Mobile inter-modulation interference (MIM) in cellular band-A due to CDMA transmissions in cellular band-B

Theoretical analysis reveals that continuous CDMA transmissions in band-B can be a source of inter-modulation interference for mobiles in band-A. Detailed laboratory testing was conducted to understand cellular band-A mobile station (MS) behavior when the MS is on a digital traffic channel (DTC) and being influenced by CDMA transmissions in band-B. Two types of 3rd order inter-modulation interference associated with CDMA signals are considered, one where both the channels are from within the CDMA channel and the other where one of the inter-modulating channels are one of the AMPS channels. A third higher order CDMA associated inter-modulation interference is also investigated and tested. Exhaustive testing in a controlled laboratory environment showed that mobile inter-modulation interference (MIM) can be reduced or eliminated if appropriate signal strength (SS) separation is maintained between band-A and band-B channels. A -30 dB C/I margin between band-A and band-B channels was found to be adequate to maintain low BER under non-faded channel conditions. A field testing procedure describes a simple drive testing scheme that can be used by field engineers to determine potential problem areas in cellular service areas where band-A and band-B utilize EIA/TIA/IS-136-A and TIA/EIA/IS-9S-A CDMA technologies, respectively. System modifications that would help maintain the appropriate BER are also discussed.