Swati Pathak
, IJRIT-109 International Journal of Research in Information
Technology (IJRIT)
www.ijrit.com ISSN 2001-5569
Reduction Of ICI With Self Cancellation Modulation & De- Modulation
Swati Pathak1, Vishal Pasi2,
1Scholar, M-Tech Digital Communication B.T.I.R.T. / R.G.T.U.
Sagar, M.P. India [email protected]
2Asst. Prof. Electronics & Communication Deptt. B.T.I.R.T. / R.G.T.U.
Sagar, M.P. India [email protected]
Abstract
In this advanced generation the accessible frequency spectrum is limited, so for high data rate services bandwidth operation must be efficient. It should be characterized by significantly enhancing the spectral efficiency in order to increases the speed of operation and network capability. In conventional FDM, the frequency sub channels are non- overlapping. This leads to in-efficient way of utilize the accessible bandwidth. In 1960’s a scheme of overlap and orthogonal frequency sub channels was planned, hence the OFDM came into subsistence.
OFDM has planned for 4G wireless communication scheme due to its spectral efficiency and robust against multipath fading. Due to Doppler shift and frequency offset among transceiver, orthogonally is lost among sub carriers. This lead to peak average power ratio (PAPR) and inter carrier interference (ICI), which restrict the performance of OFDM.
In this dissertation a new method of SELF CENCELLATION MODULATION AND DEMODULATION (SCMD) has proposed to alleviate the effect of ICI from OFDM system. This method is compared in terms Bit Error Rate (BER) under M-ARY Quadrature Amplitude modulation (QAM) scheme in AWGN channel. It has been accomplished that the SMD technique is better in terms of Eb/No under QAM modulation for BER of frequency offset 10−4. For simulation reason MATLAB tool is used for this work.
Keywords: Peak Average Power Ratio (PAPR) Inter Carrier Interference (ICI) Quadrature Amplitude modulation (QAM) Self Cencellation Modulation And Demodulation (SCMD) Bit Error Rate (BER)
1. Introduction
All wireless communication principles, accessible and under improvement, adopt or consider adopting orthogonal frequency-division multiplexing (OFDM) as the modulation technique. It is clear that OFDM has become the definitive modulation scheme in current and future wireless communication systems.
Pursuance for better ways of living has been instrumental in advancing human civilization. Communication services available at any time and place free people from the limitation of being attached to fixed devices. Nowadays, thanks to the remarkable progress in wireless technology, affordable wireless communication service has become a reality.
Mobile phone shook people up whenever and wherever they want. Digital audio and video broadcasting offers consumers high-resolution, better-quality and even interactive programs. The devices are now thin, light, small and inexpensive. Furthermore, smart mobile phones capable of multimedia and broadband internet access are showing up on the shelves.
Several projects studying wireless networks with different extents of coverage are underway. They will enable wireless access to internet backbone everywhere, either indoors or outdoors and in rural or metropolitan areas. In the following, their evolution and future developments will be introduced. The essential role that the orthogonal frequency-division multiplexing (OFDM) technique plays in wireless communication systems will also become very clear.
In the modern world, most people fill the need for information and entertainment through audio and video broadcasting. The inauguration of AM radio can be traced back to the early twentieth century, whilst analog TV programs were first broadcast before the Second World War. Around the middle of twentieth century, FM radio
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, IJRIT-110
programs became available. These technologies, based on analog communication, brought news, music, drama, movies and much more into our daily lives. To provide more and better programs, digital broadcasting techniques, such as digital audio broadcasting (DAB) and digital video broadcasting (DVB), began to replace the analog broadcasting technologies in the past several years.
DAB is among the first standards that use the OFDM technique. The DAB project started inmid-1980. Based on OFDM, DAB has one distinct benefit: a single-frequency network (SFN). In a single frequency broadcasting network, one carrier frequency can be used for all transmitters to broadcast the same radio programmer in the entire country without suffering from co-channel interference. On the other hand, in the FM system, only one out of approximately 15 possible frequencies can be used, resulting in a very inefficient frequency reuse factor of 15. A single frequency network and a multi frequency network.
In the DAB system, it is not necessary to search for radio stations as is necessary with AM/FM radios. The programs of all radio stations are integrated in so-called multiplexes. Multiplexes save on the maintenance cost of individual radio stations. In addition, variable bandwidths can be assigned to each programmer, fulfilling their respective demands for sound quality. Music radio multiplexes can transmit at a rate up to the highest-quality192 Kbps, while mono talk and news programs may use only 80 Kbps. Furthermore, the DAB system features better mobile reception quality thanks to the OFDM technique.
DVB is the European standard for digital television broadcasting. The DVB standards include DVB-S for satellites, DVB-C for cables, DVB-T for terrestrial transmission and DVB-H for low-power handheld terminals. Among them, DVB-T and DVB-H utilize OFD has the modulation scheme. DVB-T receivers started shipping in late-1990 and now digital DVB-T programs are available in many countries. As the DAB system, DVB-T/H technology also supports countrywide single-frequency networks. In addition, DVB-T/H standards offer several modes of operation that are tailored for large-scale SFN and high mobility reception.
