Intercarrier Interference Compression in OFDM Communication Systems by Using Correlat
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Intercarrier Interference Compression in OFDM Communication Systems by Using Correlative Coding
1. Orthogonal Frequency Division Multiplexing(OFDM)
Over the past few years , there has been increasing number of users of mobile/movable non wired communication than wired communication system. At present, in addition to voice services only low bit data rate are available to mobile users. The mobile radio channel is characterized by multipath reception: the signal receive by receiver is not only LoS(line-of –sight) radio wave but also a large number of reflected radio waves that arrives at receiver at different time. Delayed signal are the result of reflection from terrain features like hills , buildings, cars , trees etc. These reflected delayed waves directly interfere with direct waves and cause intersymbol interference which in turn causes significant degradation of network performance. A wireless network should be designed to minimize these type of adverse effect.
One of the remedy of the above problem is OFDM which is most efficient among other remedies. The principles of orthogonal frequency division multiplexing(OFDM) modulation have been in existence for several decades. However, in recent years these techniques have quickly moved out of text books and research laboratories and into practice in modern communications systems. Frequency division multiplexing(FDM) extends the concept of single carrier modulation by using multiple subcarriers within the same single channel.
Let’s take a look on the following figure. In this figure W is the total bandwidth and Nis no. of carrier. We can see that for a given N, W of OFDM is less than or equal to W of FDM.
1.2 Transmitter of OFDM
Representation of ak , X(0), X(1), X(2), X(3) in graphical form:
At first the transmitter converts the input data from a serial stream to parallel sets. Each set of contain one symbol Xk, for each subcarrier. For example, a set of four data would be 1 1 1 -1 -1.
Suppose that this transmission takes 4 seconds then each piece of data has a duration of 1 second. On the other hand , OFDM would send the 4 piece simultaneously, in this case, each piece of data has a duration of 4 seconds. This longer duration leads to fewer problems with ISI.
As communication systems increase their information transfer speed, the time for each transmission necessarily becomes shorter. Since the delay time caused multipath, remains constant , ISI becomes a limitation in high data rate communication.
Hence, OFDM is especially suitable for high speed communication due to its resistence to ISI as it avoids yhis problem by sending many low speed transmission simultaneously.
In practice each OFDM channel consists of 128 to 2048 sub-carriers and can occupy bandwidth from 1.25 MHz to 20MHz. in addition each of these sub-carriers is modulated using BPSK , QPSK or 64 QAM modulation depending upon the requirements of the physical channel.
Radio channel are random, fast changing and error prone. In a wireless system the variation/fluctuation in the received signal is called fading. The goal of the wireless system design is to overcome different types fading and provide reliable and efficient transmission. Generally there are two types of fading.
• Large scale fading: It is the fluctuation in the average signal strength over a large distance and is caused by terrestrial change. This occurs when a mobile travel from a lake to mountainous are to a lake area or from an open area to a tall buildings area. Large scale fading can be mitigated by controlling the transmit power.
• Small scale fading: Occurs as a result of the fluctuations in the received signal strength over a small distance and is caused by multipath and Doppler's shift. Doppler shift refers to the change on frequency of the signal because of relative motion between the transmitter and the receiver.
The Doppler shift introduces another type of interference in OFDM i.e. inter carrier interference (ICI). OFDM divided the spectrum into narrowband subcarriers and they are tightly spaced simply because they are orthogonal. One of the requirements for orthogonality is to maintain the subcarrier spacing exactly the reciprocal of the symbol period. The figure below shows the frequency shifts thus changing the subcarrier spacing which results in the loss of orthogonality. This loss of orthogonality creates interference among the signals which is called as ICI. Since the subcarriers in OFDM are usually very narrow hence the OFDM system becomes very sensitive to ICI. ICI destroys the orthogonality of the OFDM system which is overcome by the use of cyclic prefix mechanism.
2.2 different types of ICI cancellation schemes:
2.2.1Frequency Domain Equalization
Frequency domain equalizers (FEQs) have been applied extensively in multicarrier systems to enhance transmission rate by reducing transmit redundancy in the form of guard interval. The proposed equalization algorithm is able to remove intersymbol and intercarrier interference (ISI and ICI) incurred by the reduction or the absence of this redundancy by properly exploiting null subcarriers that are inherent in standardized multicarrier systems. This method is not effective one.
2.2.2 Time Domain Windowing
This approach can be regarded as an improved version of the method using correlative coding, where the data samples of the tail subcarriers in an OFDM symbol are encoded with the data samples of the head ones in the same OFDM symbol, instead of being encoded with those of the subsequent OFDM symbol for the original approach. For low complexity, the modified correlative coding parameters are optimized through theoretical analysis. a demodulation algorithm which does not need prior information of the transmit sequence is also developed. Simulation results show that the proposed scheme not only achieves better performance in ICI suppression, but also prevent error propagation through OFDM symbols. The penalty is only a slight increased in the computational complexity at the receiver.
2.2.3 Pulse Shaping
As we have seen in the OFDM spectrum that each carrier consist of a main lobs followed by a number of side lobes with reducing amplitude. As long as Orthogonality is maintained there is no interference among the carriers because at the peak of the every carrier, there exists a spectral null. That is at that point the component of all other carrier is zero. Hence the individual carrier is easily separated. When there is a frequency offset the Orthogonality is lost because now the spectral does not coincide with the peak of the individual carriers. So some power of the side lobes exists at the center of the individual carriers which is called ICI power. The ICI power will go on increasing as the frequency offset increases. The purpose of pulse shaping is to reduce the side lobes. If we can reduce the side lobes significantly we then the ICI power will also be reduced.
2.2.4 ICI Self Cancellation
ICI self –cancellation is a scheme that was introduced by Yuping Zhao and Sven-Gustav Haggman in 2001 to combat and suppress ICI in the OFDM. The ICI self-cancellation scheme is a technique in which redundant data is transmitted onto adjacent sub-carriers such that the ICI between adjacent sub-carriers cancels out at the receiver. The main idea is one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. By doing so, the ICI signals generated within a group can be ―self-cancelled‖ each other. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the proposed scheme causes a reduction in bandwidth efficiency, it can be compensated, by using larger signal alphabet sizes in modulation.