Orthogonal Frequency-Division Multiple Access (OFDMA)

With Orthogonal Frequency-Division Multiple Access (OFDMA) muliple access is achieved by assigning subsets of sub-carriers to individual users. This allows simultaneous low data rate transmission from several users.OFDMA simultaneously supports multiple users by assigning them specific subchannels for intervals of time.So in short,

  • Each terminal occupies a subset of sub-carriers
  • Subset is called an OFDMA traffic channel
  • Each traffic channel is assigned exclusively to one user at any time
  • The IEEE 802.16e/ WiMax use OFDMA as Multiple access technique
  • OFDMA allows different users to transmit over different portions of the broadband spectrum (traffic channel)
  • Simpler Reciever – Only FFT processor is required
  • Bit Error Rate performance is better only in Fading environment

Orthogonal Frequency Division Multiplexing – OFDM

OFDM – Orthogonal Frequency Division Multiplex, the modulation concept being used for many wireless and radio communications radio applications from DAB, DVB, Wi-Fi and Mobile Video.OFDM is a special case of Frequency Division Multiplexing.Orthogonal frequency division multiplexing (OFDM) has been shown to be an effective technique to combat multipath fading in wireless communications.

Concept – Divide and Rule..

  • An OFDM signal consists of a number of closely spaced modulated carriers.
  • When modulation of any form – voice, data, etc. is applied to a carrier, then sidebands spread out either side.
  • It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data.As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them. This is not the case with OFDM.
  • Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another.

Understand.. How it works?

The main concept is orthogonality. 

Area Under a sine wave is always 0
Area Under a sine wave is always 0

You would recall that the functions, sin(nx), cos(nx) : n = 1, 2, 3, … are orthogonal with respect to Riemann integration on the intervals [0, 2π], [-π, π], or any other closed interval of length 2π. This fact is a central one in Fourier series. Corresponding to it the area under a sine(or a cosine) wave will be always 0.

So by using the same concept we can transmit, using a single transmitter, a set of frequency multiplexed signals with the exact minimum frequency spacing, to make them orthogonal so that they do not interfere with each other.In OFDM, the subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other, meaning that crosstalk between the subchannels is eliminated and intercarrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver (No Filters and No Guard bands.. well theoretically ). Unlike in conventional FDM, a separate filter for each subchannel is not required.

Data in OFDM

  • The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload. This reduces the data rate taken by each carrier.
  • The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.