Techs for 4th Generation Wireless Communication

8/12/2019 Techs for 4th Generation Wireless Communication

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Thursday 2/7/13 11:05 AMThe Mobile Wireless ChannelTransmitted signal reaches Rxthrough several paths, each

path has:a) attenuation.b) phase.c) delay.Channel is described by impulse response

Parameters of the mobile multipath wireless channel:EXA:


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Sunday 2/10/13 11:03 AM

Freq. selective fading:| H(w) | is variable with (w)over the BW of the signal.

Power delay profile:is the spatial average

RMS delay spread of the channel:

EXA: find rms delay spreadfor the channel withpower delay profile.

Solution:

Flat fading : | H(w) | isapproximately constantover the BW of signal.


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Coherence BW( Bc ):Is the range of freqs. over which the ch. H(w) is consedered flat, in otherwords ( i.e. ), two freqs. are highly correlated iff2 - f1 < Bc

In the previous EXA:


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Tuesday 2/12/13 10:58 AMFollow previous EXA:-b) would this channel be suitable for

AMPS (BW = 30 kHz)GSM (BW = 200 kHz)

Solution

Can operate properlywithout equalizer (flat fading)GSM: BWsig > Bcneeds equalizer (Freq. selective)

Freq. spreading (Doppler effect)

Max. Doppler shift

Coherence time (Tc):Is the time at which h(t) is approximately unchanged.

EXA: a mobile Rx at fc = 2.2 GHz moving at speed v = 20 km/hrFind Tc.Solution


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Thursday 3/5/13 11:03 AMConventional methods for ch. fading:

1) Time - selective fading:-Solution: ch. coding & interleaving.2) Freq. - selective fading:-Solution: equalizers.Adaptive modulation & coding:Is one of the key - technology in 3G/4G system.Enables spectrally efficient transmission over time varying channels.Basic Concepts:- To estimate the ch. gain ( ) at Rx and feedback to the Tx.- Tx dynamically changes:-

1) Modulation, 2) Power & 3) Codingaccording to ch. condition ( ) within each frame (frame length < Tc).- The goal is to max. spectral efficiency.

Block diagram:

Adaptivemodulation& coding

Powercontrol

Demodulation

Channelestimation

Adaptive System


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Sunday 2/17/13 11:02 AMBs = 1/Ts (Nyquist pulse.)P : Average Tx power.

Define

SNR (Signal-to-Noise power ratio) at Rx if P power is transmitted.Adaptation params:

The Tx power for a specific ( )The data rate for a specific ( )

FindingRayleigh channel

Example on rate-variation with fixed power:Consider adaptive modulation using:-

1) QPSK (M = 4)2) 8-PSK (M =8)

To meet a target prob. of bit error Pb = 10^-3Tx works as follows:

1) if received SNR is sufficient for 8-PSK to have Pb = 10^-3 , use 8-PSK.2) if received SNR is sufficient for QPSK to have Pb = 10^-3 , use QPSK (but notsufficient for 8-PSK to have Pb = 10^-3)

3) if received SNR is not sufficient for QPSK / 8-PSK to meet target Pb DO NOTSEND DATA ...


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EXA: Find:-a) find SNR regions ( ) assigned to each modulation. (QPSK, 8-PSK, No data)b) find spectral eff. R/B for this method.b) compare to non-adaptive.

Solution


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Tuesday 2/19/13 11:04 AMFollow EXA:-

1) Variable power technique:we vary Tx-power so that SNR at Rx is constant.

Adaptation:-

(Non adaptive >> no feedback >> high error.)Adaptation method:-

There is synchronization betweenTx & Rx, so Don't send: makes sense.


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Thursday 2/21/13 11:05 AMTruncated channel inversion:

For Rayleigh channel:-

Select threshold

EXA: find the power adaptation for BPSK modulation for a target BER Pb =

10^-3. Assuming Rayleigh channel withb) Find outage prob. (Pout)c) Find spectral eff. R/B.

Solution


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2) Variable - rate, variable power schemes:Rate/Power is selected to maximize spectral eff. (R/B), while meeting targetPb.For good ch.But, optimized?Derivation:-

The goal is to maximize R/B:-


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Sunday 2/24/13 11:24 AM

(Water Filling)


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Tuesday 2/26/13 11:02 AMEXA: assume a fading ch. with an average power p, and if p was sent by Txthen Rx SNR has PDF

Assume target BER =

Solution

Spectral eff. of non-adaptive systems:BER is obtained by averaging prob. of error (AWGN) over fade distributionWhich M satisfies (Pb)

Rayleigh ch.


