This is to certify that the thesis entitled "PAPR Analysis and Channel Estimation Techniques for 3GPP LTE System" submitted by ABHIJEET SAHU and SOUMYAJYOTI BEHERA in partial fulfillment of the requirements for the award of a Bachelor of Technology Degree in Electronics & Communication Engineering at National Institute of Technology, Rourkela is an authentic work done by them under my supervision and guidance. To the best of my knowledge, the matter contained in the thesis has not been submitted to any other university/institute for the award of any degree or diploma. In submitting our thesis report on “PAPR Analysis and Channel Estimation Techniques for 3GPP LTE System”, we would like to express our gratitude and sincere thanks to our supervisor Prof.
Poonam Singh, Department of Electronics and Communication Engineering for her constant inspiration and support during the course of our work in the last one year. SCFDMA is currently a strong candidate for uplink multiple access in the Long Term Evolution of cellular systems being considered by the 3GPP. Since we have taken a slow Rayleigh fading channel in our study, we have used block type pilot regularization channel estimation which uses LS (least square), MMSE (minimum mean square error) estimator.
There are several other adaptive estimation techniques such as LMS and RLS for estimating blind channels and comb type pilot array estimation techniques for fast fading channels.
Orthogonal Frequency Division Multiplexing 26
Significance of pulse shaping filter in PAPR analysis 36
Comparison of PAPR for various kinds of SCFDMA
Analysis of PAPR by altering the roll-off factor of pulse
- Simulation by altering the pilot symbols quantity 50 5.4.2 Study and Plot of the channel characteristics 51
INTRODUCTION
- Introduction
- Evolution of 3G
- Multi carrier modulation
- Estimating the channel
- Objective and Outline of Thesis
The first mobile radio phone (car mounted) was made in 1924 by AT & T Bell Laboratories. Since then, the mobile phone industry has seen tremendous growth and promises to deliver more in the 21st century. With the development of highly reliable, miniature, solid-state radio frequency hardware in the 1970s, the wireless communications era was born. The International Telecommunication Union (ITU) has formulated a plan to implement a global frequency band in the 2000 Mhz range that will support a single and common wireless communication standard for all countries around the world.
Therefore, each OFDM symbol is a linear combination of the instantaneous signals in each of the subcarriers in the channel. This scheme facilitates efficient use of bandwidth and reduces Symbol Interference (ISI). But another problem is high peak to average power ratio (PAPR) OFDM symbols. To counter this we use a modified scheme called single-carrier FDMA (SC-FDMA). Advantages are PAPR reduction and frequency domain equalization. The cellular radio channel significantly affects the performance of the wireless communication system and needs rigorous analysis before designing any system.
The main objectives of thesis are: (1) A comparative study of OFDMA and SCFDMA used for downlink and uplink communication respectively in 3GPP LTE.
Characteristics of Mobile Radio Channel
- Introduction
- Types of Fading
- Small-scale Fading
- Critical Channel Parameters
- Types of Small-scale Fading
- Rayleigh and Ricean Distribution
If there is relative motion in the channel, then frequency shift occurs (Doppler Effect). Knowledge of all these phenomena is necessary to model the channel for the propagation of radio waves. Multipath delay propagation occurs due to the time-dispersive nature of the channel in the local area. The coherent bandwidth (f0) is analogous to the delay propagation used to characterize the channel in the frequency domain.
It is a statistical measure of the range of frequencies for which all components are passed with equal gain and linear phase. Doppler spread occurs due to relative motion between the transmitter and receiver or the motion of objects in the channel. Coherence time (Tc) is a dual time-domain Doppler spread and is used to characterize the time-varying nature of the channel's frequency dispersivity.
It is the statistical measure of the duration of the impulse response of the channel essentially constant. If the channel bandwidth is less than the frequency range over which the channel has constant gain and linear phase, then the signal undergoes flat fading. After passing, the spectral characteristics of the channel remain unchanged, but the gain changes with time.
If fm < f0 and Ts > στ, where fm : signal bandwidth and Ts : symbol period, then the channel produces flat fading. If a channel has a constant gain and a linear phase response over a range of frequencies less than the signal bandwidth, the channel produces frequency selective fading. In a fast-fading channel, the channel characteristics change many times over the symbol duration, which changes at a rate higher than the rate of the transmitted signal.
In a slow fading channel, the channel impulse response changes at a rate much lower than that of the transmitted signal. So we can say that the relative movement between mobile and receiver determines that the channel is slow fading or fast fading.
ORTHOGONAL
FREQUENCY DIVISION MULTIPLEXING (OFDM)
- Introduction
- Importance of Orthogonality
- Guard Interval
- OFDMA
- Switch to SCFDMA
- Description of Problem Statement
- LFDMA 2. IFDMA
- Mathematical Calculation for PAPR
- Significance of pulse shaping filter in PAPR analysis
The bandwidth occupied by the OFDM systems is greater than the correlation bandwidth of the fading channel and gives it an additional edge over serial communication [14]. OFDM can ultimately be viewed as a form of multicarrier modulation where the carrier spacing is carefully selected so that each subcarrier is orthogonal to the other subcarriers. Thus, maintaining orthogonality means that the OFDM signal can be defined using Fourier transform procedures [14].
