Chinese operator China Unicom commenced deployment of its CDMA 800 MHz network in 2001, with 80 per cent of its new CDMA sites co-located with its existing GSM 900 MHz sites. At one particular China Unicom CDMA/GSM co-located site in the coastal province of Shandong, the CDMA antennas were initially installed 10 metres below the GSM arrays on the same 50-metre tower; however, in time the operator raised the height of the CDMA antennas to the same level, in order to improve coverage.

With both arrays at the same elevation, the effect of CDMA interference on the existing GSM service was instantaneous, with the dropped call rate increasing 25-fold from just 0.17 per cent to 4.3 per cent. Similar issues were experienced at many of China Unicom's sites around the nation. A customized solution was developed which consisted of a CDMA downlink filter providing 75-dB selectivity for frequencies within the GSM receive band; plus a GSM uplink filter providing more then 80-dB rejection of CDMA transmit frequencies.

After installation of the filters, the dropped call rate decreased to below one per cent and the call completion rate increased to above its original rate. This illustrates the effectiveness of specially designed co-location filters, which allow operators to make optimum use of existing infrastructure.

A more common challenge confronting China Unicom during its CDMA 800 MHz roll-out, is that resulting from CDMA/GSM co-location on congested rooftop sites. At such congested sites, it is physically impossible to achieve both horizontal and vertical antenna separation needed to realise a minimum acceptable isolation between the two services. CDMA downlink and GSM uplink filters have been regularly deployed to remedy such situations across China.

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