Wideband – OFDM

Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission technique that has been successfully applied to wide variety of digital communication applications. Although the concept of OFDM has been around for a long time, it has been recently recognized as an excellent method for high speed bi-directional wireless data communication. This technology is used in broad cast systems such as Asymmetric Digital Subscriber Line (ADSL), European Telecommunications standard Institute (ETSI), radio (DAB: Digital Audio broadcasting) and TV (DVB: Digital Video broadcastingTerrestrial) as well as being proposed for wireless LAN standards.

OFDM efficiently squeezes multiple modulated carriers tightly together reducing the required bandwidth but keeping the modulated singles orthogonal so that they do not interface with each other. Any digital modulation technique can be used on separate carriers. The output of the modulated carriers is added together before transmission. At the receiver, the modulated carriers are separated before demodulation.
W- OFDM will allow the deployment of 4 G wireless networks that enable phones to transmit data at rates of up to megabits per second.OFDM segment are according to frequency. It is a technique that divides the spectrum in to a number of equally spaced tones and carriers a portion of a users information on each tone. A tone can be thought of frequency. Each tone is orthogonal to the other. OFDM is also called multi tone modulation.
OFDM can be considered as a multiple access technique, because an individual tone or groups tones can be assigned to different users. Multiple users share a given bandwidth in this manner, yielding the system called OFDMA. Each user can be assigned a predetermined number of tones when they have information to send, or alternatively a user can be assigned a variable number of tones based on the information that they have to send.W-OFDM can overcome problems of high peak-to-average signal amplitude and fading due to multipath affects. W-OFDM enables the implementation of low power multipath RF networks that minimize interference with adjacent networks.
OFDM FOR MOBILE COMMUNICATION
OFDM represents a different system design approach it can be though of as combination of modulation and multiple across schemes that segment a communications channel in such a way that many users share it. Where as TDMA segments are according to time and CDMA segments are according to spreading codes ,OFDM segments are according to frequency. It is a technique that divides the spectrum into a number of equally spaced tones and carries a portion of a users information on each tone. A tone can be thought of a frequency, much in the same way that each key on a pain represents unique frequency. OFDM has a special property that each tone is orthogonal with each other. There will be frequency guard bands b/w frequencies so that they do not interfere with each other. OFDM allows the spectrum of each tone to overlap and because they are orthogonal they donot interfere with each other. This reduces the required spectrum.
OFDM is a modulation technique that enables user data to be modulated onto the tones. The information is a modulated into a tone by adjusting the tones phase amplitude or both. In the most basic form, a tone may be present or disabled to indicate a one or zero bit of information; however, either phase shift keying (PSK) or quadrate amplitude modulation (QAM) is typically employed. An OFDM system takes a data stream and splits it into N parallel data streams each at a rate 1/N of  the original rate. Each stream then mapped to a tone at a unique freq and combined together using the inverse fast Fourier transform (IFFT) to yield the time-domain waveform to be transmitted.
For example, if a 100-tone system were used, a single data stream with a rate of 1 mega bit per second (Mpbs) would be converted into 100 streams of 10 kilobits per second (Kpbs). By creating parallel data streams, the bandwidth of modulation symbol is effectively decreased by a factor of 100. OFDM can also be considered a multiple access technique because an individual tone or groups of tone can be assigned to different users. Multiple users share a given band with in this manner, yielding the system called OFDMA. Each user can be assigned a predetermined number of tones when they have information to send, or alternatively, a user can be assigned a variable number of tones on the amount of information that they have to send.
OFDM can be combined with frequency hopping to create a spread spectrum system, realizing the benefits of frequency diversity and interference averaging property. In frequency hopping spread spectrum system, each users’ set of tones is changed after each time period. By switching frequencies after each symbol time, the losses due to frequency selective fading are minimized.OFDM therefore provides the best of the benefits of TDMA in that users are orthogonal to one another and CDMA-while avoiding the limitations of each including the need for TDMA frequency planning and multiple access interference in the case of CDMA.