Wide-Band Orthogonal Frequency Multiplexing (W-OFDM) Technical
Orthogonal Frequency Division Multiplexing (OFDM) is a multi carrier transmission technique whose history dates back to the mid1960's. Although, the concept of OFDM has been around for a long time, it has recently been recognized as an excellent method for high speed bidirectional wireless data communication. The first systems using this technology were military HF radio links. Today, this technology is used in broadcast systems such as Asymmetric Digital Subscriber Line (ADSL), European Telecommunications Standard Institute (ETSI) radio (DAB:Digital Audio Broadcasting) and TV (DVBT:Digital Video Broadcasting---Terrestrial) as well as being the proposed technique for wireless LAN standards such as ETSI Hiperlan/2 and IEEE 802.11a. There is also growing interest in using OFDM for the next generation of land mobile communication systems.
OFDM efficiently squeezes multiple modulated carriers tightly together reducing the required bandwidth but keeping the modulated signals orthogonal so they do not interfere with each other. Any digital modulation technique can be used on each carrier and different modulation techniques can be used on separate carriers. The outputs of the modulated carriers are added together before transmission. At the receiver, the modulated carriers must be separated before demodulation. The traditional method of separating the bands is to use filters, which is simply frequency division multiplexing (FDM). Fig. 1 shows a representative power spectrum for three sub channels of a FDM system.
In a classic FDM system, the sub channels are non-orthogonal and must be separated by guard bands to avoid inter channel interference. This results in reduced spectral efficiency.
Another method to achieve frequency separation, but is more spectrally efficient than FDM is to overlap the individual carriers, yet ensuring the carriers are orthogonal is to use the discrete Fourier Transform the (DFT) as part of the modulation and demodulation schemes. This is where the name orthogonal FDM (OFDM) arises. High speed, fast Fourier transform (FFT) chips are commercially available, making the implementation of the DFT a relatively easy operation. Fig. 2 shows the spectrum of an OFDM signal with three sub carriers. The main lobe of each carrier lies on the nulls of the other carriers. At the particular sub carrier frequency, there is no interference from any other sub-carrier frequency and hence they are orthogonal. In Fig.2, the sub carriers are 300 Hz apart.
The orthogonal nature of the OFDM sub channels allows them to be overlapped, thereby increasing the spectral tightly efficiency. In other words, as long as orthogonality is maintained, there will be no inter channel interference in an OFDM system. In any real implementation, however, several factors will cause a certain loss in orthogonality.
Designing a system which will minimize these losses therefore becomes a major technical focus. Another advantage to OFDM is its ability to handle the effects of multipath delay spread. In any radio transmission, the channel spectral response is not flat. It has fades or nulls in the response due to reflections causing cancellation of certain frequencies at the receiver. For narrowband transmissions, if the null in the frequency response occurs at the transmission frequency then the entire signal can be lost.
Multipath delay spread can also lead to inter symbol interference. This is due to a delayed multipath signal presents overlapping with the following symbol. This problem is solved by adding a time domain guard interval to each band OFDM symbol. Inter carrier interference (ICI) can be width avoided by making the guard interval a cyclic extension of, the OFDM symbol. There are, however, certain negatives associated with this technique. It is more sensitive to carrier frequency offset and sampling clock mismatch than single carrier systems. Also the nature of the orthogonal encoding leads to high peak to average ratio signals: or in other words, signals with a large dynamic range. This means that only highly linear, low efficiency RF amplifiers can be used.
We present here WOFDM technology, which is less sensitive to inherent OFDM problems such as frequency offset, sample clock offset, phase noise and amplifier non-linearities. WOFDM is also able to tolerate strong multipath and fast changing selective fading by using a powerful equalization scheme combined with a forward error correction scheme.