The orthogonal frequency division multiplexing (OFDM) transmission technique can be very efficient in dealing with the multi-path propagation effects for broadband radio channels. Also, this technique can be used in several multiple access schemes, such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA). When combined with the mentioned access approaches, OFDM will definitely be a breakthrough for both multi-user downlink and uplink systems.
As with other developing technologies, there are also several challenges imposed by OFDM. For one, the system design requires amplified linearity to prevent any non-linear effects that might be cause by the large peak-to-average ratio (PAR) of the signal. The OFDM which is combined with multiple access schemes should be able to address any multiple access interferences (MAI) during its carrier synchronization in its base station receiver. So far, OFDM has not been used in cellular communications. However, its potential for such industry is already in the spotlight.
Currently, the OFDM is employed in several broadcast applications and some wireless local loop standards. Basically, OFDM should be able to split the whole bandwidth into several narrowband channels. These sub-channels should be equidistant on the frequency axis. Though they can overlap each other, their sub-carriers will still be orthogonal in the receiving end and thus, can be separated using the Fourier transformation. Introduction The most common technology used today is that of GSM which started in Europe during the early 1980s.
This was released in 1990, which at that time started the first few researches for the 3rd generation (3G) of cellular communication systems. The transition from GSM to 3G is currently visible. Compared to GSM, 3G (IMT-2000, UMTS) systems provide higher data transfer rates, which range from 64 – 384 kbps. It can even go as high as 2Mbs for indoor applications or lower mobility. However, the market is constantly demanding for better technologies. It has been anticipated that users will require more data services in the future.
Not only will mobile phones be primarily used for voice services, these are also expected to provide data services at high speeds (20Mbs +) to accommodate the demand for high-end mobile applications. The figure below graphs where the future requirements of the 4G systems stand compared to WLAN and 3G. The 4G system should be adaptive and flexible enough to be able to provide transmission at different layers in the ISO-OSI stack. High-end mobile applications will require the highest layer while the radio channel for the transmission medium will employ the lowest layer.
The OFDM transmission technique is expected to meet the requirements of such technology. Because of its flexibility in system designs, OFDM can be applied to a wide-band radio channel which can be used as an air interface. Currently, this OFDM scheme has matured enough for wide-band systems to be integrated to the mobile telecommunications environment. OFDM Transmission Technique – Radio Channel Behaviour The cellular communications system design is dependent on the radio channel behaviour.
Typically, multi-path propagation occurs because of the transmitted signal always reflects at several obstacles inside the local environment. This can be best described by the figure below. There are several options for a new system design. However, what is definite is that future mobile communication systems will always require extremely huge data transfer rates, and thus larger bandwidth. If the current single carrier schemes are applied, strong interferences due to the anticipated wide-band applications will be observed. To counter these adverse effects will necessitate a powerful equalizer for each receiver.
Developing such complex equalizers will be increasingly more difficult with increasing bandwidth requirements. For this reason, the OFDM poses a very viable alternative for broadband mobile applications. OFDM does not only efficiently counter these effects in multi-path propagations but does it in much lesser computation complexities, in terms of developing the equalizer for the receivers. The current performance of OFDM in new air interface for the 4G mobile communication systems looks very promising, albeit the more selective frequency and time-variant channels.