1. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 6, August 2012
Channel Estimation using OFDM for 4G
1. Sumit Kumar Gupta 2. Ajay kumar Vishwakarma
Abstract— Research and development are taking place all over The higher data rate of 3G systems will be able to support a
the world to define next generation of wireless broadband widerange of applications including Internet access, voice
multimedia communication systems.Wireless Broadband communications and mobile videophones. In addition to this,
Technologies allow simultaneous delivery of voice, data and a large number of new applications will emerge to utilize the
video over fixed or mobile platforms. WiFi, WiMAX, 3G, UWB
are some of the emerging broadband technologies. WiMAX is
permanent network connectivity, such as wireless appliances,
standards-based (IEEE 802.16) wireless technology that provides notebooks with built in mobile phones, remote logging,
high-throughput broadband connections over long distances. The wireless web cameras, car navigation systems, and so forth. In
paper is divided into two parts channel estimation and channel fact most of these applications will not be limited by the data
tracking. In the channel estimation part, estimation of the rate provided by 3G systems, but by the cost of the
channel transfer function in the preamble using MLS method is serviceeasier or more enjoyable. Applications are virtually
down. The various effect of varying channel length and Doppler unlimited as long as the devices have the capabilities to
frequency on different modulation technique is seen. Study of the provide such type of services. Research has just recently
effects of channel. At high SN, both methods reach an error floor begun on the development of 4th generation (4G) mobile
due to the residual error produced by the ICI. This paper
develop robust and low complexity channel estimation
communication systems.Ultimately 4G networks should
algorithms and compare different modulation technique for encompass broadband wireless services, such as High
OFDMA reuse1 systems, which are in compatible with the IEEE Definition Television (HDTV) (4 - 20 Mbps) and computer
802.16e standard. network applications (1 - 100 Mbps). This will allow 4G
networks to replace many of the functions of WLAN systems.
However, to cover this application, cost of service must be
Keywords—Wireless, OFDM, 4G, Modulation, channel reduced significantly from 3G networks. The spectral
estimation, channel width, Doppler frequency. efficiency of 3G networks is too low to support high data rate
services at low cost. As a consequence one of the main
I. INTRODUCTION focuses of 4G systems will be to significantly improve the
spectral efficiency Supporting the high data rates with
Now-a 3rd generation mobile systems such as the Universal sufficient robustness to radio channel impairments, require
Mobile Telecommunications System (UMTS) are introduced. careful choosing of modulation technique. OFDM is a robust
These systems are striving to provide higher data rates than efficient modulation scheme for broadband communications.
current 2G systems such as the Global System for Mobile It combats multipath fading and narrow band interference
communications (GSM) and IS-95. Second generation efficiently. OFDMA, multiple access scheme based on
systems are mainly targeted at providing voice services, while OFDM, has lots of flexibility and when coupled with feedback
3rd generation systems will shift to more data oriented information it can achieve high data rates efficiently. The
services such as Internet access. Third generation systems use wireless MAN-OFDMA is one of the air interface standard for
Wide-band Code Division Multiple Access (WCDMA) as the NLOS communication.
carrier modulation scheme.[1] This modulation scheme has a
high multipath tolerance, flexible data rate, and allows a
greater cellular spectral efficiency than 2G systems. Third II. CHANNEL ESTIMATION: PRINCIPLES
generation systems will provide a significantly higher data
rate (64 kbps – 2 Mbps) than second generation systems (9.6 – The channel is the medium through which the signal travels
14.4kbps). from the transmitter to the receiver. Unlike wired channels
that are stationary and deterministic, wireless channels are
extremely random in nature. Some of the features of wireless
Sumit Kumar Gupta, Asst .Professor ,Dept. Of Electronics communication like mobility, place fundamental limitations
& communication, Bansal institute of engineering & on the performance in a wireless system. The transmission
technology (BIET),Lucknow, India, 9452052155) path between the transmitter and receiver can vary from line-
of-sight (LoS) to one that is severely obstructed by buildings,
Ajay kumar Vishwakarma,Sr.Lecturer Dept. Of terrain and foliage. Efficient channel estimation strategies are
Electronics & communication, Bansal institute of engineering required for coherent detection and decoding. Channel
& technology BIET,Lucknow, India,9839039699 estimates are used in diversity techniques like Maximal Ratio
combining. In opportunistic communication systems, the
channel estimate is feedback to the transmitter. The
transmitter uses the channel knowledge to exploit Multi User
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All Rights Reserved © 2012 IJARCSEE

2. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 6, August 2012
Diversity (MUD). Therefore, the channel estimator is IV. ORTHOGONAL FREQUENCY DIVISION
inevitable in any wireless communication system.[4] MULTIPLEXING (OFDM)
A channel model on the other hand can be thought of as a
mathematical representation of the transfer characteristics of In the scenario of high bit rate digital communications over
this physical medium. This model could be based on some wireless channels, one often has to deal with transmission
known underlying physical phenomenon or it could be formed channels that exhibit a phenomenon known as fading or a
by fitting the best mathematical / statistical model on the highly time-varying frequency response over the signal
observed channel behavior. Most channel models are bandwidth. This fading occurs over wireless transmission
formulated by observing the characteristics of the received media primarily because the signal in such media is
signals for each specific environment. Different mathematical transmitted across multiple paths of varying attenuation and
models that explain the received signal are then fit over the delay spread. In such a system, with the use of single carrier
accumulated data. Usually the one that best explains the modulation systems, channel equalization turns out to be a
behavior of the received signal is used to model the given very difficult task, especially in the presence of time-varying
physical channel. channels, and the discrepancies in the SNR at different
Channel estimation is simply defined as the process of frequencies lead to a low spectral efficiency, or even to a
characterizing the effect of the physical channel on the input reduction in the useful bandwidth. The idea underlying the
sequence. If the channel is assumed to be linear, the channel adoption of multicarrier modulation systems is that of dividing
estimate is simply the estimate of the impulse response of the the available channel band into a very high number of sub-
system. It must be stressed once more that channel estimation bands (sub channels) , each one so small that the channel
is only a mathematical representation of what is truly frequency response can be assumed to be constant within a
happening. single sub-band . The overall information stream is thereby
partitioned into corresponding sub streams, each one of them
being transmitted over a different sub-channel. By dividing
III. NEED FOR CHANNEL ESTIMATION the available bandwidth into several narrowband sub-channels
each of which undergoes frequency flat fading, high-rate data
Channel estimation algorithms allow the receiver to is sent through low-rate sub channels in parallel. Also, a cyclic
approximate the impulse response of the channel and explain prefix is used to make sub-channels orthogonal to one another
the behavior of the channel. This knowledge of the channel's so that inter carrier interference (ICI) and inter symbol
behavior is well-utilized in modern radio communications. interference (ISI) are avoided. With these advantages OFDM
Adaptive channel equalizers utilize channel estimates to has been adopted as the standard transmission technology for
overcome the effects of inter symbol interference. Diversity digital audio broadcasting (DAB) and terrestrial digital video
techniques (for e.g. the IS-95 Rake receiver) utilize the broadcasting (DVB-T) in Europe, and recently for wireless
channel estimate to implement a matched filter such that the local area networks (WLAN) such as HIPERLAN/2 IEEE
receiver is optimally matched to the received signal instead of 802.11a and IEEE 802.16. It is also one of the promising
the transmitted one. Maximum likelihood detectors utilize techniques for the fourth generation (4G) cellular system.
channel estimates to minimize the error probability. One of Although the theoretical basis for development of OFDM is
the most important benefits of channel estimation is that it over 40 years old, it has not been implemented successfully
allows the implementation of coherent demodulation.Coherent until recently owing to high amount of computational
demodulation requires the knowledge the phase of the signal. complexity and memory requirements of the receiver for such
This can be accomplished by using channel estimation a system.
techniques.
The channel estimation in OFDMA can be widely
classified into two categories.[7]
1. Pilot Based channel Estimation: known symbol called
pilots are transmitted.
2. Blind Channel Estimation Methods: No pilots are
required. It uses some
underlying mathematical properties of data sent. The Blind
channel estimation methods are computationally complex and (a) Time domain, The number of cycles
hard to implement. The Pilot based channel estimation
methods are easy to implement, but they reduce bandwidth (b) Frequency domain, the peak of thein Symbol differ by one
efficiency. The Pilot based methods are more popular for adjacent sinc comes at the null of other sincsub
nowadays. IEEE802.16e standards supports the Pilot based carriers waveforms.
channel estimation Fig 1. Orthogonality of subcarriers
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All Rights Reserved © 2012 IJARCSEE

3. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 6, August 2012
V. SYSTEM MODEL higher.Variation in QPSK amplitude is not much. Hence
carrier power almost remains constant.