The basic digital stream in DVB-T is the MPEG-2 transport stream that contains one or more program streams. Each stream multiplexes compressed video, audio and data signals. The DVB-T standard can support a data rate of MPEG-2 high-definition TV(HDTV), which is up to 31 Mbps. In DVB-H, high-speed IP services as an enhancement of mobile telecommunication networks are offered. Moreover, DVB standard has allowed for integration with bi-direction data connections through other access technology, thus enabling interactive applications between the viewers and the TV stations.
Mobile phones are now a necessity to several billions of people in the world. Their functionalities range from voice service to picture, video and broadband data services. The migration from the second-generation (2G) to the third- generation (3G), and then onward to the fourth-generation (4G) mobile cellular communication systems. In 2G, the GSM system is used as the European standard and CDMA One IS-95is adopted in North America. Both of them offer digital voice services at around 10 Kbps.
Afterwards, General Packet Radio Service (GPRS) and Enhanced Data rate for Global Evolution (EDGE) systems provide transmission rates of up to several hundreds of Kbps as an enhancement of the GSM standard. Similarly, CDMA2000 1X upgraded the data transmission to 300 Kbps in North America.
2. Basic of OFDM
In OFDM, a high-data rate channel is divided into N number of low data-rate sub-channels and each sub channel is modulated in different sub-carrier. These low data rate sub channels have bandwidth less than the coherence bandwidth of the channel.
By doing so each sub channel experience a flat-fading and equalization at the receiver is less complex. By selecting a special set of (orthogonal) carrier frequencies, high spectral efficiency is obtained because the spectra of the SCs overlap, while mutual influence among the SCs can be avoided.
In an OFDM system, the input bit is multiplexed into N symbol, each with symbol period of T, and each symbol stream is used to modulate the parallel sub carriers. The sub carriers are separated by 1/NTs in frequency domain, so they are orthogonal over (0, Ts).
A typical OFDM transceiver system is shown in fig First, serial to parallel converter converts the input bits stream into groups of log2Mbits, Where M is alphabet of size of digital modulation scheme used in different sub carrier. A total of N such symbols X (k) are created. Then, the N symbols are mapped to IFFT. These IFFT corresponds to the orthogonal sub-carriers in the OFDM symbol.
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, IJRIT-111
Fig. Bandwidth Comparison between OFDM and FDMA.
Fig OFDM and FDMA Signal And Transceiver System
x(n) = 1
N Xmxe
/0 ≤ n ≤ N − 1
Where Xm are the baseband symbols on each sub-carriers. The digital-to-analog (D/A) converter then creates an analog time-domain signal which is transmitted through the channel. The digital-to-analog (D/A) converter produces an analog time-domain signal which is transmitted through the channel. Before transmitting the OFDM symbol cyclic prefix must be append at the front end of symbol. At the receiving side, the cyclic prefix is removed then the signal is converted back to a discrete N point sequence y (n), for each sub-carrier. This discrete signal is demodulated using an N-point fast Fourier transform (FFT) operation at the receiver. The demodulated symbol stream is given by:
Y(m) = ∑ y(n)e
/+ W(m) 0 ≤ m ≤ N − 1
Where, W (m) corresponds to the FFT of the sample of w (n), which is AWGN channel.3. Principal of OFDM
In OFDM system, there are mainly two principles as follows:
1. The IFFT and the FFT are used for, respectively, modulating and de modulating the data constellations on the orthogonal SCs [1]. These signal processing algorithms replace the banks of I/Q-modulators and demodulators that would otherwise be required.
Note that at the input of the IFFT, N data constellation points are present, where N is the number of FFT points. (i is an index on the SC; k is an index on the OFDM symbol). These constellations can be taken according to any phase shift keying (PSK) or QAM signaling set (symbol mapping). The N output samples of the IFFT, being in TD, form the baseband signal carrying the data symbols on a set of N orthogonal SCs. In a real system, however, not all of these N possible
SCs can be used for data.
2. The second key principle is the introduction of a cyclic prefix as a GI, whose length should exceed the maximum excess delay of the multipath propagation channel [1].Due to the cyclic prefix, the transmitted signal becomes periodic, and the effect of the time-dispersive multipath channel becomes equivalent to a cyclic convolution, discarding the GI at the receiver. Due to the properties of the cyclic convolution, the effect of the multipath channel is limited to a point wise multiplication of the transmitted data constellations by the channel TF, or the FT of the channel IR that is, the SCs remain orthogonal. The only drawback of this principle is a slight loss of effective transmits power, as the redundant GI must be transmitted. Usually, the GI is selected to have a length of one tenth to a quarter of the symbol period, leading to an SNR loss of 0.5 to 1 db. The equalization (symbol demapping) required for detecting the data constellation an element wise multiplication of the FFT output by the inverse of the estimated channel TF (channel estimation). For
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, IJRIT-112
phase modulation schemes, multiplication by the complex conjugate of the channel estimate can do the equalization.
