DFE is a feedback based scheme for channel equalization in signal handling. For signal processing, the text of the high speed extension of the IEEE standard 802.11 specifies complementary code keying (CCK) as the modulation scheme for 5.5 and 11 Mbps data rates in the 2.4 GHz band. In the CCK modulation scheme, CCK code words are transmitted which correspond to the actual bits to be sent. The IEEE 802.11 CCK codes are polyphase complementary codes (or binary complementary sequences or number of pairs of like elements) with any given separation in one series being equal to the numbers of pairs of unlike elements with the same separation in the other series. The 802.11 complementary spreading codes in one form have a code length of 8 and a chipping rate of 11 M chips/s. The 8 complex chips comprise a single code word. Each CCK code word consists of eight chips. In the 5.5 Mbps case, incoming bits are divided into blocks of four bits. The MSB two bits are used to select one of the four code words and LSB two bits to perform DQPSK modulation on all the eight chips of the selected code word. Similarly, in 11 Mbps case, incoming bits are divided into blocks of eight bits, MSB six bits are used to select one of the 64 code words and LSB two bits to perform DQPSK modulation. So, each code word is called a ‘symbol’ and the eight values in each code word are called ‘chips’.
Transmitted signal can thus be represented as,
    s1=[c1, c2, c3, c4, c5, c6, c7, c8]    s2=[c1, c2, c3, c4, c5, c6, c7, c8]. . .. . .    sn=[c1, c2, c3, c4, c5, c6, c7, c8],    where s1, s2 . . . sn are the symbols and    c1, c2 . . . c8 are the chipsThe symbols s1, s2 . . . sn are transmitted sequentially.Now, in the receiver, the symbol decision, which is deciding all the eight chips of the symbol, is performed using CCK demodulation. This involves correlation with each of four code words and maximum picking in the 5.5 Mbps case and correlation with each of 64 code words and maximum picking for the 11 Mbps case. This will be used to remove inter symbol interference. But, to remove the interference caused due to the chips within a symbol, chip decision is used.Symbol decision consists in deciding s1, s2 . . . sn and chip decision consists in deciding c1, c2 . . . c8 for each symbol.
A base band processor implements the CCK waveform to achieve high data rates over wireless links. The base band processor can generally improve packet error rate performance in multi-path environments through the use of rake receiver architecture and decision feedback equalizer. However, the question of interference removal and reducing error propagation in DFE designs needs to be addressed to obtain efficient performance.
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