Intermodulation and parasitic response issues in CDMA RF system design

JasonYoo

Intermodulation and parasitic response issues in CDMA RF system design [Copy link]

In the design of CDMA base station transceivers, careful consideration should be given to cross-mixing, cross-modulation, spurious responses, and other RF issues. A major advantage of spread spectrum communication systems is that they are highly immune to interference while hiding their transmissions in the background noise. In a CDMA system based on direct sequence spread spectrum (DS-SS) technology , the desired information carrier is modulated by a digital code that consists of a pseudo-random noise (PN) sequence. The PN code signal is independent of the data and has a much higher data rate than the desired information. As a result, the bandwidth of this digital code is much larger than the minimum bandwidth required to transmit baseband data in a digital system. When the carrier information is modulated with a digital code, the carrier bandwidth can be as large as the code bandwidth. Depending on the modulation method used ( such as binary, quadrature, or minimum shift keying ) , the carrier bandwidth is expanded by a certain multiple. The receiver then "de-spreads" this signal by correlating with a synchronous replica of the original PN code, as shown in the following figure.

Figure 1 : CDMA signal transmission and reception

It can be seen that the correlator improves the processing gain by spreading and despreading the carrier signal. After down-conversion to baseband, the CDMA signal is fed to the input of the CDMA correlator together with other interference signals. When a correlator PN sequence matches the PN sequence embedded in the CDMA signal , the signal of the desired information is restored to its original bandwidth before spreading. On the other hand, the input signal that does not match this sequence ( such as receiver noise, CW interference signal or other CDMA signals with non-precise code synchronization ) is spread to a bandwidth identical to the PN code by the correlator PN sequence. A digital filter following the despreader and having the same bandwidth as the information bandwidth can select all the desired information carriers while only allowing the spread codes of some interference signals to pass.

At the transmitter output of the base station, the communication channel, pilot channel, synchronization and paging channel are multiplexed and then sent on a radio channel. Therefore, the power of each user communication channel represents a small part of the total power of this forward CDMA channel. After bandwidth limiting by a digital filter, the 3dB bandwidth of a CDMA radio channel is 1.23MHz .

Since the information of the CDMA system is bi-phase modulated, the modulated signal envelope is not fixed. The peak-to-average ratio ( PTAR ) of the forward CDMA signal is 10dB . Therefore, in order to detect and demodulate the received signal accordingly, the receiver should maintain linearity within the range allowed by the received signal power.