The individual sub-channels can be perfectly separated by FFT at the receiver in the absence of ISI and ICI caused by channel distortion. One way to prevent ISI is to create a cyclically extended guard interval, where each symbol is preceded by a periodic extension of the signal itself. Subchannelization defines the subchannels that can be assigned to subscriber stations based on channel conditions and data requirements.
Thus, the bandwidth of an LFDMA transmission is limited to a fraction of the system bandwidth. The duration of any symbol XK in the set X is T and represents one of the subcarrier set. Since the N subcarriers chosen to transmit the signal are orthogonal, so we can have fn = n∆f, where n∆f = 1/NT and NT is the duration of the OFDM data block X.
The cumulative distribution function (CDF) is one of the most regularly used parameters, which is used to measure the efficiency of any PAPR technique. CCDF) is used instead of CDF, which helps us measure the probability that the PAPR of a given data block exceeds the given threshold [18]. The CDF of the PAPR of the amplitude of a signal sample is given by F(z) = 1- exp(z). CCDF of PAPR of the data block that is desired in our case is to compare the results of different reduction techniques.
In digital communication, pulse shaping is one of the methods of changing the waveform of the transmitted pulse. It helps to limit the effective bandwidth of the transmission and also the ISI caused by the channel can also be kept under control.
Channel Estimation in OFDMA
- Introduction
- Block type of Pilot Arrangement
- Comb type of Pilot Arrangement
- Working Environment
- Mathematical analysis of the channel estimators
- MINIMUM MEAN SQUARE ERROR (MMSE) ESTIMATION
- LEAST SQUARE ERROR (LS) ESTIMATION
- MODIFIED MMSE ESTIMATOR
In both the frequency and time domains, the impulse response of the channel at different subcarriers appears uneven. Pilot-based approaches are widely applied to estimate channel characteristics and correct the corrupted channel due to multipath fading. The first, block-type pilot channel estimation, is developed under the assumption of slow fading channel and is performed by inserting pilot tones into all OFDM symbol subcarriers within a specific period [20].
Assuming that the payloads of the pilot arrangements are the same, the comb-type pilot arrangement has higher retransmission rates. Since only a few subcarriers contain the pilot signal, the channel impulse response of non-pilot subcarriers can be estimated by linear, cubic, or spline interpolation of neighboring pilot subcarriers. Since the coherence time is shorter than the fast-fading symbol period due to higher spreading, the channel characteristics fluctuate many times within the time duration of a symbol block.
As we are taking a slow fading Rayleigh channel, we have highlighted in different block type pilot arrangement of channel evaluation. In block-type pilot-based channel estimation, OFDM channel estimation symbols are transmitted periodically and all subcarriers are used as pilots. So in our work we are using a general model for a slowly fading channel where we use the MMSE (minimum mean square error) and LS (least square) estimator and a method for complexity and performance compromising modifications.
If we have a channel estimator in the receiver side, multiamplitude signaling schemes or M-ary PSK modulation schemes can be used. Performance is presented in terms of both Mean Square Error (MSE) and Symbol Error Rate (SER). Below is the general estimator structure as shown in [22]. LS uses no channel statistics, has low complexity, but estimator gives a higher mean squared error.
Since most of the energy in g is contained in, or close to, the first tap L, as shown in [23], a modification of the MMSE estimator, where only taps with significant energy are considered. As we know, the LS estimator does not use the channel statistics, it only depends on the input and output.
Simulations and Results
Comparison of PAPR for OFDMA and SCFDMA
Parameters Values
Comparison of PAPR for various kinds of SCFDMA techniques
Here we conclude that the PAPR for SCFDMA is less than OFDMA. And under SCFDMA localized FDMA has higher PAPR compared to interlaced and distributed FDMA (As we can see from both the fig 5.2 and 5.3 for 16qam and qpsk modulation respectively).
Analysis of PAPR by altering Roll-off Factor of pulse shaping filter
Here we see that the root raised cosine filter gives a better PAPR response than that of the raised cosine filter.
Channel Estimation
- Simulation by altering the pilot symbols quantity
- Study and Plot of the channel characteristics
- MSE and SER plots for various estimation techniques
Rest taps can thus be set to zero, as they will not contribute significantly to the autocovariance matrix Rgg, which forms the basis for mod-MMSE. But as we increased the number of ticks for consideration from gk, the error decreases. As can be seen from the above graphs, the same nature of estimator performance is maintained under different modulation techniques.
Here, we conclude that as the number of delay components in the channel increases, the root mean square error decreases as more delay information is considered for channel estimation. For different lead lengths, we cannot simulate with a fixed mod-MMSE scheme, as can be seen in Figure 5.16, the profile |gk| changes by changing the number of taps.
CONCLUSION
