Co-location of different wireless-communications base stations often makes economic sense, but poses a variety of technical challenges. The approach allows carriers to still use existing infrastructure while deploying new wireless networks. The practice helps reduce capital expenditures while supporting rapid network rollouts. A decision to co-locate base stations is also impacted by the scarcity of premium base-station locations and the growing demand for aesthetically pleasing and low visual impact solutions.

In its most fundamental form, collocation involves the sharing of site space and structures for the location of base station active equipment and the RF distribution system. Across the US, carriers have long co-located compatible services in the cellular and personal (PCS) bands. In such cases, the transmit and receive frequencies—both within each band and between bands—are separate enough to avoid serious interference problems.

More recently, escalating demand has led to a reallocation of spectrum and the introduction of new services. This includes the extension of the PCS band, and the reallocation of enhanced specialized mobile radio (ESMR) 800-MHz spectrum to commercial carriers using the integrated-digital-enhanced-network (iDEN) system. Moreover, the near future will bring new spectrum to carriers for universal-mobile-telecommunications-system (UMTS) services, plus 900-MHz spectrum for iDEN carriers.

This veritable cocktail of spectrum and services is leading to a more challenging interference scenario—and not just in the US. As services operating in neighboring frequency bands are co-located, significant—and initially unforeseen— interference issues can arise. This has already been observed extensively in countries such as China and Brazil, where a "cross pollination" of 900-MHz Global System for Mobile communications (GSM) and 800-MHz code-division-multiple-access (CDMA) services exists. In such cases, GSM services have suffered significant interference—and hence quality problems—as the direct result of co-location with CDMA. Similar issues are also being experienced by US carriers with other combinations of services—and such problems will only increase with increasing demands for wireless bandwidth and services.

The degradation in performance of one service co-located-with another is due to unwanted signals from the interfering transmitter arriving at the receiver of the affected service. Collocated 800-MHz CDMA and 900-MHz GSM base stations are an example of this. The close proximity of the 800-MHz CDMA downlink and 900-MHz GSM uplink frequency bands (Fig. 1) leads to interference in the GSM receiver, thereby decreasing its sensitivity and resulting in dropped calls.

Two main sources of co-location interference exist: transmitter spurious emissions and high-power interfering signals. Spurious emissions are caused by unwanted transmitter effects: both discrete (harmonics, intermodulation products) and wideband signals that fall outside the transmit band. If these fall within the receive band of a co-located service, they manifest as wideband noise and raise the noise floor of the receiver.

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

The scenario of Fig. 1 for CDMA/GSM co-location is similar to that which can arise in the US when commercial cell based mobile radio services in the 800-MHz ESMR/public safety band (such as iDEN carriers) are co-located with cellular services. In one possible scenario, the low end of the transmit band of the 800-MHz commercial mobile radio carriers (currently spread over 851 to 866 MHz) is in close proximity to the upper end of the cellular receive band (824 to 849 MHz). In a co-location situation, this can have detrimental impact on cellular services, particularly those in the cellular B-band (846.5 to 849 MHz).

This problem will be alleviated by the current rebanding of 800-MHz spectrum, when a much greater guard band will exist between the ESMR commercial mobile radio transmit band (862 to 869 MHz) and the cellular B-band. However, the additional 900-MHz spectrum allocated to the same commercial mobile radio carriers includes a receive band of 896 to 901 MHz, which could in turn be affected by collocated cellular services transmitting in the band, 869 to 894 MHz (notably the cellular B-band, which transmits at 891.5 to 894 MHz).

It is also predicted that US carriers could experience interference as the result of co-location between UMTS and PCS services, once UMTS spectrum is auctioned in the 1700-MHz and 2100-MHz bands. This is a rather special case generated by transmitter intermodulation products, rather than the proximity of interfering transmit and affected receive bands. Figure 2 illustrates the scenario, where the third order intermodulation products generated by the co-located UMTS and PCS transmitters fall within the UMTS receive band (1710 to 1755 MHz), causing interference and degradation of the UMTS service.

Clearly, this significant degradation of services when co-located is unacceptable for carriers and consumers alike. A practical solution lies in the judicious application of specially designed RF filters—in both the interfering downlink and affected uplink—to minimize the unwanted signals being received by the affected base station.

The installation of a bandpass filter in the interfering downlink to filter out of-band spurious emissions—particularly those that fall within the receive bands of co-located services—reduces by up to 75 dB the magnitude of wideband noise received by the affected 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 affected uplink. This filter mitigates the real power of the interferer falling just outside the receive band of the affected service. Depending on the transmitting power of the interfering base station, these uplink filters need to achieve a minimum selectivity of as much as 50 dB.

The exact scenario for a particular co-located site will depend on the channels allocated to each base station. Generally, the bandpass filters used for collocation applications 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 guard band decreases.

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Figure 3 shows the filter characteristic of a premium performance bandpass filter, which has a passband of 898.5 to 960.0 MHz and provides 50-dB rejection of unwanted signals at frequencies below 894 MHz. The three cross-couplings within this 10-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.

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

In other words, to a large extent, co-location filters need to be customized on a case-by-case basis—taking into account the specific guardband and passbands involved. Notably, the guardbands available between the cellular B-receive and 800-MHz commercial mobile radio transmit bands, plus the 900-MHz commercial mobile radio receive and cellular B-transmit bands, are of the order of just 2 MHz. Such cases require sophisticated filters.

The exact on-site location of the installed filters also needs to be considered, and may introduce its own challenges (Fig. 4). 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 often not allocated for co-location filters, leading to their frequent installation outside the base stations—for example, on the mast itself. Alternatively, if the filter is installed on the antenna side of the duplexer, the passband needs to accommodate the entire uplink and downlink frequencies, while at the same time rejecting those which interfere.

As global wireless 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. Moreover, it is almost impossible to predict what new technologies will take off and how future spectrum will be allocated. Yet now that the challenges associated with co-location interference are better understood— and the solutions for combating it are available—carriers 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.