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Thursday 2/28/13 11:05 AMEXA (previous)Find the spectral eff. for non-adaptive system.Solution

3) channel inversion with fixed rate:Assume MQAM, we achieve fixed SNR that satisfies Pb.Choice of M?

We need to max. R/B

Power assignment

Rate eff. (fixed)

EXA: assume the channel

Find power/rate policy and achievable spectral eff., if we use truncated ch.inversion with fixed rate.

Solution


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Sunday 3/3/13 11:04 AMDiscrete - rate adaptation:MQAM , power is cont., M is discrete, Mo = 0 (no transmission) ,M1 = 2 ,M2=4,

Optimal choice of

Regions of

EXA:

Spectral eff.

Algorithm:


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EXA: given a ch. with:-

Solution


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Tuesday 3/5/13 11:06 AMDiscrete - rate adaptation / ch. inversion:DefineTotal inversion:-

Achievable SNR

Truncated inversion:

Algorithm:-

EXA:Solution


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Chapter

Orthogonal FrequencyDivision Multiplexing

(OFDM)


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OFDM is now widely used in standards for broadband comm.Due to its ability to work efficiently in freq. - selective chs.Basic idea:Is to divide a high rate data stream into N - parallel substreams, eachmodulated on a separate carrier, which are all orthogonal

OFDM increases symbol duration (Ts) by a factor of (N) so that:-


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Thursday 3/7/13 11:02 AMMulticarrier comm. system:

Disadvantage:-High complexity in implementation of LPFs, multipliers ... Especially forlarge (N).OFDM using complex multipliers:


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Standard OFDM (using IFFT - FFT):Instead of sending continuous time x(t) (band-limited to BW) we can sendsampled signal (Nyquist rate).

Sample period

N-samples will be sufficient:-1- reconstruct x(t) at Rx.2- more importantly to recover the data


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Sunday 3/10/13 10:57 AM(Standard OFDM):-

OFDM with cyclic prefix:Cyclic prefix: extending the OFDM symbol by copying it's last part into it'sbeginning.

Reasons for CP:-1- prevent interference of adjacent OFDM symbols.2- converts Linear-convolution into circular-convolution to maintainorthogonality between channels.


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Tuesday 3/12/13 11:03 AM

Detection of the OFDM signal:

Assume timing error of ( ):


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Thursday 3/14/13 11:04 AM

Error in starting point of OFDM symbol:Case 1:- Extimated starting point is the exact staring point.

orthogonality is preserved. No ICI.Case 2:- Estimated start is before exact start but after length L.

orthogonality is preserved. No ICI.Case 3:- Estimated start is within 0 to L.Case 4:- Estimated start is after exact start.- case 3 & 4 :- orthogonality is destroyed. ICI + ISI.

Block diagram:

EXA: given OFDM system with:-

Find: a) sub channel BW.b) overhead ratio of CP.c) Data rate if


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Solution


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Sunday 3/17/13 11:03 AMEXA: IEEE 802.11a WLAN

Spectral eff. of OFDM:Ts: OFDM symbol duration.

N: no. of subcarriers.Tch: sampling period = Ts/N.


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PSD of OFDM and reduction of out-of-band radiation:

Reduction techniques:1) Digital filteringDisadvantages:-1- complexity.2- filters impulse response is added to overall ch.

2) Raised cosine windowing.

3) Zero carriers at the edges.


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Tuesday 3/19/13 11:05 AMDesign guidelines for OFDM system:(choice of OFDM parameters)

1) Gaurd interval Tcp (Duration of CP)

2) Ts

Effects of wireless ch. on OFDM system:Time varying >> orthogonality is destroyed.

1) Carrier freq. offset (CFO).2) Doppler spread.3) Sampling clock offset.Time varying ch. effect:OFDM Tx - signal

The demodulated signal:


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Fixed channel:

Effect of CFO:


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Tuesday 3/21/13 11:06 AM


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Case 1:

Case II:

We get circular shiftof detected data.


