RADIO FREQUENCY FILTERS: Filtering the compromise

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.