Research On Capacity And Coverage For Broadband Wireless Access -VSF-OFCDM

This paper proposed broadband wireless access based on VSF-OFCDM(variable spreading factor-orthogonal frequency and code division multiplexing) in NTT DoCoMo. Configuration of air interface and key technologies have been introduced. We proposed theoretical analysis of the capacity and the coverage, and developed expression of the capacity and the coverage for VSF-OFCDM in both downlink and uplink.Through link budget we developed maximum output power of base station (BS). Power Spectral Density (PSD) of BS in VSF-OFCDM was much less than that of WCDMA and thus saved valuable power resources (72% compared to WCDMA). A novel method was proposed for average link loss from the view of loading and power in VSF-OFCDM downlink. Through extremum of a multivariate function (proposed by Lagrange multipliers), we got the maximum of capacity and coverage both in downlink and uplink from the viewpoint of multi-cell and isolated-cell environments, respectively.The development of capacity and introduction of multivariate function is a main innovation for VSF-OFCDM and we got the important conclusion: when each subcarrier gets the same output power from BS maximum transmit power, namely, BS maximum transmit power is averagely distributed for all subcarriers, the maximum of capacity is achieved. The same conclusion will be applied for all multi-carrier CDMA systems.Simulation results show that in downlink, throughput performance of voice can achieve 38.9 Mbit/s and the coverage radius can reach 2.88 km while the throughput performance of data can achieve 56.8 Mbit/s and the coverage radius can reach 3.46 km in the multi-cell envirerement. Simulation results also show that in uplink, throughput performance of voice can achieve 7.11 Mbit/s and the coverage radius can reach 2.80 km while the throughput performance of data can achieve 9.49 Mbit/s and the coverage radius can reach 2.09 km in the multi-cell envirenmet. In isolated-cell environment, throughput performance of the whole system can reach higher capacity because of no inter-cell interference. From comparison among GSM, WCDMA and VSF-OFCDM, we get the conclusion that the spectrum efficiency of VSF-OFCDM is 3.5 times that of WCDMA, and 5.4 times more than the spectrum efficiency of GSM. Finally, we analyse the effecting factors for capacity, load and coverage, and open out the essence among capacity, load and coverage. Capacity and coverage restrict one another: when the transmit power is fixed, decreasing in capacity means extension in coverage, and vice versa.