We saw previously that the channel capacity of a WCDMA system is severely limited by the Multiple Access Interference (MAI). Now let us consider the case where the 5MHz channel is divided equally among 20 users such that each user has a bandwidth of 250kHz. Keeping the transmit power for the narrow band signal the same as the wideband signal the signal to noise ratio would improve tremendously (since there is no MAI and the noise power is reduced by a factor of 20). Thus each narrowband channel would have a capacity of 2.99Mbps giving a combined capacity of 59.83Mbps for the 5MHz channel. The main drawback of an FDMA system is that the capacity of each narrowband channel is fixed at 2.99Mbps. However the capacity of a WCDMA user is dependent upon the number of users varying from 38Mbps for a single user to 5Mbps for two users (10Mbps combined) to 370kbps for 20 users (7.4Mbps combined). The higher capacity of FDMA when combined with dynamic allocation of channels builds the case for OFDMA based techniques.
Note: For the narrowband case the signal power is -80dBm and noise power is -116dBm resulting in an SNR of 36dB.
Author: Yasir
More than 20 years of experience in various organizations in Pakistan, the USA, and Europe. Worked with the Mobile and Portable Radio Group (MPRG) of Virginia Tech and Qualcomm USA and was one of the first researchers to propose Space Time Block Codes for eight transmit antennas. Have publsihed a book “Recipes for Communication and Signal Processing” through Springer Nature.
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In a Heterogeneous network (LTE) OFDMA downlink, can I use the whole bandwidth (B) for each macro and femtocell and divide B according to their number of users? If there is 20 users under a macrocell and i want to divide the bandwidth by 20 and for a femtocell that contains 5 users, i want to divide the bandwidth by five? and after a while when there will be a change in the user number, i want to divide it by the new number. Is it possible?? what if I want to assign a bigger portion of the B to a special user (like femtocell user). Is it possible in OFDMA?