTION The paper is organized as follows. In Section II, we sum- Vehicular communications play an important role in vehicle marize the prior work on wireless access in vehicular envi- safety and transportation efficiency. The objective of vehicular ronments, OFDM and DC. Section III presents a literature re- communication is to ensure vehicle safety for the drivers and view of the related work. Our approach, DC-OFDM is pre- passengers and to reduce time and fuel consumption, among sented in Section IV, including an analysis of the system per- other services. A few of the primary applications envisioned for formance. Section V presents results of the performance of DC- vehicular networks are emergency notifications for automotive OFDM. Finally, in Section VI, we present our conclusions and safety, notification, and prevention of vehicles during collision, future research directions. location-based information and vehicle tracking services, high- speed tolling, real-time traffic updates, and Internet access with multimedia streaming. II. LITERATURE REVIEW For short and medium range communications, wireless access in vehicular environments (WAVE)/IEEE 802.11p systems have A. Wireless Access in Vehicular Environments been devised using the wireless local area network (WLAN) technologies in these systems. These systems have decent trans- WAVE is a combination of both IEEE 802.11p and IEEE mission range and power, although, they are limited in terms of 1609.x working in the dedicated short range communications coverage distance. WAVE systems are complex as the vehicular (DSRC) band, especially for both the physical (PHY) and the environment is dynamic. Therefore, it is important to maintain medium access control (MAC) layers. a stable and reliable wireless connection for a significant pe- The IEEE 802.11p standard PHY layer is similar to the riod of time. Moreover, WAVE, which is a wireless scheme that 802.11a standard, but with more specific requirements to meet provides vehicle-to-vehicle (V2V) communication and vehicle- the communication requirements between vehicles or betweena to-infrastructure (V2I) communication, has as its primary appli- vehicle and the infrastructure in current vehicular environments. cation in providing emergency safety measures for vehicles. The key points that drove the 802.11p standard are the rela- IEEE 802.11p uses orthogonal frequency division multiplex- tive speed (distances) between the vehicles, the maximum possi- ing (OFDM) [1] to transmit information. OFDM is a widely ble coverage distance ( 1000 meter radius), varying multipath ∼ used technology in fourth generation wireless networks (4G) channel effects in multiple environments and most importantly, and 802.11a/g/n WLANs that achieve high transmission rates the reliability and the security of the message broadcasted in the over dispersive channels by transmitting serial information network. The 5.85–5.925GHz band was chosen to minimize the through multiple parallel carriers. The transmission bandwidth interference and overcrowding present in the operating band- width of 802.11a WLANs. The 75 MHz bandwidth of 802.11p Manuscript received September 12, 2012. is divided into seven 10 MHz channels each with a 5 MHz mar- This research has been partially supported by NSF Grant IIP-1217306. gin at the lower end. Since the channel bandwidth is halved (as The authors are with the Department of Electrical Engineering, Univer- compared to 802.11a channel bandwidth of 20 MHz), the data sity of South Florida, Florida 33620, USA, email: [email protected], [email protected]. rates, carrier spacing, and other parameters are also be halved. Digital Object Identifier 10.1109/JCN.2013.000026 While doubling the symbol duration (8 µs) helps prevent inter- 1229-2370/13/$10.00 c 2013 KICS ARROBO AND GITLIN : IMPROVING THE PERFORMANCE OF OFDM-BASED VEHICULAR... 133 symbol interference (ISI), it also helps in reducing the effects single carrier system. That is, the averagesymbol error probabil- of the multipath channel in rural, urban and sub-urban environ- ity for an OFDM symbol is equal to the symbol error probability ments under study. of a subcarrier. Pser =PserOFDM , B. Orthogonal Frequency Division Multiplexing N 1 The OFDM is a technique that transmits a serial high data Pser = PserSubCarrier . (5) N i i=1 rate stream in parallel over multiple subcarriers with much lower data rates per subcarrier. Since PserSubCarrier = PserSubCarrier , i Similar to the IEEE 802.11a architecture, the IEEE 802.11p i ∀ has a total of 64 carriers that are divided into 48 subcarriers to Pser =PserSubCarrier . (6) carry the data, 4 pilot carriers to make the signal robust against frequency offset, and the remaining 12 subcarriers are null. The However, the OFDM signal has a lower SNR compared to the subcarriers subjected to inverse fast Fourier transform (IFFT) single carrier signal due to the cyclic prefix. convert from the frequency domain to the time domain. The When a multipath channel is assumed (e.g., Rayleigh dis- output of the IFFT process is the OFDM symbol. To this sym- tributed), the symbol error probability for a quadrature PSK bol, guard intervals (GIs) are added to prevent the symbol from (QPSK) modulated OFDM signal can be calculated as [5]: ISI and inter-carrier interference (ICI). For high-speed scenar- 3 γ¯ 1 −1 γ¯ ios, the channel is difficult to predict and there is an increase in Pser = 1 tan (7) 4 − 1+¯γ − π 1+¯γ the Doppler shift, which in turn degrades the quality of the sig- nal and increases the bit error rate (BER). The choice of having where a longer guard interval is a tradeoff between the bandwidth of Eb the data and the reliability as longer GI reduces the throughput γ = 2 2 . (8) π fd of the channel. In the case of IEEE 802.11p standard, the guard 3∆f 2 Eb + N0 interval is observed to be longer than the maximum excess de- ∆f is the subcarrier spacing, f is the maximum Doppler lay in all the environments. The minimum transmission rate for d shift and is calculated as: IEEE 802.11p is 3 Mbps for BPSK modulation with 1/2 coding rate up to a maximum of 27 Mbps for 64 quadrature amplitude vfc fd = (9) modulation (QAM) with 3/4 coding rate. c The BER of an OFDM-based system for M-QAM modulation where v is the relative speed between transmitter and receiver, scheme in additive white gaussian noise (AWGN) and Rayleigh and c is the speed of wave (typically the speed of light in the vac- channels, respectively, are given by [3]: uum). The probability of having δ symbol errors in an OFDM symbol can be calculated using the binomial probability mass 2(M 1) E 6log M P = Q b 2 , (1) function: e −M 2 Mlog2 N0 M 1 − n n−δ δ Pδ = (1 Pser) Pser . (10) Eb 3log2M δ − 2 M 1 N0 M −1 Pe = − 1 (2) M Eb 3log2M Mlog2 − 2 +1 And the probability of having no symbol errors (δ = 0) in an N0 M −1 n-subcarrier OFDM symbol can be calculated using (10): where M is the order of the modulation scheme and Eb/N0 is n Pδ=0 = (1 Pser) . (11) the energyper bit to noise power spectral density ratio. The sym- − bol error probability (Pser) for a M-QAM and M-phase-shift The probability of symbol error of a coded OFDM system, keying (PSK) modulated OFDM signals, respectively, under an using block coding (n,k,t), is given by [5]: AWGN channel are given by [4]: n 1 P = δP . (12) sercoded n δ 1 Es 3 δ=t+1 Pser =4 1 Q − √M N0 M 1 − By implementing DC in vehicular communications that use 2 1 2 Es 3 OFDM-based technologies, such as IEEE 802.11p systems, the 4 1 Q , (3) bandwidth is utilized in an efficient manner, the reliability of − − √M N0 M 1 − M− π π the communication is improved and lost information can be ( 1) sin2 M − Es M 1 N 2 recovered in different vehicular communication scenarios. For 0 sin θ (4) Pser = e dθ.
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