 of 48 complex numbers. Each such group is associated with one OFDM symbol. In each group, the complex numbers are numbered 0 to 47 and mapped hereafter onto OFDM sub-carriers numbered -26 to -22, -20 to -8, -6 to -1, 1 to 6, 8 to 20 and 22 to 26. The sub-carriers -21, -7, 7 and 21 are skipped and, subsequently, used for inserting pilot sub-carriers. The 0 sub-carrier, associated with center frequency, is omitted and filled with the value 0. The total number of the sub-carriers is 52 (48 + 4). k) For each group of sub-carriers -26 to 26, convert the sub-carriers to the time domain using inverse Fourier transform. Prepend to the Fourier- transformed waveform a circular extension of itself, thus forming a GI, and truncate the resulting periodic waveform to a single OFDM symbol length by applying time domain windowing. 1) Append the OFDM symbols one after another, starting after the SIGNAL symbol describing the RATE and LENGTH fields. m) Up-convert the resulting complex baseband waveform to a radio signal according to the center frequency of the desired channel and transmit. Transmission Convolution Interleaving encoder Mapping insertion modulation Reception Deinterleaving Convolution demodulation removal Demapping encoder Figure 3.2. Transmission and reception chain 3.1.2.2. Scrambler The DATA field, composed of SERVICE, PSDU data unit, tail and pad parts, shall be scrambled with a length-127 scrambler (Figure 3.3). The same scrambler is used to scramble transmit data and to descramble receive data. When transmitting, the initial state of the scrambler shall be set to a pseudo-random non-zero state. 802.11a/g Interfaces The seven least significant bits (LSB) of the SERVICE field shall be set to all zeros prior to scrambling to enable estimation of the initial state of the scrambler in the receiver. X Superscript(1) Scrambled Figure 3.3. Scrambler diagram 3.1.2.3. Convolutional encoder The convolutional encoder shall use the generator polynomials g0 = 1338 and g1 = 1718 and produce two sequences from the scrambled D