It’s very interesting to know that 3G is just roll out in the developed and developing countries and the whole engineering community is started to think about the 4G. Engineers from the world started to work on 4G. These next generation wireless systems are intended for Intelligent Transportation Systems (ITS). Intended ITS applications are broadband communications to high-speed trains, including real-time video security, video advertising and broadband wireless Internet. OFDM is one of those techniques which are proposed for this next generation wireless communication systems.
Orthogonal Frequency Division Multiplexing (OFDM) is a method that allows to transmit high data rates over extremely hostile channels at a comparable low complexity. Orthogonal FDM’s (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multi path-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information. OFDM is sometimes called multi-carrier or discrete multi-tone modulation.
3. OFDM AND ORTHOGONAITY PRINCIPLE
In OFDM, a rectangular pulse is used as sub carrier for transmission. It facilitates the process of pulse forming and modulation by implementing efficiently with simple IDFT (inverse discrete fourier transform) along with IFFT (inverse fast fourier transform). To reverse this operation at receiver, an FFT (fast Fourier transform) is needed. According to the theorems of the Fourier Transform the rectangular pulse shape will lead to a sin(x)/x type of spectrum of the sub carriers (fig.1). But the spectrums of the sub carriers are not separated but overlap. The information transmitted over the carriers can still be separated because of the ‘orthogonality relation‘. By using an IFFT for modulation the spacing of the sub carriers is chosen in such a way that at the frequency where the received signal is to be evaluated (indicated as arrows), all other signals are zero.
For this orthogonality, the receiver and transmitter must be perfectly synchronized i.e. both must assume the same modulation frequency and the same time-scale for transmission. Their components must be of high quality. And there should be no multi path channel. In general, quality of components is not critical as ‘perfect synchronization’ is. For this very sophisticated receivers are required.
Figure 1: OFDM and the orthogonality principle.
This approach is being choosen to combat the multipath channel. Fortunately there’s an easy solution for this problem: The OFDM symbols are artificially prolonged by periodically repeating the ‘tail’ of the symbol and precede the symbol with it (fig.1). At the receiver this so called guard interval is removed again. As long as the length of this interval D is longer than the maximum channel delay Tmax all reflections of previous symbols are removed and the orthogonality is preserved. Of course this is not for free, since by preceding the useful part of length Tu by the guard interval we lose some parts of the signal that cannot be used for transmitting information. Taking all this into account the signal model for the OFDM transmission over a multipath channel becomes very simple: The transmitted symbols at time-slot l and subcarrier k are only disturbed by a factor Hl,k which is the channel transfer function (the fourier transform of the cir) at the subcarrier frequency, an by additional white Gaussian noise n
zl,k = al,k * Hl,k + n (1)
The influence of the channel can easily be removed dividing by Hl,k.
As far as the analog components are concerned experience has shown that in the broadcasting applications under consideration here, they are not so critical. What remains is to establish ‘perfect’ synchronization. This requires a very sophisticated receiver.
4. Single carrier approach
In fig 2 the general structure of a single carrier transmission system is depicted. The transmitted symbols are pulse formed by a transmitter filter. After passing the multipath channel in the receiver a filter matched to the channel is used to maximize signal to noise ratio a the device used to extract the data.
Figure 2: Basic structure of a single carrier system
In case of DVB-T (digital video broadcasting telivision) receiver, the receiver is characterized by the following conditions:
Transmission Rate: R = 1/T = 7.4 Msym/s Maximum channel delay: Tmax = 224 ms
For the single carrier system this results in an ISI of: Tmax / T ≈1600 (2)
The complexity involved in removing this interference in the receiver is tremendous. In the scenario under consideration here, using such an approach will only lead to sub-optimal results. This is the main reason why the multi carrier approach presented in the next section has become so popular.
5. Multi carrier approach
Fig 3. shows the general structure of a multicarrier system.
Figure 3: Basic structure of a multicarrier system
The original data stream of rate R is multiplexed into N parallel data streams of rate Rmc = 1/Tmc = R/N each of the data streams is modulated with a different frequency and the resulting signals are transmitted together in the same band. Correspondingly the receiver consists of N parallel receiver paths. Due to the prolonged distance in between transmitted symbols the ISI for each sub system reduces to
Tmax/ Tmc = Tmax / N.T (3)
In the case of DVB-T we have N=8192 leading to an ISI of
Tmax/ Tmc = 0.2 (4)
Such little ISI can often be tolerated and no extra counter measure such as an equalizer is needed. Alas as far as the complexity of a receiver is concerned a system with 8192 parallel paths still isn’t feasible. This asks for a slight modification of the approach which leads us to the concept of OFDM.
OFDM has various advantages over the previous generation’s access techniques. In OFDM, interference’s within the cell are averaged by using allocation with cyclic permutations. OFDM enables orthogonality in the uplink by synchronizing users in time and frequency, multi path mitigation without using Equalizers and training sequences, enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage, spatial diversity by using antenna diversity at the Base Station and possible at the Subscriber Unit. OFDM also enables adaptive modulation for every user, making the 4G backward compatible with 2.5G and 3G wireless mobile systems. OFDM offers Frequency diversity by spreading the carriers all over the used spectrum, time diversity by optional interleaving of carrier groups in time. OFDM also enables the usage of Indoor Omni Directional antennas for the users. In OFDMA systems the medium access control layer complexity is same as that of TDMA systems.