techs for 4th generation wireless communication
TRANSCRIPT
-
8/12/2019 Techs for 4th Generation Wireless Communication
1/69
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:
-
8/12/2019 Techs for 4th Generation Wireless Communication
2/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
3/69
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:
-
8/12/2019 Techs for 4th Generation Wireless Communication
4/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
5/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
6/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
7/69
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 ...
-
8/12/2019 Techs for 4th Generation Wireless Communication
8/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
9/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
10/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
11/69
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:-
-
8/12/2019 Techs for 4th Generation Wireless Communication
12/69
Sunday 2/24/13 11:24 AM
(Water Filling)
-
8/12/2019 Techs for 4th Generation Wireless Communication
13/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
14/69
-
8/12/2019 Techs for 4th Generation Wireless Communication
15/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
16/69
-
8/12/2019 Techs for 4th Generation Wireless Communication
17/69
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:
-
8/12/2019 Techs for 4th Generation Wireless Communication
18/69
EXA: given a ch. with:-
Solution
-
8/12/2019 Techs for 4th Generation Wireless Communication
19/69
Tuesday 3/5/13 11:06 AMDiscrete - rate adaptation / ch. inversion:DefineTotal inversion:-
Achievable SNR
Truncated inversion:
Algorithm:-
EXA:Solution
-
8/12/2019 Techs for 4th Generation Wireless Communication
20/69
Chapter
Orthogonal FrequencyDivision Multiplexing
(OFDM)
-
8/12/2019 Techs for 4th Generation Wireless Communication
21/69
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:-
-
8/12/2019 Techs for 4th Generation Wireless Communication
22/69
Thursday 3/7/13 11:02 AMMulticarrier comm. system:
Disadvantage:-High complexity in implementation of LPFs, multipliers ... Especially forlarge (N).OFDM using complex multipliers:
-
8/12/2019 Techs for 4th Generation Wireless Communication
23/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
24/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
25/69
-
8/12/2019 Techs for 4th Generation Wireless Communication
26/69
Tuesday 3/12/13 11:03 AM
Detection of the OFDM signal:
Assume timing error of ( ):
-
8/12/2019 Techs for 4th Generation Wireless Communication
27/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
28/69
Generated by CamScanner
Solution
-
8/12/2019 Techs for 4th Generation Wireless Communication
29/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
30/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
31/69
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:
-
8/12/2019 Techs for 4th Generation Wireless Communication
32/69
Fixed channel:
Effect of CFO:
-
8/12/2019 Techs for 4th Generation Wireless Communication
33/69
Generated by CamScanner
Tuesday 3/21/13 11:06 AM
-
8/12/2019 Techs for 4th Generation Wireless Communication
34/69
Case 1:
Case II:
We get circular shiftof detected data.
-
8/12/2019 Techs for 4th Generation Wireless Communication
35/69
-
8/12/2019 Techs for 4th Generation Wireless Communication
36/69
Generated by CamScanner
Sunday 3/24/13 11:00 AM
-
8/12/2019 Techs for 4th Generation Wireless Communication
37/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
38/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
39/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
40/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
41/69
Rate maximization algorithm:-
Margin maximization:Minimize P subject to fixed target bit rate (fixed BER Pb.)
-
8/12/2019 Techs for 4th Generation Wireless Communication
42/69
Algorithm:
-
8/12/2019 Techs for 4th Generation Wireless Communication
43/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
44/69
Power Assaignment:-
Rate Assaignment:-
Channel Estimation:
-
8/12/2019 Techs for 4th Generation Wireless Communication
45/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
46/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
47/69
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:-
-
8/12/2019 Techs for 4th Generation Wireless Communication
48/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
49/69
Given LS - estimation of pilot chs. :Define
Define the MMSE estimation:-Ch. vector to be interpolated.
To estimate some chs. &improve other chs.
-
8/12/2019 Techs for 4th Generation Wireless Communication
50/69
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)
-
8/12/2019 Techs for 4th Generation Wireless Communication
51/69
Generated by CamScanner
Tuesday 4/9/13 11:05 AM
This example is an applicationfor video broadcasting ======>>
-
8/12/2019 Techs for 4th Generation Wireless Communication
52/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
53/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
54/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
55/69
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.
-
8/12/2019 Techs for 4th Generation Wireless Communication
56/69
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
-
8/12/2019 Techs for 4th Generation Wireless Communication
57/69
Tuesday 4/23/13 11:16 AMSolution of 1st exam:Q1:-
Q2:-
-
8/12/2019 Techs for 4th Generation Wireless Communication
58/69
Generated by CamScanner
Thursday 4/25/13 11:01 AM
-
8/12/2019 Techs for 4th Generation Wireless Communication
59/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
60/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
61/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
62/69
Tuesday 4/28/13 11:04 AM
Channel capacity (waterfilling):
Algorithm:
EXA: find the ch. capacity for:-
Solution
-
8/12/2019 Techs for 4th Generation Wireless Communication
63/69
EXA: find the capacity for:-
Solution
-
8/12/2019 Techs for 4th Generation Wireless Communication
64/69
ChapterSpace - TimeCoding
-
8/12/2019 Techs for 4th Generation Wireless Communication
65/69
Transmitt diversity:-
Time delay diversity:-
Block (Alamouti) ST (Space Time) code:
-
8/12/2019 Techs for 4th Generation Wireless Communication
66/69
Tuesday 4/30/13 11:04 AM
Real space - time coding:Code rate = 1
Square matrix
(Quantization to nearest pt. in constellation)
-
8/12/2019 Techs for 4th Generation Wireless Communication
67/69
R = K No. Of symbols transmitted.
No. Of time instants.
Non-Square:
-
8/12/2019 Techs for 4th Generation Wireless Communication
68/69
Generated by CamScanner
-
8/12/2019 Techs for 4th Generation Wireless Communication
69/69
The End