PHY LAYER : IEEE 802.11p

The lower layer of IEEE 802.11p is the base standard for DSRC, this layer follows the same frame structure, modulation scheme and training sequences as done by IEEE 802.11a PHY layer. If compared to cellular communication DSRC provides higher transfer rates and smaller communication latencies for small communication zones defined by communication radius of the technology. It supports communication between the nodes that travel with a speed upto 200Km/h. Besides all these advantages this standard needs to provide better path loss and fading than its counterparts. Path loss refers to signal strength variations due to environment, whereas fading refers to multipath effects.

DSRC uses OFDM modulation scheme to multiplex data. The technology works by splitting the radio signal to multiple smaller sub-signals. Main reason for using OFDM is its high spectral efficiency. All of the orthogonal subcarriers are simultaneously transmitted. OFDMA- Orthogonal frequency division multiple access provides shared access to multiply users by assigning subsets of subcarriers to individual users. Theoretically sub-channelization could be made in thousands of sub channels.

However, high mobility can cause negative effects like failure to receive message or packet errors. The first is because during transmission of safety related messages, some of the receivers may move out of the transmission range with regards to sender. The second negative impact refers to high packet error rates and consequently lower channel capacity because high mobility makes worse Doppler’s spread on OFDM. [2]

Note that in the physical layer, the warning alerts are sent on a different channel than the permanent beacons. Similar, the Bidirectional Info is sent on a different channel than the Bidirectional Autonomous information.

In Table 2 the specifications for the some of the 802.11 standards are presented:

Wi-Fi standards Modulation Frequency [GHz] Channel Bandw. [MHz] No. of all/non- overlap channel Bandwidth (max) [Mbit/s] Transmission range (outdoor) [m]
802.11a OFDM 5,725–5,850 20 12/8 54 30
802.11b DSSS 2,400–2,485 22 14/3 11 250
802.11g OFDM 2,400–2,483 22 14/3 54 250
802.11p US OFDM 5,850–5,925 10 (20) 7/7 54 1000
802.11p EU OFDM 5,875–5,905 10 7 54 1000

Table 2: Specifications for the some of the 802.11 standards [1]

The physical layer is an interface between the MAC and the media layer, which helps in sending and receiving the frames. It is responsible for hardware specification, bits conversion, signal coding and data formatting.  As mentioned earlier PHY layer of 802.11p standard is similar to 802.11a. . It is composed of two sub layers as shown in Figure 3.

Figure 3: WAVE Protocol Stack and the sub layers of PHY
Figure 3: WAVE Protocol Stack and the sub layers of PHY

As can be seen in the figure 3, PHY in 802.11p consist of two sublayers:

  • Physical Layer Convergence Protocol (PLCP): Responsible for communicating with the MAC layer and is also a convergence process that transforms the Packet Data Unit (PDU) arriving from the MAC layer to compose an OFDM frame.
  • Physical Medium Access (PMD): It is an interface to the physical transmission medium such as radio channels and fiber links. Its task is to manage data encoding and perform the modulation.
Figure 4: IEEE 802.11p PHY layer PPDU Frame structure

The Protocol Packet Data Unit (PPDU) composed of a preamble, signal field and a payload component containing the useful data as shown in Figure 4.

  • The preamble field marks the beginning of the physical frame. It is used to select the appropriate antenna and correct the frequency and timing offset.
  • The signal field (SIG) is used to specify rate and length information.
  • The data field is intended to carry the over load data which are OFDM symbols

As told earlier, here OFDM technique with 64 sub-carriers is used for transmission . Out of which, only the inner 52 carriers are utilized. Among 52 carriers 48 contains the actual data and 4 sub-carriers, called pilot sub-carriers, transmit a fixed pattern, used to disregard frequency and phase offset at the receiver side. Each of 48 data sub-carriers can be modulated with different modulation techniques like BPSK, QPSK, 16QAM or 64QAM. All together with different coding rates, this leads to a nominal data rate of 6 to 54 Mb/s, if full clocked mode with 20 MHz bandwidth is used. IEEE 802.11p uses the half clocked mode with 10 MHz bandwidth, in order to make signal more robust against fading, resulting in corresponding data rate reduction. Some other differences between IEEE 802.11a and IEEE 802.11p due to reduced sampling rate are highlighted in Table 3.

Table 3: Comparison of PHYs implementations in IEEE 802.11a and IEEE 802.11p
Table 3: Comparison of PHYs implementations in IEEE 802.11a and IEEE 802.11p [1]

WAVE networks have to be extremely robust and high speed response, because their failure may cause the loss of life and property. Irrespective of various advantages, there are many challenges faced by Physical layer which was not encountered in other wireless communication application like:
Collision Avoidance in WAVE technology between the high mobility vehicles which effects the Quality of Service.

Tight Latency Requirement, IEEE defined that the latency for safety applications in VANET should be 50 ms and not exceeds 100 ms, however, for other applications more than 100 milliseconds is allowed.

Doppler’s spread on OFDM, which is caused due to high packet error rates and consequently lower channel capacity because of high mobility as OFDM is too much sensitive to carrier frequency offset.

To overcome this problem of IEEE 802.11p, some Physical layer parameter have been changed. For example: Sub-carrier spacing has been halved. In other words, one IEEE 802.11a OFDM channel uses 52 sub-carriers and out of that 48 used for transmit data and rest for pilot carrier, but in one IEEE 802.11p use same number of sub-carriers but the bandwidth per channel has been halved from 20 MHz to 10 MHz, this also means that various parameter in time domain is doubled in 802.11a and since by doing this Doppler spread decreases due to small bandwidth per channel, interference also decreased.  Data rate in 802.11p has been halved to 3 to 27 Mbps against 6 to 54 Mbps

Still there are several other challenges faced in 802.11p like:

 1. Effect of Noise in Bit and Symbol Energy

  • Effect of unused sub-carriers on symbol energy
  • Effect of Cyclic Prefix on symbol energy

2. Multipath Effects

  • Rayleigh fading
  • Frequency Selective Fading
  • Delay Spread

3. Channel variation and channel estimation

4. Network Coverage Range

5. Bit rate enhancement techniques

Though for each challenges have been partially addressed in legacy cellular systems. The question arises is whether solutions proposed for Cellular Communication are applicable for VANETs. In conclusion I can say that VANETs have emerged as a new technology that helps in providing vehicles safety and driving comfort and the PHY is key factor in achievement of the objectives of these network. In my next block I will throw in length light on another key layer i.e. MAC layer of IEEE 802.11p. Till then dasvidanya…!! 🙂
References:

[1] Abdeldime M.S. Abdelgader, Wu Lenan, “The Physical Layer of the IEEE 802.11p WAVE Communication Standard: The Specifications and Challenges” Proceedings of the World Congress on Engineering and Computer Science 2014 Vol II WCECS 2014, 22-24 October, 2014, San Francisco, USA.

[2] Radu Popescu-Zeletin, llja Radusch, Mihai Adrian Rigani, “Vehicle-2-X Communication- State-of-the-Art Research in Mobile Vehicular Ad hoc networks”- Springer-Verlag Berlin Heidelberg 2010

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