CDMA - Interferences

A CDMA signal experiences high interference signals other than the CDMA users. This takes two forms of interference — interference from other users in the same minicell and interference from the adjacent cells. The total interference also includes the background noise and other spurious signals.

CDMA is based on the use of a spread spectrum form of modulation to encode a signal for its transmission and retrieval.

Noise Sources

In the spread spectrum technology, the radio signals are distributed on a single 1.23 MHz wide frequency band. Each subscriber has assigned PN codes. Signals corresponding to the PN codes are decoded and processed. Signals that do not contain the code matches are treated as noise and ignored.

Signal Processing: Receive

CDMA starts with an encoded narrowband signal; this spreads with the use of the PN codes to a bandwidth of 1.23 MHz.

When the signal is received, it is filtered and processed to recover the desired signal. A correlator eliminates sources of interference because they are uncorrelated with the desired signal treatment. Using this method, the number of CDMA calls can occupy the same frequency spectrum simultaneously.

Frame Error Rate

The number of transmission errors, measured in terms of frame error rate (FER). It increases with the number of calls. To overcome this problem, the minicell and mobile site can increase the power until either the mobile or the minicell site can power up more further to reduce FER to an acceptable amount. This event provides a soft limit calls from a particular minicell and depends on −

  • The noise floor naturally occurring and man-made interference.
  • Interference from calls on this minicell.
  • Interference from calls on other cells.

Power per Walsh Code

The power control bit is used during call processing to maintain the relative power of each individual active traffic channel and power up or down to maintain acceptable FER measurements by the mobile on the channel. This power is expressed in terms of digital gain units.

The following actions can be seen in the transmit path −

  • The low bit rate digital voice packet from PSU2 (packet switch unit 2 in the 5ESS switch) is spread by a Walsh code in the minicell.

  • The RF transmit carrier frequency is modulated by the spread signal.

  • The direct sequence spread spectrum signal is transmitted.

The following actions can be seen in the receive path −

  • The direct sequence spread spectrum signal is received.

  • The signal is demodulated by using the RF receive carrier frequency.

  • The signal dispreads by using the same Walsh code.

  • A bit detector restores the decoded signal to a reasonable representation of original speech pattern.

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