Wireless Channel Impairment Mitigation Techniques

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Wireless Channel

Impairment Mitigation Techniques

Reference: Haesik Kim, “ Wireless Communications Systems Design- From Theory to Design”, Ch. 5, 2015

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There are many types of wireless channel impairments such as noise, path loss, shadowing, and fading and impairment Mitigation techniques should be adopted according to system requirements and channel environments.There are many techniques to mitigate wireless channel impairments. For example: For the purpose of mitigating delay spreads, Global System for Mobile Communications (GSM) system uses adaptive channel equalization techniques and Code Division Multiple Access (CDMA) system uses a rake receiver.

Introduction

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Diversity TechniquesDiversity techniques mitigate multipath fading effects and improve the reliability of a signal by utilizing multiple received signals with different characteristics.

Space diversity uses multiple antennas

Time diversity uses different time slots

Frequency diversity uses different frequency slots.

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Diversity channels

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Space diversitySpace diversity uses multiple antennas and is classified into macroscopic diversity and microscopic diversity. Macroscopic diversity mitigates large scale fading ‐caused by log normal fading and shadowing. To achieve macroscopic diversity, antennas are spaced far enough and we select an antenna which is not shadowed. Thus, we can increase the signal to noise ratio.

Microscopic diversity mitigates small scale fading ‐caused by multipath. To achieve microscopic diversity, a multiple antenna technique is used as well and an antenna is selected to have a signal with small fading effect.

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Space diversity

Example of channel responses and the average of two channel responses

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Time diversity

Time diversity uses different time slots. Basically, consecutive signals are highly correlated in wireless channels. Thus, a time diversity technique transmits same signal sequences in different time slots. The time sequence difference should be larger than the channel coherence time. An interleaving technique is one of time diversity techniques.

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Time diversity

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Frequency diversity

Frequency diversity uses different frequency slots. It transmits a signal through different frequencies or spreads it over a wide frequency spectrum. Frequency diversity is based on he fact that the fading effect is differently appeared in different frequencies separated by more than the channel coherence bandwidth. When the channel coherence he transmission bandwidth is greater than bandwidth (namely, it is a broadband system), the frequency diversity Technique is useful.

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Frequency diversity

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Combining Techniques for DiversityMaximal Ratio Combining(MRC)Equal Gain Combining (EGC)Selection Combining (SC)

The signal, s(t), is transmitted through L different channels. The each received signal, rl(t), through different channels is represented by Channel: gain

(αl) and phase rotation (φl) nl(t) : Gaussian noise

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MRC technique

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The received signal, r(t), is weighted by wl

SNR, γ, of the receivedsignal, r(t),

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Schwartz’s inequality

The equality holds iffor all l, where K is an arbitrary complex constant

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The maximum SNR, γmax, can be found when

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EGC techniqueCombining all signals using phase estimation and unitary weight to achieve a high SNR

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the SNR, γ, of the received signal, r(t),

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SC technique

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Summary: Diversity techniques

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Multi-Input Multi-OutPut TechniquesMIMO techniques use the multiple antennas at a transmitter and receiver.

They are very effective to mitigate the degradation of fading channels and enhance the link quality between a transmitter and a receiver. Especially, they improve Signal to Noise Ratio (SNR), Signal to Interference plus Noise Ratio (SINR), spectral efficiency, and error probability.

The MIMO techniques are classified into spatial diversity techniques, spatial multiplexing techniques, and beamforming techniques.

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Spatial diversity•Spatial diversity techniques target to decrease the error probability. A transmitter sends multiple copies of the same data sequence and a receiver combines them.

MIMO as spatial diversity technique

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Transmit diversity: The transmitter has multiple antennas and pre processing blocks for combining the ‐multiple same data sequences. We typically assume the receiver has channel knowledge. Several well‐known spatial diversity techniques are Space Time ‐Block Codes (STBCs) and Space Time Trellis Codes ‐(STTCs) for improving the reliability of the data transmission. The STBC provides us with diversity gain only. However, the STTC uses convolutional codes and provides us with both code gain and diversity gain.

Receiver diversity: The receiver has multiple antennas and combining techniques such as MRC, EGC, and SC.

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Spatial multiplexingSpatial multiplexing techniques substantially increase spectral efficiency. A transmitter sends Nt data sequences simultaneously in the different antennas and same frequency band and a receiver detects them using an interference cancellation algorithm.

MIMO as spatial multiplexing technique

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This technique improves spectral efficiency because multiple data sequences are transmitted in parallel. Thus, the spectral efficiency is improved by increasing the number of the transmit antennas (Nt). In addition, It can be expanded into multi user MIMO ‐ or Space ‐Division Multiple Access (SDMA). Multi user MIMO ‐techniques assign each data sequence to each user.