Differential detection can be applied as well, where the symbol constellations of adjacent SCs or subsequent OFDM symbols are compared to recover the data.
4. Factor Causes ICI in OFDM System Phase Noise
Phase noise has two effects. First, it introduces a random phase variation that is common to all sub carriers. If the oscillator line width is much smaller than the OFDM symbol rate, then the common phase error is strongly correlated from symbol to symbol, so tracking technique is used to minimize the effect of this common phase noise error.
The second which is the most disturbing effect of phase noise is ICI, because the sub carrier is no longer orthogonal.
The amount of ICI is calculated and translated into degradation in SNR that is expressed as:
= 4
(4.1)Where β is the -3db one sided bandwidth of the power density spectrum of the carrier.
Frequency offset
OFDM sub carriers are orthogonal only when they all have deferent integer number of cycles within the FFT interval. if there is frequency offset, then the number of cycles in the FFT interval is not an integer anymore and hence causes ICI after FFT. The FFT output for each sub carrier will contain interfering terms from all other sub carriers. The amount of ICI in the middle sub carrier if OFDM spectrum is approximately double than the sub carriers at the band edges because the sub carriers in the middle have interfering sub carriers on both sides.
Fig. ICI Arises Due To Frequency Offset The degradation in SNR due to frequency offset is expressed as:
= ( ∆ )
where f is the frequency offset. The impact of frequency offset can be seen as an error in the frequency instants, where the received signal is sampled during demodulation by the FFT. Fig shows this effect. The amplitude of the desired sub carrier is reduced (” + ”), and ICI increases from adjacent sub carriers (”O”).
.
5. Problem Formulation Method
ICI Canceling Modulation
The ICI self-cancellation scheme the transmitted signals be constrained such that assignment of transmitted symbols allows the received signal on subcarriers k and k + 1 to be written as
and the ICI coefficient S' (l-k) is denoted as
S' (l-k) = S(l-k)-S(l+1-k)
ICI coefficients S(l-k) Vs subcarrier shows a comparison between |S‘(l-k)| and |S(l-k)| on a logarithmic scale. It is seen that |S'(l-k)| << |S(l-k)| for most of the l-k values. Comparison of |S(1-k)|,|Sǁ(1-k)|, and |S'' (1-k)| .
ICI Canceling Demodulation
ICI modulation introduces redundancy in the received signal since each pair of subcarriers transmit only one data symbol. This redundancy can be exploited to improve the system power performance, while it surely decreases the bandwidth efficiency. To take advantage of this redundancy, the received signal at the (k + 1) th subcarrier, where k is even, is subtracted from the k th subcarrier. This is expressed mathematically as
Y’’ (k) = Y'(k) – Y'(k+1)
Subsequently, the ICI coefficients for this received signal becomes
S''(l-k) = -S(l-k-1)+2S(l-k)-S(l-K+1)
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, IJRIT-113
When compared to the two previous ICI coefficients |S(l-k)| for the standard OFDM system and |S'(l-k)| for the ICI cancelling modulation, |S''(l-k)| has the smallest ICI coefficients, for the majority of l-k values, followed by |S'(l-k)| and
|S(l-k)|. The combined modulation and demodulation method is called the ICI self-cancellation scheme. The reduction of the ICI signal levels in the ICI self-cancellation scheme leads to a higher CIR in [14].
From , the theoretical CIR can be derived as
6. Mitigation ICI by Self Cencellation Modulation And Demodulation (SCMD)
In this section analyzed the ICI analysis with AWGN for and in different values of N which is number of QAM points with the proposed method SCMD at different value of frequency offset (ep) and IFFT. Analysis in graph or figure that change become in ICI mitigation with AWGN. Our proposed method work well there for the mitigation of ICI.
Table 5.1 Parameters for proposed work
PARAMETERS VALUE ICI WITHOUT
PROPOSED METHOD
ICI WITH PROPOSED METHOD
N 128
60 40
IFFT 1024
FREQUENCY OFFSET (ep) .4 at 10-5 GUARD INTERVAL .5
N 256
Above 60 Still approx 40
IFFT 2048
FREQUENCY OFFSET (ep) .4 at 10-5 GUARD INTERVAL .5
Fig. Sub-Carriers for N = 128.
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, IJRIT-114
Fig. Sub-Carriers for Index |S (l-k)|, |S’ (l-k)|, and |S’’ (l-k)| at N=128.
Fig. ICI Comparison With and Without Proposed Method
Fig. Sub-Carriers for N = 256.
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, IJRIT-115
Fig. Sub-Carriers for Index |S (l-k)|, |S’ (l-k)|, and |S’’ (l-k)| at N=128.
Fig. ICI Comparison With and Without Proposed Method
7. Conclusion
In this report, three methods of ICI mitigation SCMD have analyzed. This method is compared in terms of BER/CIR in AWGN channel under QAM modulation. Results show that the SCMD is better in all conditions. But the software and hardware implementation of SCMD is more complex. Also it requires large time to calculate the estimated frequency offset at the receiver
.
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