Walsh Codes

Orthogonal codes are easily generated by starting with a seed of 0, repeating the 0 horizontally and vertically, and then complementing the 1 diagonally. This process is to be continued with the newly generated block until the desired codes with the proper length are generated. Sequences created in this way are referred as “Walsh” code.

The Walsh code is used to separate the user in the forward CDMA link. In any given sector, each forward code channel is assigned a distinct Walsh code.
 
 

Ø      64-order Walsh function is used as a spreading function and each Walsh code is orthogonal to other.

Ø      A Walsh can be presented by Wim where ith (row) is the position and m is the order. For example, W24 means 0101 code in W4 matrix

Ø      In forward direction, each symbol is spread with Walsh code

Ø      Walsh code is used to distinguish the user in forward link

Ø      For IS95A/B, in the reverse, every 6 symbols correspond to one Walsh code. For example, if the symbol input is 110011,the output after spreading is W5164  (110011=51).

Ø      For CDMA2000, in the reverse, Walsh function is used to define the type of channel (RC 3-9)

 

Orthogonal Functions:

Orthogonal functions (that is, signals or sequences) have zero cross-correlation. Zero correlation is obtained if the product of two signals, summed over a period of time, is zero. For the special case of binary sequences, the values 0 and 1 may be viewed as having opposite polarity. Thus when the product (XORing in this case) of two binary sequences results in an equal number of “1’s and “0’s, the cross-correlation is zero.

 

Creating Orthogonal Functions:

Orthogonal codes are easily generated by starting with a seed of 0, repeating the 0 horizontally and vertically, and then complementing the 0 diagonally. This process is continued with the newly generated block until the desired codes with the proper length are generated. Sequences created in this way are referred to as Walsh codes.

 

 

Variable Walsh codes:

Enhancements of CDMA2000 include the use of Walsh spreading factor to attain high data rates on the forward link. Variable Walsh spreading uses the tree structure for recursively constructing Walsh codes of the longer lengths. Higher data rates for the user can be obtained by using the shorter Walsh code. However, use of one of the shorter codes precludes using any longer code that is derived from it. 

 

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