What is Channel EstimationThe wireless radio channel can be parameterised as a combination of paths, each characterized by a delay and complex amplitude. The signal gets distorted or various noises are added to the signal while the signal goes through the channel. To properly decode the received signal without much errors or to remove the distortion and noise applied by the channel from the received signal. To do this, the first step is to figure out the characteristics of the channel that the signal has gone through. The technique/process to characterize the channel is called 'channel estimation'. The channels in mobile radio systems are usually multipath fading channels, which are causing inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, different kind of equalizers can be used. Detection algorithms based on trellis search (like MLSE or LMS) offer a good receiver performance, but still often not too much computation. Therefore, these algorithms are currently quite popular.However, these detectors require knowledge on the channel impulse response (CIR), which can be provided by a separate channel estimator. Usually the channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst. Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples. The channel effects like a filter. The filtering nature of the channel is caused by the summation of amplitudes and delays of the multiple arriving waves at any instant of time.Channel estimation is to estimate the filter coefficient through received signal and other known information (such as modulation type).

Fading and Multipath fading effectsThe term fading, or, small-scale fading, means rapid fluctuations of the amplitudes, phases, or multipath delays of a radio signal over a short period or short travel distance. This might be so severe that large scale radio propagation loss effects might be ignored.In principle, the following are the main multipath effects: Rapid changes in signal strength over a small travel distance or time interval. Random frequency modulation due to varying Doppler shifts on different multipath signals. Time dispersion or echoes caused by multipath propagation delays.

The following physical factors influence small-scale fading in the radio propagation channel: Multipath propagation Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. The effects of multipath include constructive and destructive interference, and phase shifting of the signal. Speed of the mobile The relative motion between the base station and the mobile results in random frequency modulation due to different doppler shifts on each of the multipath components. Speed of surrounding objects If objects in the radio channel are in motion, they induce a time varying Doppler shift on multipath components. If the surrounding objects move at a greater rate than the mobile, then this effect dominates fading. Transmission Bandwidth of the signal If the transmitted radio signal bandwidth is greater than the bandwidth of the multipath channel (quanti- fied by coherence bandwidth), the received signal will be distorted.

Thus we see that unlike the normal wired communication the impulse response or the characteristics of the channel in case of wireless communication changes due to above mentioned factors and hence there arise the need to model and estimate the wireless radio channel.

What is Multi Carrier Modulation/SystemMulti-carrier modulation (MCM) is a method of transmitting data by splitting it into several components, and sending each of these components over separate carrier signals. The individual carriers have narrow bandwidth, but the composite signal can have broad bandwidth. When the overall transmission is received, the receiver has to then re-assemble the overall data stream from those received on the individual carriers.There are many forms of multicarrier modulation techniques that are in use of being investigated for future use. Some of the more widely known schemes are summarised below. Orthogonal frequency division multiplexing, OFDM: OFDM is possibly the most widely used form of multicarrier modulation. It uses multiple closely spaced carriers and as a result of their orthogonality, mutual interference between them is avoided. Generalised Frequency Division Multiplexing, GFDM: GFDM is a multicarrier modulation scheme that uses closed spaced non-orthogonal carriers and provides flexible pulse shaping. It is therefore attractive for various applications such as machine to machine communications. Filter Bank Multi Carrier, FBMC: FBMC is a form of multicarrier modulation scheme that uses a specialised pulse shaping filter known as an isotropic orthogonal transform algorithm, IOTA within the digital signal processing for the system. This scheme provides good time and frequency localisation properties and this ensures that inter-symbol interference and inter-carrier interference are avoided without the use of cyclic prefix required for OFDM based systems.The advantages of MCM include relative immunity to fading caused by transmission over more than one path at a time (multipath fading), less susceptibility than single-carrier systems to interference caused by impulse noise, and enhanced immunity to inter-symbol interference. Limitations include difficulty in synchronizing the carriers under marginal conditions, and a relatively strict requirement that amplification be linear.

Standard Multi-Carrier Transmitter g(t): Raised Cosine PulseDescription:- In the transmitter path, binary input data is first sent to a serial-to-parallel buffer. Then the data is mapped using different modulating schemes and then the parallel binary bits can be modulated onto subcarriers. In an MCM based communication system, the modulated carriers are summed for transmission, and must be separated in the receiver before demodulation.Standard Multi-Carrier Receiver

Descritption:- The multi-carrier receiver used is a coherent receiver at different frequencies. The receiver has to estimate frequency offset and symbol timing.

MCM OrthogonalitySubcarriers at f0 +i/Tn and f0+j/Tn are orthogonal.

Drawbacks of Multi-Carrier Modulation Requires N modulators and N demodulators which leads to difficult hardware implementation Sharp BPF at the receiver not practical which leads to intercarrier interference. Sub-channel Bandwidths are larger SolutionThe solution is to use overlapping signals and Fourier Transforms which forms the basis for Orthogonal Frequency Division Multiplexing. Using the Nyquist criteria the signals can be assumed to be sampled at B rates per second at the transmitter where B is the double sided available bandwidth and also twice the maximum frequency satisfying the Nyquist Criteria. Hence this gives a resemblance to an expression similar to that of an Inverse Fourier transform. Thus instead of using N modulators and N demodulators we can use IDFT at the transmitter for mapping the split data symbol in to different frequency sub-carriers and DFT at the receiver for demodulating from sub-carriers and then summing up the split data in to one once again.

Orthogonal Frequency Division MultiplexingThe basic idea underlying OFDM systems is the division of the available frequency spectrum into several subcarriers. To obtain a high spectral efficiency, the frequency responses of the subcarriers are overlapping and orthogonal, hence the name OFDM. This orthogonality can be completely maintained with a small price in a loss in SNR, even though the signal passes through a time dispersive fading channel, by introducing a cyclic prefix (CP).OFDM Transmitter

The binary information is first grouped, coded, and mapped according to the modulation in a signal mapper. After the guard band is inserted, an N-point inverse discrete-time Fourier transform (IDFTN) block transforms the data sequence into time domain (note that N is typically 256 or larger). Following the IDFT block, a cyclic extension of time length TG, chosen to be larger than the expected delay spread, is inserted to avoid intersymbol and intercarrier interferences. The D/A converter contains low-pass filters with bandwidth 1/TS, where TS is the sampling interval. The channel is modeled as an impulse response g(t) followed by the complex additive white Gaussian noise (AWGN) n(t), where m is a complex values and 0 mTS TG.

OFDM Receiver

At the receiver, after passing through the analog-to-digital converter (ADC) and removing the CP, the DFTN is used to transform the data back to frequency domain. Lastly, the binary information data is obtained back after the demodulation and channel decoding.Let X=[Xk]T and Y=[Yk]T (k=0,1,.. N-1)denote the input data of IDFT block at the transmitter and the output data of DFT block at the receiver, respectively. Let g=[gn]T and n=[nn]T denote the sampled channel impulse response and AWGN, respectively. Define the input matrix X=diag(X)and the DFT-matrix

Multipath Induced ISI in OFDM

The last few bits of the previous data symbol interferes with the starting bits of next data symbol. In order to remove this interference we use Cylic Prefix technique. ISI Removal The Cyclic Prefix The Cyclic Prefix (CP) is a block of symbols added at the beginning of each data block CP converts a linear convolution channel into a circular convolution channel. It helps to remove the ISI. Drawbacks of CP are a reduction in data rate due to the CP overhead and waste of power in the cyclic prefix samples. The latter can be avoided by transmitting all zeros (no power).

At the receiver data blocks of length N: y[0],y[1]..y[N-1] are unaffected by ISI.

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