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CDMA综述外文翻译

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CDMA综述外文翻译

附录 1. 英文原文 An Overview of CDMA CDMA Basics CDMA systems spread a slow ination bit rate with a fast chip sequence, transmit it over the air and retrieve the original ination. How to actually spread and retrieve the ination is standardized in details in IS-95. Three main tools are used Walsh codes, 64-chip orthogonal sequences A short code 215 - 1 32767 chips long, which has the property of being orthogonal to any nonzero offset of itself. A long code 242 chips long, used to generate unique sequences, which are pseudo-orthogonal to one-another. The following sections go into further details on where these tools are used in various aspects of the IS-95 air interface on forward links, reverse links, in access mode, and in traffic mode. Walsh Codes Forward LinkWalsh codes are orthogonal codes, IS-95 uses them to multiplex several mobile communications and control channels on the forward link. In the forward link, each mobile uses a specific Walsh code sequence; all sequences are multipld together in a total combined sequence wheresi represents the ination symbol 1 or -1, gi is the individual channel gain. That sequence is manipulated further and sent over the air; on the receiver side, that sequence is decoded by simply integrating for each channel for channel k, the ination bit is retrieved from the sign of the integral Reverse Link In the reverse link of IS-95, Walsh codes are not used in that manner but simply encode bits in a 64-ary encoding scheme that s each 6-bit sequence is mapped to a 26 64 bit Walsh code. In further evolutions like IS-2000 and IS-856, the reverse link multipls several channels similarly to the forward link, and Orthogonal properties of the Walsh codes provide the multiplexing scheme Short Code Short codes are bit sequences with very specific autocorrelation functions. IS-95 uses a 215 - 1 32767 chip short code. For a first example let us use here a 22 - 1 3 bit long short code 100. Permutated, that short code has the following property if 1 is counted for every bit of the permutated sequence that is identical to the bit of the original sequence, and -1 is counted when that bit is different, every permutated sequence totals -1, whereas the original sequence obviously totals its length . If we note S,Snthe above computation, we have for a short code sequence This is a remarkable property of autocorrelation between bit sequences that is used in CDMA. As another example, verify the following two short code sequences of length 23 - 1 7 verify the same property as the table above s1 0011101 and s2 1110010. These two sequences are used for Gold codes in other CDMA standards. And another short code of length 25 - 1 31 is s3 0000101011101100011111001101001. IS-95 uses a much longer such sequence 215 - 1 32768 chips which provides many possible sequences orthogonal to one another, used to differentiate between sectors on the forward link. Long Code

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