OFDM (Orthogonal Frequency Division Multiplexing) uses multiple carriers that are mutually orthogonal; OFDM includes multiplexing in the phase domain as well as the frequency domain. OFDM uses multiple carriers that are mutually orthogonal; OFDM includes multiplexing in the phase domain as well as the frequency domain. Likewise, OFDM is a means to divide up a single user’s message for conveying it over a set of orthogonal frequencies. Its multiple access methods (OFDMA) are used in latest wireless technologies including LTE and LTE-A (fourth generation of mobile wireless networks), WiFi (IEEE 802.11), WiMAX (IEEE 802.16) as well as in in digital subscriber lines (DSL).
Since the subcarriers in OFDM do not interfere with each other due to their orthogonality, the spectrum of each tone is allowed to be overlapped, thus the amount of required spectrum for OFDM compared to conventional FDM. OFDM used in conjunction with PSK or QAM modulation techniques overcomes the limitations of FDM; particularly, OFDM does not require the use of expensive bandpass filters required in conventional FDM systems. Nonetheless, OFDM requires strict frequency synchronization.
Given that the total channel bandwidth W is divided up into K slots, each one having a subcarrier frequency of Fk with spacing given that the total channel bandwidth W is divided up into K slots, each one having a subcarrier frequency of Fk with spacing ∆F=F(k+1)+Fk (k should be read as a subscript here)
then Fk=k(W/K). Likewise, assume symbol rate to be r=1 / T = ∆F, where T is the symbol duration such that the modulated subcarriers will be mutually orthogonal, or
∫cos(2π.Fk.t).cos(2π.Fj.t).dt = 0 ∇j,k; j≠k (integrated from 0 to T, again, j and k should be read as subscripts).
Thus, with orthogonal frequency division multiplexing (OFDM) we have a method of sending K symbols concurrently without interference. In other words, we take a given message that consists of a set of frames where each frame consists of K symbols, so that, instead of sending the entire frame over the channel at a bandwidth and rate of Rs = 1 = Ts = K/T = W, each symbol is sent over a separate subcarrier frequency Fk and thus send K symbols in parallel over the channel at the slower rate of R = Rs/K Hz, so then, symbol duration becomes T=KTs (s is a subscript here as k and j before).
An important consequence of this lower data rate is the corresponding reduction in ISI (InterSymbol Interference), and multipath effects like it happens in 3GPP’s UMTS WCDMA (Wideband Code Division Multiple Access), where multipath interference is a big issue thus forcing implementation of complex use of multiple antenna configuration called “rake receivers” where the output of each of the incoming signal multipliers is then fed to a diversity combiner that consists of a set of adjustable delay and gain elements so that the outputs add constructively, since the signal from each finger will have identical delays. The signal from each path is scaled so as to increase the level of signals from the fingers with high signal-to-noise ratios and reduce the level of those with low signal-to-noise ratios. OFDMA in conjunction with a matrix an antennas named MIMO (Multiple Input Multiple Output) is much more efficient than WCDMA and Rake Receivers, thus used in LTE in favour of CDMA methods.