An OFDM signal set contains multiple subcarriers, each of which is a smaller percentage of the total frequency bandwidth than in a single carrier system. As a result, phase noise is a smaller percentage of the bandwidth in a single-carrier system. For this reason, phase noise degrades the performance of an OFDM system more than in a single carrier system. Phase noise effects in an OFDM system can be separated into two categories: phase noise maintained within one subcarrier spacing, and phase noise that extends across subcarrier spacings. Phase noise that extends across subcarrier spacings is considered extreme and results in demodulation errors. Phase noise within one subcarrier spacing essentially has a similar but scaled effect as for the single carrier system. The phase noise results in phase uncertainty in the constellation point producing an arc-shaped noise pattern in the constellation of each subcarrier.

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In order to help the OFDM system handle phase noise, pilot subcarriers are often used. These pilot subcarriers are generated by the IFFT and can be used to provide a stable phase reference for the receiver circuitry. Adding these pilots lowers the available data rate of the system because these subcarriers are no longer available to transmit data.

Non-linear circuits in the transmitter and receiver All transmitters and receivers in communications systems contain devices such as amplifiers and mixers that have non-linear transfer functions. These non-linearities create an additional performance limitation. The receiver performance is typically limited by distortion generated in the input amplifier or mixer in the presence of strong undesired signals. The transmitter performance is limited primarily by power amplifier linearity. An OFDM signal is made up of multiple simultaneous signals that, for a given average power, have a higher peak signal level. OFDM signals result in an increase in the peak-to-average ratio (PAR) of the signal. For multi-carrier systems, the PAR value is often expressed in terms of statistics because the probability that all subcarriers will simultaneously reach peak amplitude is low, even though the simultaneous peak amplitude value is large. These higher peak amplitude levels will create more severe distortion than a single carrier case even if the average power levels of each are the same. The higher distortion will increase the SNR needed to maintain adequate performance. Linearity requirements in both the receiver and transmitter must be adjusted or "backed off" to account for this increase in PAR value. The PAR value, and also the amount of linearity compensation, will depend on a number of parameters including the number of subcarriers and the level of SNR that must be maintained.

Modern applications

OFDM has been chosen for several current and future communications systems all over the world. It is well-suited for systems in which the channel characteristics make it difficult to maintain adequate communications link performance. Asynchronous digital subscriber line (ADSL) provides a method of delivering high speed data over the phone line. The system uses OFDM techniques, calling their variation discrete multi-tone (DMT). DMT includes features for allowing the removal of subcarriers and for adjusting modulation format (from 1 to 15 bits per symbol) on a per subcarrier basis to best suit the transmission channel characteristics. The system also permits "dynamic allocation" of these parameters.

European digital television is based on the DVB-T (digital video broadcast - terrestrial) standard that uses either 2048 (2K) or 8192 (8K) subcarriers within a standard 8 MHz TV channel. The system specifications and coding were specifically designed to allow multipoint repeater signaling that creates co-channel signals. Discussions are ongoing in the U.S. to look at a similar system and Japan is close to adopting a similar standard for their future digital TV broadcast system.

The next generation of radio broadcast may also make use of OFDM techniques. In the U.S., the system under consideration will initially "co-exist" in the same frequency slot as the current analog broadcast. OFDM allows the system designers to shape the digital spectrum by disabling the subcarriers that correspond to the current analog spectrum during the co-existence period. After the co-existence period the subcarriers can be enabled and the subsequent data rate increased.

Various high-speed wireless networking standards in the 5 GHz frequency region employ OFDM modulation. The U.S. IEEE 802.11a and European ETSI Hiperlan/2 standards utilize similar physical layer structures with 64-carrier OFDM and modulation ranging from BPSK to 64-QAM per subcarrier. Various data rates from 6 to 54 Mbps are possible. OFDM works well in home and office environments for handling wall reflections and movement within the structure.

Conclusions

OFDM techniques are quickly becoming a popular method for advanced communications networks. Advances in VLSI technology have made it possible to efficiently implement an FFT block in hardware. Despite the advantages OFDM can offer, the hardware to implement it can still make up a sizeable and expensive portion of the design. OFDM should not be considered for every communication system because of its increased complexity and higher transmitter and receiver demands. However, for certain systems, modern digital signal processing techniques now make it possible to use this modulation system to improve the reliability of the communications link.

REFERENCES

1. Bingham, J.A.C., Multicarrier Modulation for Data Transmission: An idea whose time has come, IEEE Communications Magazine, Vol. 28, no. 5, pp. 5-14, May 1990.
2. J.M. Cioffi, A Multicarrier Primer, in ANSI T1E1.4 Committee Contribution, No. 91-157, Boca Raton, FL, Nov. 1991.
3. Weinstein, S.B., Ebert, P.M., Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform, IEEE Transactions on Communication Technology, Vol. COM-19, no. 5, pp. 628-634, October 1971.
4. J. Stott, The Effects of Phase Noise in COFDM, EBU Technical Review, Summer 1998
5. P. Shelswell The COFDM Modulation System, The Heart of Digital Audio Broadscasting, BBC Research and Development Report, BBC RD 1996/8