ter and is split into N bit stream of rate R/N bps. Each of these bit streams drives one modulator, which maps each bit or symbol onto a point in the modulation constellation being used (Section "Digital Modulation Technique, p. 95"). The N resulting amplitude and phase points drive the inputs of an Inverse Fast Fourier Transform (IFTT), the output of which is the sum of the subcarriers, each modulated according to the individual input bit streams. At the receiver, after removing any guard interval, a Fast Fourier Transform (FFT) determines the amplitude and phase of each subcarrier in the received signal. The amplitude and phase are adjusted using information gathered from the pilot tones. A demodulation decision is made by mapping this amplitude and phase onto the modulation constellation and the corresponding input bit or bits are generated. The resulting N parallel R/N bps bit streams are then combined in a parallel to series converter to give the original R bps bit stream. The IEEE 802.11a/g standards uses OFDM in the unlicensed 2.4 and 5 GHz ISM bands respectively to provide data rates up to 54 Mbps. modulation Serial bit or Inverse symbol stream Serial to Guard Digital to analogue parallel Data coding Fourier interval conversion and RF up conversion (QAM) conversion Transform insertion filtering (IFFT) Sub-carrier phase Noise, multi path and Transmission and amplitude other interference channel Serial bit or symbol stream Channel Parallel Guard Fourier Analogue to digital to serial equalization RF down interval conversion and and data Transform conversion conversion removal filtering decoding (FFT) demodulation Figure 4-19: Schematic Block Diagram of an OFDM Transmitter and Receiver Chapter Four The system uses 52 subcarriers of which 48 are used to carry data and are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM) or 64-QAM. The remaining four subcarriers are used as pilot tones. Space Division Multiple Access Space division multiple access (SD