Performance improvement in a multi-band GSM network by appropriate frequency allocation in BCCH and TCH bands

Abstract

To meet the increasing traffic demand and requirements for an improved Radio Quality in a GSM network, the various new technologies have been introduced. Multiband Cell or MBC is such a technology where two co-located 900-MHz and 1800-MHz cells share the same BCCH. Normally, each cell requires one BCCH for signaling. When 900-MHz and 1800-MHz cells were used simultaneously for the first time, those cells had different BCCHs with different signaling channels. To meet more traffic demand and to improve the radio network quality, MBC was implemented where both the cells shared the same BCCH. In such MBC cells, BCCH was used from the 900-MHz frequency band and TCHs were used from both the 900-MHz and 1800-MHz frequency bands. However, to ensure acceptable network quality in highly dense areas, the design requires a very low site to site distance (BTS to BTS distance of 250-300 meters) with an average antenna height of 20m. As a result, in the 900-MHz band, an operator has very limited spectrum of only 12 channels for BCCH use. Consequently, the operators have to use 4/12 re-use scheme and thus the co-channel re-use distance becomes very low creating a higher co-channel interference even with the application of aggressive down-tilt. Also, this scheme has other limitations inherent in it. For example, additional sites cannot be installed for unacceptable co-channel interference due to overshooting and further coverage area cannot be improved by reducing the antenna height, which is limited by the average height of buildings. To overcome these problems, it has been proposed in the current research to use BCCH from the 1800-MHz band and TCH from both the 900-MHz and 1800-MHz bands. As path loss in the 1800-MHz is more compared to that of the 900-MHz band, interference due to overshooting is reduced in the proposed scheme. On the other hand, available spectrum in the 1800-MHz band is comparatively more, therefore, use of more relaxed frequency reuse schemes have been made possible. Thus the overall radio Quality, like the Carrier to Interferer Ratio (C/I), Bit Error Rate (BER), Received Signal Quality (Rx Qual) have been improved.