Spread spectrum disadvantages; CDMA implementation

Disadvantages of spread spectrum systems

• Bandwidth inefficient for a single user – a signal of a few kHz is spread over MHz.
• Near-far problem – a nearby user's strong signal swamps far users; precise power control is needed.
• Self-jamming – codes are not perfectly orthogonal, so users interfere with each other.
• Complex receivers – code acquisition, tracking, RAKE receiver and high-speed chip processing.
• Strict synchronisation of PN codes is required.

Implementation of a CDMA system

In DS-CDMA every user transmits on the same carrier at the same time. Each user's data is multiplied by a unique high-rate PN (spreading) code. The receiver multiplies the received sum by the wanted user's code; only that user is despread back to narrowband, while all others stay as wideband noise.

 Transmitter (user k)
 data d_k(t) --(x)--(x)--> sum of all users
               ^    ^         in the channel
             c_k(t) carrier
 Receiver for user 1
 r(t) --(x)--(x)--[Integrate over Tb]--> d1
          ^    ^
       carrier c_1(t)

Steps:

1. Data at bit rate R_b is multiplied by the code c_k(t), whose chip rate R_c >> R_b. Processing gain G_p = R_c / R_b.
2. The spread signal modulates the carrier (BPSK/QPSK) and is sent.
3. The receiver gets r(t) = Σ d_k(t)·c_k(t) + noise.
4. Multiplying by c_1(t): since c_1·c_1 = 1, user 1's data is recovered; cross-terms d_k·c_k·c_1 remain spread because the codes have low cross-correlation.
5. Integrating over one bit gives d_1; interference is reduced by G_p.

Example (chip level): Let code of A = +1 −1 +1 −1, code of B = +1 +1 −1 −1 (orthogonal).

• A sends bit +1 → +1 −1 +1 −1; B sends bit −1 → −1 −1 +1 +1.
• Channel sum = 0 −2 +2 0.
• Receiver A: (0)(+1)+(−2)(−1)+(+2)(+1)+(0)(−1) = 4 → 4/4 = +1 (A's bit).
• Receiver B: (0)(+1)+(−2)(+1)+(+2)(−1)+(0)(−1) = −4 → −1 (B's bit).

Real systems (IS-95) use 1.2288 Mcps, Walsh codes for channel separation, PN short codes for cell separation, power control and a RAKE receiver.