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Sunday 3/24/13 11:00 AM


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Tuesday 3/26/13 11:05 AMPerformance optimization of OFDM systems:Loading of parallel channels:-1) Rate maximization loading.2) Energy minimization loading.1) Rate maximization loading:Given fixed total power P Maximize bit-rate.

Pi: power assigned to ith ch.P : total power

Np: noise power in each ch.

Assume MQAM with target prob. bit error (Pb)Rx - SNR =

Maximize No. Of bits/OFDM symbol:

The Rx-SNR at the ith ch. ifunity power is transmitted.

Subject to the constraint


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Rate maximization algorithm:-

Margin maximization:Minimize P subject to fixed target bit rate (fixed BER Pb.)


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Algorithm:


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Thursday 3/28/13 11:06 AMEXA: 4- ch. (N=4) OFDM system with a two-tap ch.

Noise power in each ch. = 0.4The available Tx power P = 8Target BER Pb =

Det. The optimum (waterfilling) power assignment for each subcarrier andthe total no. of bits/OFDM symbol.


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Power Assaignment:-

Rate Assaignment:-

Channel Estimation:


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Sunday 3/31/13 11:06 AMEstimation of ch. requires sending pilot symbols (known data) at somesubcarriers.

Interpolation is used to estimate the ch. at other subcarriers.Pilot structure:1) block type:-Pilots transmitted on all subcarriers for one OFDM symbol.

St must satisfyTime - domain interpolation is used to estimate the ch. at other OFDMsymbols.

Suitable for highly freq-selective fading, but not fast-fading.2) comb. type:-

Freq. domain interpolation is used to find ch. estimation at other subcarriers.Sf must satisfySuitable for fast - fading, but not for high - selectivity in freq.

3) Lattice type:-


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Tuesday 4/2/13 11:06 AMBasic definitions:Training data (pilot):-

Least - squares ch. estimation:We want to find

that minimizes

Minimum Mean Squared Error (MMSE) estimation:Used to find:-a) chs. at the same pilots.b) chs. at other chs.


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Given LS - estimation of pilot chs. :Define

Define the MMSE estimation:-Ch. vector to be interpolated.

To estimate some chs. &improve other chs.


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Sunday 4/7/13 11:04 AMMMSE ch. estimator (Weiner estimator):

we need to minimize

Minimizing is eqnt. to solving:-

Elements of

Power DelayProfile

(Principle of orthogonality)


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Tuesday 4/9/13 11:05 AM

This example is an applicationfor video broadcasting ======>>


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Tuesday 4/16/13 11:06 AMDecision directed channel estimation:Used with block type plots.

For slowly varying ch.1) ch. at the kth subcarrier is estimated by LS.

2) assuming ch. has not changed (approx. the same) in the next symbol, wecan estimate data at t = (n+1) using the estimate at (n)

3)

4) process is repeated for OFDM symbols until the next pilot comes.Linear / 2nd order interpolation for comp type pilots:

2nd order interpolation:

DFT - band ch. estimation:This method improves LS-estimate.


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Synchronization in OFDM systems:

1- Sample time offset (STO):-STO width means finding the starting point of the OFDM symbol.2- carrier freq. offset (CFO) estimate:-Error in the local oscillator compensated to Tx oscillator.

Once the STO, CFO are estimated they will be compensated for & Rx is nowsynchronized.


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Thursday 4/18/13 11:21 AM1) Estimation of STO:1- Using cyclic prefix:-We use the similarity of CP with the end portion of the OFDM symbol.We find point of maximum similarity.

Let estimated STO be

Absolute deference

here is within CP


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Tuesday 4/23/13 11:16 AMSolution of 1st exam:Q1:-

Q2:-


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Thursday 4/25/13 11:01 AM


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Tuesday 4/28/13 11:04 AM

Channel capacity (waterfilling):

Algorithm:

EXA: find the ch. capacity for:-

Solution


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EXA: find the capacity for:-

Solution


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ChapterSpace - TimeCoding


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Transmitt diversity:-

Time delay diversity:-

Block (Alamouti) ST (Space Time) code:


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Tuesday 4/30/13 11:04 AM

Real space - time coding:Code rate = 1

Square matrix

(Quantization to nearest pt. in constellation)


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R = K No. Of symbols transmitted.

No. Of time instants.

Non-Square:


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Techs for 4th Generation Wireless Communication