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Mitigating interference
between LTE and 2G/3G networks Radio interference is one of the key factors affecting the quality of wireless networks. As wireless communications technologies have developed, networks of different frequency bands and standards operating in the same region have become commonplace. Interference between wireless networks has grown more troublesome. When operators start to deploy LTE networks, how can operators address the more complex type of interference between wireless networks? By Sun Jingfei
Challenges of wireless interference
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ll of the signals emitted by a w i re l e s s n e t w o rk a re interference to another and this is particularly true when the networks use adjacent frequencies. This kind of interference will affect the quality of networks, impair user experience and
even cause network failures in extreme circumstances. Currently, many operators are trying to cope with interference between their own 2G and 3G networks. The frequency bands used by some 2G networks often overlap those of other 2G networks and 3G networks. For example, the CDMA downlink frequency band partially overlaps the uplink frequency band for Extended GSM
(EGSM), causing marked interference. Many operators use 900MHz for UMTS, but the frequency bands of GSM900 and UMTS900 are close to each other, as those of GSM1900 and UMTS1900, and interference is inevitable in such situation. In the near future, most operators in the world will face a more complex situation with 2G, 3G and LTE networks coexisting. LT E h a s m a n y s p e c t r a l o p t i o n s . DEC 2009 . ISSUE 53
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Solution Mitigating interference between LTE and 2G/3G networks Frequency bands specified for LTE include 700MHz, 900MHz, 1800MHz, 2100MHz and 2600MHz. These frequency bands are very close to those of existing 2G and 3G networks and some even partially overlap those of 2G and 3G networks. As a result, radio interference inevitably exists between 2G, 3G and LTE networks. Along with the rapid growth of wireless communications, spectrums have become increasingly scarce. More and more spectrums will be refarmed, causing greater co-channel and adjacent channel interference between wireless networks. Interference between wireless networks has become a prime concern for operators and equipment suppliers. Many large companies are currently researching the issue and solutions are evolving.
Solutions for typical scenarios When a new LTE network is built, it is necessary to analyze the current use of frequency bands in 2G and 3G networks. The analysis is intended to give early warnings to possible interference scenarios and help formulate preventive measures. If there are interference risks, the most direct mitigation measure is to reserve a guard band between two wireless communication networks. This can minimize interference, but it may also waste frequency resources. The actual width of a guard band depends on the anti-interference capability of equipment and the amount of interference from existing networks. Filters and spatial isolation are needed in some extreme situations. For example, the CDMA850 frequency band already interferes with that of EGSM880, but LTE requires EGSM880. Co-sites
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and co-channels are also among the typical scenarios that cause interference.
Co-site interference Operators have invested heavily in 2G, 3G and LTE networks. Costs can be considerably reduced if it is possible to utilize the resources of existing 2G and 3G networks by sharing sites with the LTE network. In this scenario, the most pressing problem for operators is to combat co-site interference. Solutions have matured, like the co-site adjacent channel interference solution. Take UMTS and GSM for example. The bandwidth assigned to UMTS and GSM are standard-compliant and also adjacent. UMTS is given the 5MHz, while GSM is given the 200KHz band. In this case, no guard band is provided, but interference is controllable and network performance is not affected. In a co-site adjacent channel construction of UMTS900 and GSM900 networks, because the spectrum resources are limited, a solution with an operator bandwidth of 4.2MHz can be used by UMTS900, to ensure the capability and quality of GSM networks. The decreased bandwidth of a guard band can increase available frequency bands to GSM900 and the performance loss due to interference is acceptable. 60% of operators in Europe use 12.5MHz for the 900MHz frequency band, while 50% of operators in the Asia-Pacific region use 6–10MHz for the 900MHz frequency band. In this case, if a 3G network uses a frequency band for a 2G network, the 3G network can use a bandwidth smaller than the standard one, while ensuring the capacity and quality
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of the 2G network. Along with network evolution, the method for controlling cosite interference between GSM and UMTS can also be used in the co-site construction for GSM, UMTS and LTE networks to control interference between the three.
Co-channel interference Geographical isolation zones can be used to counter interference where different wireless networks use the same frequency bands. For example, after the frequency band of GSM900 is refarmed to UMTS900 in rural areas, the same spectrum is used by UMTS900 in rural areas and GSM900 in urban areas, causing a great deal of interference. It is possible to have an isolation zone using another frequency between the two areas. GSM900 networks in urban areas can then gradually evolve to UMTS900 or LTE in the future. To improve 3G coverage in r ural areas, Optus in Australia has refarmed part of the GSM900 frequency band to UMTS900. Compared with 2100MHz, the 900MHz frequency band has many inherent advantages, such as better indoor coverage, and double the coverage of 2100MHz. The 900MHz frequency band can notably reduce the number of base stations, facilitating network construction and development. Because UMTS900 networks deployed in rural areas will strongly interfere with
GSM900 networks in rural areas, isolation zones are set up between GSM900 and UMTS900 networks. With this approach to building UMTS900 networks, performance is guaranteed and interference controlled. After implementation, operators can not only streamline their operations but also gain an advantageous strategic position. Experience in 3G deployment can be used to deploy LTE networks beginning with hotspots. In the future, subscribers using GSM900 networks in urban areas will gradually decrease or turn to other networks, so the GSM900 frequency band can be retained for LTE. The existing antiinterference solutions are inherited and existing investment is protected.
Other interference There are still many other complex scenarios involving interference. Take the new UMTS2100 network at Vivo in Brazil for example. The network was found being affected by existing networks such as GSM1900, CDMA1900 and DECT1900. After analysis, the interference problem was resolved with five customized filters. When Vietnam’s HT Mobile swapped its CDMA800 network to EGSM, the uplink frequency of EGSM was adjacent to the downlink frequency of CDMA800 of other operators. To eliminate the interference between the two types of
networks, HT Mobile used filters in some places after analyzing frequency assignment. In the proposed UMTS850 (refarming of the AMPS spectrum) network construction, Thailand’s DTAC realized that the downlink frequency was adjacent to the uplink frequency of the existing GSM900 network. The interference between the two networks was then effectively eliminated with corresponding solutions. LTE network building will create more complex interference problems in the future. Huawei has conducted in-depth research and gained rich experience in eliminating interference caused by the coexistence of 2G, 3G and LTE networks, and the co-channel and adjacent-channel interference between wireless networks. Huawei provides optimal anti-interference solutions through spatial isolation, equipment isolation, guard bands and other approaches. Inter ference has long been a key concern for both operators and equipment suppliers. Fortunately, when 2G, 3G and LTE networks coexist in the future there are viable solutions to eliminate interference. To save money and maximize benefits during the construction of LTE networks, it is highly important for operators to tailor optimal solutions for all issues and potentialities from the very beginning. Editor: Wang Hongjun
[email protected]
If there are interference risks, the most direct mitigation measure is to reserve a guard band between two wireless communication networks. This can minimize interference, but it may also waste frequency resources. The actual width of a guard band depends on the anti-interference capability of equipment and the amount of interference from existing networks. DEC 2009 . ISSUE 53
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