Wireless networking based on IEEE 802.11a/g is designed for stationary in-door use (in a Wireless Local Area Network or “WLAN”, also known as a “Wi-Fi” network). Nevertheless, the standardizing body has decided to re-use the Physical Layer (PHY) from IEEE802.11a for Vehicular Ad-hoc Networks. The specification for this enhanced standard is defined in an amendment known as IEEE802.11p. The new standard is expected to find application in areas such as Intelligent Transportation Systems (ITS), both for safety and non-safety related purposes. The differences between IEEE 802.11a and IEEE 802.11p are primarily in the Medium Access Control (MAC) Layer.
The IEEE 802.11a physical layer specification uses Orthogonal Frequency Division Multiplexing (OFDM) to transmit data. Within the OFDM signal, the standard defines a preamble as well as 4 pilot sub-carriers in each OFDM symbol. The preamble occurs at the start of each packet (in the time domain). The pilot sub-carriers are present in every OFDM symbol at fixed locations in the spectrum (in the frequency domain). The preamble can be used for channel estimation at the start of a packet. During the packet, the pilot sub-carriers can be used for channel estimation. This may comprise updating the channel estimate generated from the preamble.
Notably, in the IEEE802.11p physical layer specification, no extra pilot sub-carriers were added. This makes it challenging to respond to the fast time-varying channel conditions that can be experienced under outdoor mobile conditions. The 4 pilots that are present in the transmitted signal may be too sparsely distributed in the signal spectrum to be able to track channel variations, in some circumstances.
In FIG. 1, the distribution of pilot carriers and data carriers over the spectrum is shown for an IEEE802.11a signal. The overall bandwidth is 16.25 MHz. Each sub-carrier making up the OFDM signal is indicated as a large vertical arrow. The small vertical arrows indicate neighbouring frequency intervals, in which no sub-carriers are transmitted. The transmitted sub-carriers are labelled from −26 to +26. Carrier numbers −21, −7, 7, and 21 are pilot sub-carriers. The remaining carriers are data sub-carriers. The sub-carrier spacing is 312.5 KHz, as indicated.
For IEEE802.11p, the bandwidth and carrier spacing is half as large—that is, the bandwidth of one channel is 8.125 MHz (10 MHz channel spacing) and the carrier spacing within the channel is 156.25 kHz. Without appropriate measures, proper packet reception will fail under mobile conditions leading to high Packet Error Rate (PER). For safety related ITS applications, a high PER is unacceptable and therefore approaches for improving reception quality have been proposed. For example, P. Alexander et. al. (“Outdoor Mobile Broadband Access with 802.11”, IEEE Communications Magazine, pp. 108-114, November 2007) have proposed a method for artificially making pilot sub-carriers that can be used for providing input to the channel estimation algorithms. The artificial pilots are made by re-encoding demodulated and decoded data. The philosophy behind this approach is that, after demodulation and Forward Error Correction (FEC) decoding, the decoded data has a low number of errors and therefore after re-encoding it can provide a reference for channel estimation. Whereas conventionally a pilot is a sub-carrier that is always modulated in a fixed/known way, the method of Alexander et al. uses data sub-carriers whose modulation is not known in advance. Instead the modulation is determined by decoding the data message—the decoded data is then assumed to be correct and on this basis the modulation of the corresponding sub-carriers is treated as “known”.
According to the method disclosed by Alexander et al., each symbol is decoded twice, using two separate decoders. An early decoder is used in the process of obtaining the channel estimate. The resulting channel estimate is then used for interference cancellation, to allow a delayed version of the received signal to be decoded in the main decoder. The intention is that the delayed version of the signal that has been subject to interference cancellation based on the improved channel estimate will exhibit a higher signal-to-noise ratio and will therefore yield a lower error rate after decoding.