12-01-2013, 03:00 PM
A NEW OFDM STANDARD FOR HIGH RATE WIRELESS LAN IN THE 5 GHZ BAND
A NEW OFDM STANDARD.doc (Size: 390 KB / Downloads: 87)
INTRODUCTION
In July 1998, the IEEE 802.11 standardization group decided to select OFDM as the basis for a new physical layer standard extension to the existing 802.11 MAC standard [1-3]. The new standard is targeting a range of data rates from 6 up to 54 Mbps in the 5 GHz band. Following the IEEE decision, ETSI BRAN in Europe and MMAC in Japan also based their new standards on OFDM, with the goal of creating a single world-wide physical layer standard for wireless LAN in the 5 GHz band.
OFDM PARAMETERS
Table 1 lists the main parameters of the draft OFDM standard. A key parameter which largely affected the choice of the other parameters is the guard interval of 800 ns. This guard interval provides robustness to root-mean-squared delay spreads up to several hundreds of nanoseconds, depending on the coding rate and modulation used. In practice, this means that the modulation is robust enough to be used in any indoor environment, including large factory buildings. It can also be used in outdoor environments, although directional antennas may be needed in this case to reduce the delay spread to an acceptable amount and to increase the range.
OFDM SIGNAL PROCESSING
A general block diagram of an OFDM transceiver is shown in figure 2. In the transmitter path, binary input data is encoded by a rate 1/2 convolutional encoder. The rate may be increased to 2/3 or 3/4 by pucturing the coded output bits. After interleaving, the binary values are converted into QAM values. To facilitate coherent reception, 4 pilot values are added to each 48 data values, so a total of 52 QAM values is reached per OFDM symbol, which are modulated onto 52 subcarriers by applying the Inverse Fast Fourier Transform (IFFT). To make the system robust to multipath propagation, a cyclic prefix is added. Further, windowing is applied to get a narrower output spectrum. After this step, the digital output signals can be converted to analog signals, which are then upconverted to the 5 GHz band, amplified and transmitted through an antenna.
CHANNELIZATION
Figure 1 shows the channelization for the lower and middle Unlicensed National Information Infra-structure (UNII) bands. Eight channels are available with a channel spacing of 20 MHz and guard spacings of 30 MHz at the band edges in order to meet the stringent FCC restricted band spectral density requirements. The FCC also defined an upper UNII band from 5.725 to 5.825 GHz, which carries another 4 OFDM channels. For this upper band, the guard spacing from the band edges is only 20 MHz, since the out-of-band spectral requirements for the upper band are less severe as those of the lower and middle UNII bands. Notice that different carrier frequencies may be used in Europe and Japan, but the channel spacing will be the same, while also most of the bands are expected to overlap.