1、附录 1. 英文原文 An Overview of CDMA CDMA Basics CDMA systems spread a slow information bit rate with a fast chip sequence, transmit it over the air and retrieve the original information. How to actually spread and retrieve the information is standardized in details in IS-95. Three main tools are used: Wa
2、lsh 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 fur
3、ther 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)
4、on the forward link. In the forward link, each mobile uses a specific Walsh code sequence； all sequences are multiplexed together in a total combined sequence: (wheresi represents the information symbol (+1 or -1), gi is the individual channel gain.) That sequence is manipulated further and sent ove
5、r the air； on the receiver side, that sequence is decoded by simply integrating for each channel: for channel k, the information 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 encod
6、ing 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 multiplexes several channels similarly to the forward link, and Orthogonal properties of the Walsh codes provide the multiplexing scheme: Short Code Short cod
7、es 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 sequenc
8、e 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 the above computation, we have for a short code sequence: This is a remarkable property o
9、f 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
