1 Orthogonal Frequency- Division Multiplexing (OFDM) Used in DSL, WLAN, DAB, WIMAX, 4G

2 Definition Orthogonal frequency division multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands. A subcarrier carrying a portion of the user information is transmitted in each band. Each subcarrier is orthogonal (independent of each other) with every other subcarrier.

3 Overview OFDM is a modulation (multitone modulation) and multiple-access technique that has been explored for more than 20 years. Only recently has it been finding its way into commercial communications systems. Used for a variant of digital subscriber line (DSL) European digital broadcast television standard digital radio in North America fixed wireless systems and wireless local-area network (LAN) products

4 Characteristics and Evolution of Data Communications for a Mobile Environment Data traffic differs from voice in at least three important ways data traffic is much more unpredictable (as bursty) data has very different requirements in terms of reliability data traffic encompasses a much different and wider range of services than voice

5 Overview of the Wireless Environment The wireless channel is distinct and much more unpredictable than the wire-line channel because of factors such as multipath and shadow fading, Doppler spread, and time dispersion or delay spread.

6 Multipath a transmitted signal is reflected by objects in the environment between the base station and a user The reflected signals arrive at the receiver with random phase offsets The result is random signal fades

7 Time dispersion distortion to the signal and is manifested by the spreading in time of the modulation symbols Occurs when the coherence bandwidth of the channel is smaller than the modulation bandwidth leads to inter-symbol interference (ISI) It also leads to fading

8 Doppler spread the random changes in the channel introduced as a result of a user's mobility and the relative motion of objects in the channel. shifting, or spreading, the frequency components of a signal. fast fading: When the rate of change of the channel is higher than the modulated symbol rate, Slow fading

9 Fading models Rayleigh fading: no line of sight (LOS) component present in the received signal Ricean distribution: there is a LOS component present More frequently no direct LOS path to a mobile a large dynamic range of signals (as much as 70 dB

10 Overview of Techniques used in Mobile Wireless Systems equalization, channel or error-correction coding, spread spectrum, interleaving, and diversity. Diversity has long been used to help mitigate the multipath-induced fading that results from users' mobility. spatial diversity: two or more receive antennae at a base station that are separated by some distance

11 frequency diversity: Spread spectrum (signal is spread over a much larger bandwidth than is needed for transmission) frequency hopping (FH) code division multiple access (CDMA)

12 OFDM for Mobile Communications It is a technique that divides the spectrum into a number of equally spaced tones and carries a portion of a user's information on each tone. each tone is orthogonal with every other tone OFDM allows the spectrum of each tone to overlap the overall amount of spectrum required is reduced

13 OFDM Tones

14 OFDM is a modulation technique in that it enables user data to be modulated onto the tones. can greatly reduce, or even eliminate, ISI OFDM can also be considered a multiple- access technique, the system called OFDMA OFDM can be combined with frequency hopping to create a spread spectrum system to minimise losses due to frequency selective fading

15 Data stream at R bps Total bandwidth: Nf b Central frequency:f 0 N substreams, R/N bps Subcarriers spaced f b Base frequency, f b Other Subcarriers, 2f b, 3f b, 4f b, … Nf b

16 Distribute date over multiple carriers at precise frequencies. The precise relationship among the subcarriers is referred to as orthogonality The peaks of the power spectral density of each subcarrier occurs at a point at which the power of others is zero. The subcarriers can be packed tightly together because there is minimal interference between adjacent subcarriers. The set of OFDM subcarriers is further modulated to a higher frequency band.

17 Advantages of OFDM frequency selective fading only affects some subchannels and not the whole signal. If the data stream is protected by a forward error- correcting code, this type of fading is easily handled. More importantly, OFDM overcome intersymbol interference (ISI) in a multipath environment.

18 OFDM256 OFDM256 air interface 256 subcarriers, 192 used for user data, 56 for guard bands 8 used for pilot tones Cyclic prefix can be from depending on environment (channel delay spread) there is also OFDM2048 and scalableOFDMA