Physical Layer Technologies
Although IEEE 802.11b has been demonstrating some capabilities for the communication between mobiles at high speed in ITS (Intelligent Transportation Systems), a new standard was introduced: IEEE 802.11p [1]. The lower layer of IEEE 802.11p is the base st
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Physical Layer Technologies
Although IEEE 802.11b has been demonstrating some capabilities for the communication between mobiles at high speed in ITS (Intelligent Transportation Systems), a new standard was introduced: IEEE 802.11p [1]. The lower layer of IEEE 802.11p is the base standard for the new coming DSRC (Dedicated Short Range Communications), which involves vehicle-to-x communication. The frequency allocation in US (5,850–5,925 GHz) was done from 2004, while in Europe, EU DSRC was adopted in August 2008 with the frequency band within the range of 5,875–5,905 GHz [2]. Currently, a newly formed Wireless Access in Vehicular Environments (WAVE) study group works on the migration of IEEE 802.11 standards toward 802.11p [3, 4]. The WAVE study group is working on more standards: IEEE P1609.3 that specifies the overall communication architecture and the IEEE 802.11p, IEEE P1609.1, IEEE P1609.4, IEEE P1609.2 which focus on the architecture’s details. The 802.11p PHY layer follows the same frame structure, modulation scheme and training sequences of the IEEE 802.11a PHY layer [3–5]. In comparison to cellular communications, DSRC can provide higher transfer rates and smaller communication latencies for small communication zones defined by the communication radius of the technology [6]. It will support communication between nodes that travel with a speed of up to 200 km/h [7–8]. Besides the above features that the new 802.11p needs to provide, the “normal” wireless issues like: path loss and fading needs to be minimized [3–5]. The path loss refers to signal strength variation due to environment: it can attenuate faster or slower than it does in free space. The fading refers to the multi-path effect which also affects the signal strength. The DSRC physical layer uses an orthogonal frequency division multiplex (OFDM) modulation scheme to multiplex data [3–5]. The technology works by splitting the radio signal into multiple smaller sub-signals. These sub-carriers typically overlap in frequency, but are designed not to interfere with each other: sub-carriers are orthogonal to each other and they are separated using a Fast Fourier Transform (FFT) algorithm. Main reasons for using OFDM are its high spectral efficiency [5], its good performance in multi-path fading environments and the simple transceiver design. Besides reducing multipath interference, it can actually increase the signal strength by processing the reflected packets to increase gain. This technique also improves non-line of sight delivery (where the sender and the R. Popescu-Zeletin et al., Vehicular-2-X Communication, C Springer-Verlag Berlin Heidelberg 2010 DOI 10.1007/978-3-540-77143-2_7,
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7 Physical Layer Technologies
receiver do not see each other). OFDM divides the input data stream into a set of parallel bit streams and each bit stream is then mapped onto a set of overlapping orthogonal subcarriers for data modulation and demodulation. All of the orthogonal subcarriers are simultaneously transmitted. Orthogonal Frequency-Division Multiple
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