Multicarrier modulation schemes are gaining momentum in the communications marketplace due to bandwidth efficiency. In particular, orthogonal-frequency-division-multiplex (OFDM) systems allow carriers to be very tightly packed by maintaining strict orthogonality between the various signals across the band. Efficient digital-signal-processing (DSP) implementations of the required signal processing are making this modulation type fast and cost-effective for use in consumer systems. This complex modulation scheme does present some design challenges, however. Fortunately, modern simulation tools can be used to solve some of these design challenges.

Frequency-domain multiplexing using orthogonal carriers was first proposed by Chang.¹ Many independent data channels could be packed into a surprisingly small bandwidth. Orthogonality prevents the carriers from interfering with each other, thus obviating the need for guard bands. Although this technique promised great improvements in effective transmission rates, its cost and size limited OFDM to military applications for many years. One of the first OFDM modems was designed for military high-frequency (HF) radios.² Recently, advances in electronic components and hardware have enabled this and other communications technology, such as code-division multiple access (CDMA), to enter the commercial world.

OFDM is something of a misnomer, since the transmission technique is often employed to spread a single data stream over a band of carriers, with substreams transmitted in parallel. The IEEE 802.11a data transmission standard is designed to carry packetized data over parallel channels at information rates approaching 12 b/s/Hz. The spectrum of an IEEE 802.11a signal is shown in the center of Fig. 1. In IEEE 802.11a wireless-local-area-network (WLAN) systems, 12 carriers at the ends of the spectrum are left "unloaded" to produce a power-spectral density with a steep decline at the band edges.

The high rate efficiencies associated with IEEE 802.11a are made possible through the use of high-order modulation schemes being applied to the individual carriers. Quadrature-amplitude modulation (QAM) of various levels is applied to each carrier. The modulation is normally applied to 64 carriers simultaneously with a computationally efficient Inverse Fast Fourier Transform (IFFT).³ Demodulation can be performed with the FFT. Data-transmission rates corresponding to various options for modulation and coding are shown in the table. These rates are for the HiperLAN 2 standard which employs IEEE 802.11a as the basic modulation type.

Whereas OFDM offers potential for high-data-rate transmission, it also requires additional signal processing for synchronization and other auxiliary receiver (Rx) functions. Some disadvantages of OFDM include susceptability to oscillator phase noise and sensitivity to transmission amplitude/phase distortion. The following section will briefly explore this latter issue.

In an IEEE 802.11a system, data is transmitted in frames, with each frame containing a header which is used for synchronization and control. Although the full description of the header is beyond the scope of this article, suffice it to say that the header contains a correlation word for detection of the start of the frame and also an embedded pilot sequence used for "sounding" the amplitude/phase variation across the wideband OFDM channel.