Design of CR-OFDM in 900 MHz Band

Abstract

Research shows that there are few GSM channels available for the assignment of control channels and remaining channels dynamically are available for service as secondary channels users of CR network. For the design of CR-OFDM in 900 MHz GSM band, it is necessary that the channel shall be considered as frequency selective and time variant within the scope of practical limits. The OFDM parameters need to be calculated within the availability of vacant GSM bands, that is, 200 kHz which is very narrow in comparison with the existing bands deploying OFDM technology that is above and in multiples of 1.25 MHz. The results have been obtained and compared using three methods: (a) Measurement of OFDM efficiency at 2600 MHz band and GSM spectral efficiency at 900 MHz band both at Chakan, Pune to ascertain the geographical location. This will help to map 2600 MHz data to 900 MHz band for OFDM. The Atoll planning tool is used with the same geographical location parameters at 2600 MHz OFDM technology and is validated with the obtained results. The GSM 900 MHz RF parameters were used for simulation of Atoll parameters using OFDM technology, (b) Parameters pertaining to frequency selective and time-variant channel, viz., coherence bandwidth and Doppler delay were calculated based on the practical data available for Indian Geographical locations and matched with the ITU Table for same parameters. The OFDM parameters for GSM 900 MHz band with 200 kHz bandwidth was calculated subject to the limitation imposed by OFDM technology. (c) The obtained result was verified using Atoll planning tool and finally, the spectral efficiency was calculated. Although OFDM subcarriers are very low in size to combat coherence bandwidth, not much research work has been done to use OFDM technology for narrow bandwidth. The present work shows the design of cognitive OFDM system for frequency selective and time-variant channel in 900 MHz GSM band. The suggestions are based on the calculation done and the design has been verified using data collected for live network and results are summarized on the basis of adaptive CP length, finally, the result has been calculated using three methods, i.e., from live network, from mathematical calculations, and from Atoll tool has been summarized.