7.7,
we inserted an LPF after despreading, in order to further filter the wideband interference and noise
for illustrative purposes.
3G WIDEBAND CDMA 347
FIGURE 7.7 DS-CDMA system principles.
+
Spreading
Despreading
|B(f)|
|W( f )|
|N( f )|
|Y( f )|
Desired
CDMA User
Another
CDMA User
Narrowband
Interference
LPF
Let us briefly describe this PN sequence generator. The output chip sequence is usually generated
by linear maximal??“length sequence generators. These sequences are easy to generate and, depending
on the generator polynomial, have very interesting properties. We will discuss the properties of these
polynomials, in more detail, later in this chapter; suffice it to say that the following three properties
are desirable [1??“4]:
1. Looks like random data. There should be a balance of 1s and 0s, where the period is defined
as T 2N 1, where N is the number of shift registers or order of the polynomial.
2. Cyclic addition. If one were to cyclically shift the PN sequence by X shifts, it will produce another
code word which, when added to a code word of Y shifts, results in still a third code word, Z.
3. Good correlation properties. Both autocorrelation and cross-correlation are important if one
wants to confidently decide whether this is your PN code word or not.
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