The OFDM employs sub-carrier frequencies that are not affected in any way by each radio channel transmissions. This means that such signals are orthogonal in the receiving device at the selected frequency of the output radio channel. The radio channel may interfere the phases and amplitudes but not the sub-carriers of the all the received sub-channel frequencies. This is one important property of the OFDM transmission technique. This makes it possible for the sub-carrier signals to be mutually orthogonal even at the receiver.
These signals can then be directly split into several sub-channels using the Fourier transformation, which can then be demodulated by a single tap with an equalizer in the receiver. Another great advantage of OFDM is in its high degree when it comes to adaptability and flexibility. OFDM has the ability to divide available bandwidth into several sub-bands which are non-selective frequencies. The specific sub-carriers adaptation into transmit parameters can be efficiently allowed in accordance with the measured radio channels.
OFDM systems provide higher flexibility when it comes to resource allocations compared to the existing systems. 4G Downlink Interface Various multiple access systems have been analysed, and compared to come up with the best downlink interface for future mobile telecommunications systems. These multiple access systems are TDMA, FDMA and CDMA combined with the OFDM transmission technique. The table below summarizes the findings, with the bit error rate (BER) mapped against the signal-to-noise ratio (SNR) for each system.
In the above situations, a single cell was considered for the best time and carrier synchronizations. The figure shows that best performance was achieved with the OFDM-FDMA design which takes advantage of the selective frequency fading on the mobile channel. The radio channel was allocated in the best possible sub-carrier for each user. Note, however, that this combination would necessitate a good prediction on the transfer function considered. If the FDMA used a non-adaptive carrier, performance would be likely the same to the OFDM-TDMA curve. 4G Uplink Interface
An OFDM-based downlink interface is quite popular and easily conceived. An uplink procedure for communication systems is a bit more complex. An OFDM-based uplink transmission system is still under rigorous research. Basically, an OFDM system will have cells which allocate exclusively to a deterministic subset of each available sub-carrier. This flexibility allows the total bandwidth to be shared in all mobile terminals. Assuming there is perfect synchronization between carrier and all resulting sub-carriers, multiple access interferences in the receiver can be totally avoided.
In the downlink procedure, the base stations will have to exert some effort to compute for the transmit amplitude signal to reduce the peak-to-average ratio (PAR), and in effect minimizing the interferences. For the uplink case, however, it is necessary to create a design which will transmit the signal at an already low PAR to avoid all possible interferences which may be caused by the non-linear effects during the amplification process. Taking this into account, the OFDM-FDMA system with an equidistant selection for its sub-carriers will be the most efficient method in reducing the PAR. OFDM-Based and Synchronized Cellular Network.
OFDM-based systems for uplink and downlink transmissions are definitely in the way. To broaden these concepts from individual links, ODFM also plays a huge role in the overall cellular network in the future. As discussed, multi-path propagation allows interferences, however, with an efficiently designed OFDM system; these undesired effects can be totally eliminated. If ODFM transmission techniques be employed in the cellular communications environment, fixed resource allocations can be surpassed. ODFM allows for a synchronized network to be established, where all available resources can be used simultaneously.