We derive a linear system model which relates the transmitted iv) QAM:-
OFDM symbol (in frequency domain), [ ] 1 2 1 , ,... ,........ − = For same bit error the BW required by QAM is reduced
k N x x x x x and the received OFDM symbol [ ] 1 2 1 , ,.... to half as compared to BPSK. Less bit error probability.
,......... − = k N y y y y y . Assuming that most of the energy in
the impulse response is concentrated in L taps, the channel VI. SIMULATIONS PARAMETERS
vector is given by [ ] 1 2 , 1 , ,..... 0,..0 L NX h = h h h .
Assume that channel remains constant over one OFDM A system level simulator, developed in MATLAB, for single
symbol and L is less than the cyclic prefix.[4] cellular, single user environment has been used. The
Simulation parameters are shown in the table below
Fig 2. OFDM System Model
VI. MODULATON SCHEMES
It is defined as the process by which some characteristic of a
signal called carrier is varied in accordance with modulating
signal.Used modulation technique are
i). Binary Phase Shift Keying (BPSK)
ii).Differential Phase Shift Keying (DPSK)
iii).Quadrature Phase Shift Keying (QPSK)
iv).Quadrature Amplitude Modulation (QAM)
i) BPSK:- Table 1.
Less Probability of error.It need a complicated synchronizing
ckt at the receiver for generation of local carrier. Ambiguity in The fading channel is implemented with the Jakes Method
the output signal because use square law device in receiver (Sum of Sinusoids method). The fading channel model, with
side. power delay profile corresponding to PED B, 1 is used. The
channel parameters are listed below. Only sample spaced
ii) DPSK:- channels are considered. The non-integer samples of the PDP
DPSK does not need carrier at it receiver. This means that the are mapped to the nearest sample position. The pilots are
complicated circuitry for generation of local carrier is QPSK modulated and they have unit energy.
avoided. The BW requirement of DPSK is reduced compared
to that of BPSK.The probability or error rate of DPSK is
higher than that of BPSK. Noise interference in DPSK has
more.
iii). QPSK:-
For the same bit error rate, the BW required by QPSK is
reduced to half as compared to BPSK. Because of reduced
BW, the information transmission rate of QPSK is
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All Rights Reserved © 2012 IJARCSEE

4. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 6, August 2012
10
PSK
QPSK
0
QAM
Normalized MeanSquareError
-10
-20
-30
-40
-50
-60
5 10 15 20 25 30 35 40
Table 2. Symbol Index
VII. RESULTS:-
Fig-4
Channel Length=6 and Doppler Frequency 40 Hz
Parameter Modulation
Technique 10
PSK
Doppler PSK QPSK QAM 0
QPSK
QAM
frequency
NMSE NMSE NMSE
Normalized MeanSquareError
Hz -10
in dB in dB in dB
-20
20 -40 -42 -42
-30
40 -46 -41 -47
-40
60 -41 -38 -49
-50
70 -45 -38 -49
-60
5 10 15 20 25 30 35 40
Symbol Index
Table 3.
For fixed channel length and varying Doppler frequency 10
PSK
10
PSK QPSK
0
QPSK QAM
0 QAM
Normalized MeanSquareError
-10
Normalized MeanSquareError
-10
-20
-20
-30
-30
-40
-40
-50
-50
-60
-60 5 10 15 20 25 30 35 40
5 10 15 20 25 30 35 40 Symbol Index
Symbol Index
Fig -5
Fig-3 Channel Length=6 and Doppler Frequency 60 Hz
Channel length=6 and Doppler Frequency=20 Hz
Fig-6
Channel length=6 and Doppler Frequency=70 Hz
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5. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 6, August 2012
VIII. CONCLUSION
The pilot based channel estimation and tracking algorithms
are investigated for reuse1 OFDMA system, which is in
compatible with IEEE 802.16e standard. We studied and
simulated MSL method for sample spaced channels. The FFT
based method is affected by the ―smearing effect‖ due to the
pulse shaping in the OFDMA symbol because of this it’s
performance curve reaches an error floor at high SNR. MLS
method is always superior to the other. We studied the
performance of MLS method under the channel varying inside
an OFDMA symbol. This introduces ICI and causes the MLS
method to reach an error floor at high SNR.
I have seen the effect of varying Doppler frequency on
different modulation technique. We get that QAM is better Sumit Kumar Gupta, B.Tech, M.Tech (Digital Communication) from MANIT
Bhopal(M.P). I have five year teaching experience.
than other modulation technique for varying Doppler
frequency.
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