RADIO FREQUENCY FILTERS: Filtering the compromise

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In the quest to utilise existing infrastructure while deploying new cellular networks, Indian operators are choosing to co-locate base stations and their associated antennas. This is driven primarily by the need to keep capital expenditure down, apart from the need to complete network rollouts quickly. The decision is also impacted by the scarcity of premium base station locations and the growing demand for minimal environmental impact solutions.

In its most fundamental form, co-location involves the sharing of site space and structures for the location of base station active equipment and the RF distribution system. In the early days of digital cellular, the most common combinations of services were global system for mobile communications (GSM) 900 MHz and 1800 MHz, or code division multiple access (CDMA) 800 MHz and 1900 MHz (often known as personal communications service, PCS).

More recently, the 'cross pollination' of GSM into the Americas and CDMA into Asia and Eastern Europe has led to a more challenging scenario. As GSM and CDMA services operating in neighbouring frequency bands are co-located, significant-and initially unforeseen-interference issues arise. Many GSM 900 MHz operators have found their hitherto premium service suddenly and dramatically degraded in quality by the introduction of a co-located CDMA 800 MHz system. Similarly, new GSM 900 MHz services overlaid at a CDMA 800 MHz base station site can suffer serious quality problems.

Transmit, receive
The close proximity of the CDMA downlink and GSM uplink frequency bands (see Figure 1) leads to interference in the GSM receiver, thereby decreasing its sensitivity and resulting in dropped calls. Two basic sources of CDMA-generated interference exist: CDMA spurious emissions and high-power interfering CDMA signals.

Figure 1 : The co-location challenge

Spurious emissions are caused by unwanted transmitter effects; CDMA transmitters can generate both discrete (harmonics, intermodulation products) and wideband signals that fall outside the transmit band. If these fall within the GSM receive band, they manifest as wideband noise and raise the noise floor of the receiver. Considering a 30- to 40-dB isolation between the antenna systems, a worst case scenario would yield a noise signal of around -50 dBm. The impact of this would be a dramatic and unacceptable degradation of the sensitivity of the GSM receiver. The situation is rarely this severe, but increases of 50 dB in the noise floor can be typical.

The other main source of interference is the CDMA transmitted signal itself. If the strength of the signal into the GSM receiver is higher than a certain level (known as the 'blocking' level), it generates intermodulation products that can lead to interference, again degrading receiver sensitivity.

Filtering out interference
Clearly, this significant degradation of GSM services when co-located with CDMA services is unacceptable for operators and consumers alike. A practical solution lies in the judicious application of specially designed filters-in both the CDMA downlink and GSM uplink-to minimise the unwanted CDMA signals being received by the GSM base station.

Practically, the top end of the CDMA 800 MHz transmit band is 894 MHz; the GSM 900 MHz receive band starts as low as 890 MHz (or even 880 MHz in enhanced GSM (E-GSM)), although the exact spectrum used varies from country to country. Similarly, the exact scenario for a particular co-located site will depend on the channels allocated to each base station.

The installation of a bandpass filter in the CDMA downlink to filter out-of-band spurious emissions-particularly those that fall within the GSM receive bands-reduces by up to 75 dB the magnitude of CDMA wideband noise received by the co-located base station. A filter in this location is critical in many applications.

Perhaps even more critical is the installation of a bandpass filter in the GSM uplink. This filter mitigates the real power of the CDMA interferer falling just outside the GSM receive band. Depending on the transmitting power of the CDMA base station, these uplink filters need to achieve a minimum selectivity of up to 50 dB.

Tight tolerances
The bandpass filters used for co-location applications generally need to exhibit sharp attenuation of out-of-band frequencies, owing to the tight tolerances between frequency bandwidths. It follows that the complexity of the filter (measured by the number of poles and cross-couplings) increases as the guardband decreases.

Figure 2: Filter characteristic of a premium performance bandpass filter

Figure 2 shows the filter characteristic of a premium performance bandpass filter, which has a passband of 898.5 to 960 MHz and provides 50-dB attenuation at frequencies less than 894 MHz. The three cross-couplings within this 9-pole filter generate the sharp notch below 894 MHz, which corresponds to the 4.5-MHz guardband currently available in Brazil for co-locating CDMA 800 MHz with GSM 900 MHz. The scenario in India promises to be even more challenging, with guardbands of just 1 MHz available at some locations.

In cases where the guardband is wider, the roll-off can be less severe and the filter consequently less complex (smaller number of poles). Selectivity of more than 50 dB would be difficult to achieve for the narrow 4.5-MHz guardband; but where the guardband is greater than 10 MHz, greater rejection of CDMA frequencies can be achieved.

Figure 3 : GSM/CDMA co-location filtering architecture

In other words, to a large extent, co-location filters need to be customised for a specific application-taking into account the specific guardband and passbands involved.

The exact on-site location of the installed filters also needs to be considered, and may introduce its own challenges. In most cases to-date, the interference issues associated with co-location have been revealed only upon completion of the base stations, where real-estate is at a premium. Space is usually not allocated for co-location filters, leading to their frequent installation outside the base stations-for example, on the mast itself (Figure 3).

If the GSM filter is installed on the antenna side of the duplexer, the passband needs to accommodate the entire GSM downlink in addition to the uplink frequencies. The same is true for the CDMA downlink filter.

As global cellular penetration continues to escalate and data services rise to prominence, the number of co-located base stations are bound to increase-whether combinations of 2G/2G or 2G/3G. Now that the challenges associated with co-location interference are better understood-and the solutions for combating it are available-network operators and OEMs can consider the issues during the planning and building stage. This may not eliminate the problem all together, but it will ensure that disruption to existing services is minimized when new networks come to town.   

Sukant Chakravarty Country Manager and André Doll, Vice President, RFS

Case Study: China Unicom

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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