Multiuser MIMO

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Beamforming techniquesThey are signal processing techniques for directional transmission The beamformer with Nt antenna elements combines Radio Frequency (RF) signals of each antenna element to have a certain direction by adjusting phases and weighting of RF signals. Thus, this technique brings antenna gain and suppresses interferences in multiuser environments.

In the modern wireless communications, the cell size is getting smaller and the number of cells is getting bigger. The interferences among cells became serious problem. Thus, the interference mitigation technique among cells is an essential part of wireless communication systems. The beamforming technique is very effective to mitigate interferences.

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MIMO as beamforming technique

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Fundamentals of MIMO Techniques

Spatial diversity techniques: MISO

Alamouti scheme with 2 × 1 antennas

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Signal mapping of Alamouti scheme with 2 × 1 antennas

Each symbol experiences different channel responses (h1 and h2). The received symbols

“*” = complex conjugate

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In matrix form

The combining technique in the receiver is performed. Based on the orthogonal properties of H matrix

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The combined symbols are sent to ML detector and we estimate the transmitted symbol

ML detection

The combined symbols

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Spatial diversity Techniques: SIMOWe transmit s[t] and receive yi[t] via receive antenna i at the time index t.

The receiver collects yi[t] using combining techniques and obtains more reliable received symbols.

When dealing with spatial diversity techniques, it is important to maintain uncorrelated antennas. Under the uncorrelated condition, we can obtain diversity gain which means SNR or SINR increases. If antennas are strongly correlated, we cannot obtain diversity gain.

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Spatial diversity technique with 1 × Nr antennas

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Spatial multiplexing techniques: BLAST & D-BLAST

BLAST : Bell laboratories layered space timeD-BLAST: Diagonal BLAST V-BLAST: Vertical BLAST

It is especially useful for uplink systems due to the limited number of antennas at mobile stations. Sometime, this is called collaborative MIMO or collaborative spatial multiplexing.

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D BLAST and V BLAST transmitter ‐ ‐with four antennas

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D BLAST and V BLAST transmitted data ‐ ‐sequences

( Four-antenna Example)

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Homework 1

Show with mathematical analysis how Channel Impairment Mitigated in BLAST system.

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• MIMO: A communications technology that enhances the transmission• data rate without increasing the

signal bandwidth by• combining multiple transmitters and

receivers.

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Beamforming TechniquesThey use array gain and control the direction of signals by adjusting the magnitude and phase at each antenna array. The array gain means a power gain of multiple antennas with respect to single antenna.

Simple beamformerPlane wavedeparts from omnidirectional antennas

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The delay of departure among antennas

Each signal si(t) at each omnidirectional antenna

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a = Antenna array steering vector controls the direction of the signals

Add the weighting vector w

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Thus, the signal power is strengthened in the desired direction and weakened in the undesired direction. The beamforming performance depends on finding the suitable weighting vector w, antenna array arrangement, distance d between antenna arrays, and signal correlation.

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Summary: MIMO

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Orthogonal Frquency-Division Multiplexing

The OFDM technique is based on Frequency Division Multiplexing (FDM) which transmits multiple signals in multiple frequencies simultaneously.

FDM with three carriers

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One disadvantage of the FDM is a long guard band between the carriers which makes spectral efficiency of the FDM system worse. The OFDM uses the similar concept but increases the spectral efficiency by reducing the guard band between the subcarriers. This can be achieved by orthogonality characteristic of the OFDM system.

The OFDM system uses multiple subcarriers. Thus, it needs multiple local oscillators to generate them and multiple modulators to transmit them. However, a practical OFDM system uses Fast Fourier Transform (FFT) to generate this parallel data sequences.

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OFDM with three subcarriers

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Advantages and Disadvantages of OFDM

The OFDM equalizer is much simpler to implement than those in Code Division Multiple Access (CDMA). The OFDM system is almost completely resistant to multipath fading due to very long symbols.

The OFDM system is ideally suited to MIMO techniques due to easy matching of transmit signals to the uncorrelated wireless channel.

Advantages

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It is sensitive to frequency errors and phase noises due to close subcarrier spacing.

It is sensitive to the Doppler shift which creates interferences between subcarriers.

It creates a high peak to average power ratio.

It is more complex than other communicationsystems when handling interferences at the cell edge.

Disadvantage

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System Modeling and implementationUsing Fast Fourier Transform (FFT) to implement OFDM system is a big benefit because a local oscillator is expensive and not easy to implement.

