OFDM
OFDM(Orthogonal frequency-division multiplexing), also sometimes called discrete multitone modulation (DMT), is a complex modulation technique for transmission based upon the idea of frequency-division multiplexing (FDM) where each frequency channel is modulated with a simpler modulation. In OFDM the frequencies and modulation of FDM are arranged to be orthogonal with each other which almost eliminates the interference between channels. Although the principles and some of the benefits have been known for 40 years, it is made popular today by the lower cost and availability of digital signal processing components.
The main idea behind OFDM is that since low-rate modulations (i.e modulations with relatively long symbols compared to the channel time characteristics) are less sensitive to multipath, it should be better to send a number of low rate streams in parallel than sending one high rate waveform. This is exactly what OFDM is doing. It divides the frequency spectrum in subbands small enough so that the channel effects are constant (flat) over a given subband. Then a "classical" IQ modulation (BPSK, QPSK, M-QAM, etc) is sent over the subband. If designed correctly, all the fast changing effects of the channel (multipath) disappear as they are now occurring during the transmission of a single symbol and are thus treated as flat fading at the received.
Classical signal processing such as channel coding, power allocation, adaptive modulation and coding can be applied for a given subband or over the subbands. Multiuser allocation is also possible, either using time, coding or frequency separation of the users.
Comparison to FDM
In FDM, multiple signals are sent out at the same time, but on different frequencies. Most people are familiar with FDM from radio and television: normally, each station broadcasts on a particular frequency band (range of frequencies) or channel.
- OFDM takes this concept further: In OFDM, a single transmitter transmits on many (typically dozens to thousands) different orthogonal frequencies (i.e. frequencies that are independent with respect to the relative phase relationship between the frequencies). Also, because the frequencies are so closely spaced, each one only has room for a Narrowband signal.
- This modulation technique coupled with the use of advanced modulation techniques on each component, results in a signal with high resistance to interference.
Coupling with "channel coding"
OFDM is almost always used in conjunction with channel coding—an error correction technique—to create coded orthogonal FDM or COFDM. It is a complex technology to implement, but it is now widely used in digital telecommunications systems to make it easier to encode and decode such signals. The system has been used in broadcasting as well as certain types of computer networking technology. This is particularly due to the fact that such signals show good resistance to multipath fading, best known as the source of "ghosting" on analog television broadcasts.
According to Stott, 1997 [1], "The 'COFDM magic' is achieved by the use of channel-state information (CSI). In the presence of CW interferers and/or a selective channel, some OFDM carriers will be worse affected than others." The channel coding thus allows the receiver to integrate information about the physical S/N ratios of the subchannels into the error correction of its Viterbi decoder, yielding significantly better performance than uncoded OFDM can attain with similar channel characteristics.
Characteristics
An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, with baseband data on each sub-carrier being independently modulated commonly using some type of quadrature amplitude modulation (QAM) or phase-shift keying (PSK). This composite baseband signal is typically used to modulate a main RF carrier.
Benefits
The benefits of using OFDM are many, including high spectrum efficiency, resistance against multipath interference (particularly in wireless communications), and ease of filtering out noise (if a particular range of frequencies suffers from interference, the carriers within that range can be disabled or made to run slower). Also, the upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose. Some forms of Rate-adaptive DSL use this feature in real time, so that bandwidth is allocated to whichever stream needs it most.
An extremely important benefit from using multiple sub-carriers is that because each carrier operates at a relatively low bitrate, the duration of each symbol is relatively long. If one sends, say, a million bits per second over a single baseband channel, then the duration of each bit must be under a microsecond. This imposes severe constraints on sychronization and removal of multipath interference. If the same million bits per second are spread among N subcarriers, the duration of each bit can be longer by a factor of N, and the constraints of timing and multipath sensitivity are greatly relaxed. For moving vehicles, the doppler effect on signal timing is another constraint that causes difficulties for some other modulation schemes.
OFDM modulation and demodulation are typically (as of 2001) implemented using digital filter banks generally using the Fast Fourier Transform (FFT).
Although highly complex, COFDM has high performance under even very challenging channel conditions.
By combining the OFDM technique with error-correcting codes, adaptive equalization and reconfigurable modulation, COFDM has the following properties:
- resistance against link dispersion
- resistance against slowly changing phase distortion and fading
- resistance against multipath using guard interval and cyclic prefix
- resistance against frequency response nulls and constant frequency interference
- resistance against burst noise
COFDM also generally has a nearly 'white' spectrum, giving it benign electromagnetic interference properties with respect to other signals.
Some COFDM systems use some of the sub-carriers to carry pilot signals, which are used for frequency synchronization. (Loss of synchronization causes errors in the decoded data).
In wide area broadcasting, receivers can benefit from receiving signals from several spatially dispersed transmitters simultaneously, since transmitters will only destructively interfere with each other on a limited number of subcarriers, whereas in general they will actually reinforce coverage over a wide area. This is very beneficial in many countries, as it permits the operation of national single frequency networks, and avoids the replication of program content on different carrier frequencies which is necessary with FM or other forms of radio broadcasting. Also, because effectively the bit rate is slowed down on each sub-carrier, the effects of "ghosting" are much reduced. Such single frequency networks utilise the available spectrum more effectively than existing analogue radio networks.
Disadvantages of OFDM
However, OFDM suffers from time-variations in the channel, or presence of a carrier frequency offset. This is due to the fact that the OFDM subcarriers are spaced closely in frequency. Imperfect frequency synchronization causes a loss in subcarrier orthogonality which severely degrades performance.
Because the signal is the sum of a large number of subcarriers, it tends to have a high peak-to-average power ratio (PAPR). Also, it is necessary to minimise intermodulation between the subcarriers, which would effectively raise the noise floor both in-channel and out of channel. For this reason circuitry must be very linear. This is demanding, especially in relation to high power RF circuitry, which also needs to be efficient in order to minimise power consumption.
