OFDMA is often compared to the combination of OFDM with
statistical time-division multiplexing. The advantages and disadvantages summarized below are further discussed in the
Characteristics and principles of operation section. See also the list of
OFDM key features.
Advantages • Allows simultaneous low-data-rate transmission from several users. • Pulsed carrier can be avoided. • Lower maximal transmission power for low-data-rate users • Shorter delay and constant delay • Contention-based multiple access (collision avoidance) is simplified. • Further improves OFDM robustness to fading and interference • Combat narrow-band interference. • Flexibility of deployment across various frequency bands with little needed modification to the air interface • Averaging interferences from neighboring cells, 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.
Disadvantages • Higher sensitivity to frequency offsets and phase noise • Asynchronous data communication services such as web access are characterised by short communication bursts at high data rate. Few users in a
base station cell are transferring data simultaneously at low constant data rate. • The complex OFDM electronics, including the
FFT algorithm and
forward error correction, are constantly active, thus consuming power, independent of the data rate, while OFDM combined with data packet scheduling may allow FFT algorithm to hibernate at times. • The OFDM diversity gain and resistance to
frequency-selective fading may partly be lost if very few sub-carriers are assigned to each user, and if the same carrier is used in every OFDM symbol. Adaptive sub-carrier assignment based on fast feedback information about the channel, or sub-carrier frequency hopping, is therefore desirable. • Dealing with co-channel interference from nearby cells is more complex in OFDM than in
CDMA. It would require
dynamic channel allocation with advanced coordination among adjacent base stations. • The fast channel feedback information and adaptive sub-carrier assignment is more complex than CDMA fast power control. == Characteristics and principles of operation ==