In the transmitter of the OFDM system, the data sequences are passed to Inverse FFT (IFFT) and these data sequences are converted into parallel data sequences which are combined by multiple subcarriers with maintaining the orthogonality between subcarriers.

In the receiver, the parallel data sequences are converted into the serial data sequences by FFT.

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Although the OFDM system overcomes interferences in frequency domain by orthogonality, the interferenceproblem still exists in time domain.

One of the major problems in wireless communication systems is Inter ISI‐ . This is caused by multipath and considered one important reason is a distorted original signal.

In the OFDM system, a Cyclic Prefix (CP) or Zero Padding (ZP) is used to mitigate the effects of multipath propagation. This can be represented as a guard period which is located just in front of the data and is able to mitigate delay spreads.

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OFDM transmitter with N parallel data sequence

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The baseband modulated symbol of the OFDM

Xk is the baseband modulated symbol such as BPSK, QPSK, or QAM and N is the total number of subcarriers

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The subcarrier spacing

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In addition, we can regard this signal as a discrete OFDM symbol when sampling the signal in every Ts/N.

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The OFDM transmitter can be represented using IFFT (Inverse Discrete Fourier Transform, IDFT)

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Cyclic prefix as a guard interval

When a cyclic prefix is inserted as a guard interval

Tg is a cyclic prefix length

The OFDM symbol

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Summary: OFDM

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Transmitted OFDM SignalThis baseband signal is up converted to a carrier frequency ‐ fc and we obtain the following the transmitted OFDM signal

The complex baseband signal x(t)

The received signal

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Up conversion ‐from the base band signal to the passband signal

Down‐conversion from the pass band signal to the baseband signal

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Synchronization process is performed at trhe receiver using the baseband signal y(t). The OFDM signal is very sensitive to synchronization errors such as ISI and inter carrier interference. ‐Thus, this process is very important and should be implemented very carefully. Generally, the synchronization of the OFDM system is composed of three stages which are symbol Timing synchronization, carrier frequency/phase offset synchronization, and sampling clock/sampling frequency synchronization.

The orthogonal frequency division multiple access (OFDMA) is a multiple access scheme based on the OFDM technique. The subcarriers in the OFDM system are shared by multiple users in the OFDMA system.

60OFDM based communication system‐

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Equalization

Due to a time dispersive channel by multipath fading, Inter Symbol Interference (ISI) occurs and an ‐ equalizer plays an important role in ISI compensation.

Channel model for equalization

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The frequency response of the equalizer should be designed to satisfy the following equation

F(f) = Combined frequency response of the transmit filter, channel, and receive filter

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Zero Forcing EqualizerOne of simple equalizers using the inverse of the channel frequency response.

Channel model for zero forcing equalizer

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This technique basically ignores AWGN.

The complexity is low but the performance is not good.

However, the equalization in the OFDM system is not much important because multipath fading is compensated in the OFDM system itself. Each subcarrier of the OFDM symbol experiences a flat fading channel. Thus, the OFDM system does not need a strong equalization and the one tap zero forcing equalizer in ‐frequency domain is enough to compensate subcarrier distortions.

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In order to estimate how well an equalizer works, the Mean Squared Error (MSE) is used as the metric. It is defined as the mean squared error between the received signal and the desired signal as follows

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Channel Estimation estimate the channel response Hc(f )

The first type is to use training symbols (preambles) or pilot symbols and the other type is a blind channel estimation. The method using preambles in the OFDM system is to reserve several OFDM symbols. The receiver knows which symbol the transmitter sends without interpolation.

Frame structure including preambles and data symbols

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Types of Pilot Structures in the OFDM System

Block type pilot structure in the OFDM system‐

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Comb type pilot structure in the OFDM system‐

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Lattice type pilot structure in the OFDM system‐

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Block type pilot structure‐Suitable for a frequency selective channel. The pilot interval in time domain should be satisfied by

fd = Doppler spread and TOFDM = OFDM symbol duration

Comb type pilot structure‐Suitable for a time selective channel

τmax = Maximum delay spread Δf = Subcarrier spacing in frequency domain

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Lattice type pilot structure in the OFDM system‐Combination of both the block type pilot structure ‐ and the comb type pilot structure‐ . It allocates a pilot signal to a part of subcarriers and time slots as maintaining specific interval. This structure is suitable for a frequency and time selective channel. The pilot interval inntime domain and frequency domain should be satisfied by the following both equations

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Homework 2

The Least Square (LS) estimation and the MMSE estimation are important channel estimations based on the training symbols or pilot symbols.

Discuss with aid of mathematical analysis

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Summary: Equalization