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
10、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 Long code is a seq
11、uence of bits used for its pseudo-orthogonal properties； that much longer sequence does not have the perfect autocorrelation properties as the short code, but a similar one such that: This pseudo orthogonal property is used in IS-95 reverse link in a 1.2288Mcps chip sequence to differentiate between
12、 mobiles. On the forward link the long code is decimated down to a 19.2kbps bit stream for a unique user mask. IS-95A Reverse Traffic Channel Detection Demo The IS-95A Reverse Traffic Channel Detection demo simulates modulation and spreading of the data symbols at the mobile station transmitter, as
13、well as despreading and noncoherent demodulation at the base station receiver. At the transmitter, the symbols are modulated by a Walsh modulator and spread, and a randomized gating is applied to the transmit bursts. At the receiver, the noncoherent rake demodulator recovers the data. The bit error rate for the data is displayed in the simulation. You can open the model by following the instructions in Opening a Demo, or by typing is95revchdetection2 at the MATLAB prompt. Library Blocks in the Demo
