2.5.4 Orthogonal Frequency-Division Multiple Access (OFDMA)

2.5.4 Orthogonal Frequency-Division Multiple Access (OFDMA):

We can say that’s the main difference between Long Term Evolution (LTE) and other systems.

            And its very similar to Orthogonal frequency Division Multiplexing(OFDM) in function ,with the main difference being that the instead of being allocated all of the available subcarriers ,the base station allocates a subset of carriers to each user in order to accommodate multiple transmissions simultaneously.

            It is Flexibility of deployment across various frequency bands with little needed modification to the air interface, Averaging interferences from neighboring cells, by using different basic carrier permutations between users in different cells. Interferences within the cell are averaged by using allocation with cyclic permutations.

            Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage, Offers Frequency diversity by spreading the carriers all over the used spectrum. Allows per channel or per subchannel  power control.

            A Problem with Ofdma as a compared to OFDM is its high sensitivity to frequency offset.

            In OFDM since all the frequencies of the subcarriers were generated by one transmitter, maintaining orthogonality of subcarriers is relatively easy.

            In OFDMA, since many users transmit simultaneously, each with their own estimates of the subcarriers frequencies, a frequency offset is inevitable and multiple access interference occurs as users’ power leaks into subcarriers bands.

            The key difference between both transmission methods is that OFDM allows only one user on the channel at any given time whereas OFDMA allows multiple access on the same channel.

             To enable multiple access subchannelization is used. A subchannel is a group of subcarriers that can be allocated dynamically to different subscribers. Depending on the channel conditions and data requirements modulation and coding is set individually for each subscriber.

            Transmit power can be adapted separately as well, which optimizes the use of network resources. Because of subchannelization OFDMA signals are more complex than OFDM signals and offer better performance and scalability.