通訊系統(二)第六單元 6-1 第六單元 數位信號經AWGN通道傳輸 單元要點: 1. 信號空間 2. 匹配濾波器與相關函數 3. 信號距離與錯誤機率計算 4. 基頻數位傳輸 4.1 二位元信號傳輸 4.2 多振幅信號傳輸 4.3 多維度信號傳輸 5. Regenerative repeater 1. 信號空間 Sec. 5.1, 5.2, pp. 309-317, Haykin 有許多正交函數集合可用來表示某一時間區間內之函數,稱為函數之基底 函數,例如 { } ,... 2 , 1 , 0 , 0 ± ± = n e t jnω , Walsh functions, Legendre polynomials, Laguerre functions等。其中eternal functions之集合是傅氏分析的基底。而Walsh functions 在實用上很重要,因為它易於用邏輯電路實現。針對M個信號求其 ) ( M N ≤ 個基 底函數:Gram-Schmidt orthogonalization process。 練習題1-1:(C-12-6) Two signals which might be used in a digital communication system are sketched below: For these two signals, determine (a) each signal's energy (b) a single set of orthonormal basis functions which can be used to represent each signal (c) the distance between the signal in signal space. 練習題 1-2:(Prob. 5.1, p. 338, Haykin) In Section 3.7 we described line codes for pulse-code modulation. Referring to the material presented therein, formulate the signal constellations for the following line codes: (a) Unipolar nonreturn-to-zero code Signal #1 Signal #2 1 1 -1 -1 0 0 .25 .25 .5 .5 .75 .75 1 1 t t
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Two signals which might be used in a digital communication system are sketched below:
For these two signals, determine (a) each signal's energy (b) a single set of orthonormal basis functions which can be used to
represent each signal (c) the distance between the signal in signal space.
練習題 1-2:(Prob. 5.1, p. 338, Haykin)
In Section 3.7 we described line codes for pulse-code modulation. Referring to the material presented therein, formulate the signal constellations for the following line codes:
where iA = 1± , 3± , 5± , 7± . Formulate the signal constellation of ( ){ }81=ii ts .
練習題 1-4:(Prob. 5.3, p. 338, Haykin) The following figure displays the waveform of four signals ( )ts1 , ( )ts2 , )(3 ts and
)(4 ts . (1) Using the Gram-Schmidt orthogonalization procedure , find an orthonormal
basis for this set of signals. (2) Construct the corresponding signal-space diagram.
練習題 1-5:(Prob. 5.4, p. 338, Haykin) (a) Using the Gram-Schmidt orthogonalization procedure, find a set of orthonormal
basis functions to represent the three signals s1(t), s2(t), and s3(t) shown in Figure P5.4.
(b) Express each of these signals in terms of the set of basis functions found in part (a).
FIGUR
t t t
( )ts2 ( )ts1 ( )ts3 ( )ts4
1 1 1 1
0 3T 0 3
T T 0 T 0 32T
t
通訊系統(二)第六單元
6-3
E P5.4 練習題 1-6:(Prob. 5.6, p. 339, Haykin)
A source of information emits a set of symbols denoted by { }Miim 1= . Two
candidate modulation schemes, namely, pulse-duration modulation (PDM) and pulse-position modulation (PPM), are considered for the electrical representation of this set of symbols. In PDM, the ith symbol is represented by a pulse of unit amplitude and duration (i/M)T. On the other hand, in PPM, the ith symbol is represented by a short pulse of unit amplitude and fixed duration, which is transmitted at time t = (i/M)T. Show that PPM is the only one of the two that can produce an orthogonal set of signals over the interval 0 ≤ t ≤ T. 練習題 1-7:(Prob. 5.7, p. 339, Haykin)
A set of 2M biorthogonal signals is obtained from a set of M orthogonal signals by augmenting it with the negative of each signal in the set.
(a) The extension of orthogonal to biorthogonal signals leaves the dimensionality of the signal space unchanged. Why?
(b) Construct the signal constellation for the biorthogonal signals corresponding to the pair of orthogonal signals shown in Figure P5.5.
FIGURE P5.5 練習題 1-8:(Prob. 5.8, p. 339, Haykin) (a) A pair of signals si(t) and sk(t) have a common duration T. Show that the inner
product of this pair of signals is given by
( ) ( ) k
T Tiki ssdttsts∫ =
0
where si and sk are the vector representation of si(t) and sk(t), respectively. (b) As a followup to part (a), show that
( ) ( )( ) 2
0
2 |||| k
T
iki ssdttsts∫ −=−
練習題 1-9:(Prob. 5.9, p. 339, Haykin)
Consider a pair of complex-valued signals s1(t) and s2(t) that are respectively
通訊系統(二)第六單元
6-4
represented by s1(t) = a11ψ1(t) + a12ψ2(t), -∞ < t < ∞ s2(t) = a21ψ1(t) + a22ψ2(t), -∞ < t < ∞
where the basis functions ψ1(t) and ψ2(t) are both real valued, but the coefficients a11, a12, a21, a22 are complex valued. Prove the complex form of the Schwarz inequality:
( ) ( ) ( ) ( ) dttsdttsdttsts2
2
2
1
2*21 ∫∫∫
∞
∞−
∞
∞−
∞
∞−≤
where the asterisk denotes complex conjugation. When is this relation satisfied with the equality sign?
統。 針對 rectangular pulse of amplitude A and duration T之 matched filter 的特
例(Ex. 4.1, p. 252, Haykin),
−
Λ=
−Π==
TTtTkAtg
T
Ttthtg
o2)(
2)()(
因只觀察 t=T時之輸出,可用 Fig. 4.3, p. 253, Haykin 之 integrate-and-dump電路(積分器後接在時間 t=T取輸出之開關)實現。下題以 RC低通濾波器實現,比較 S/N比,看其性能可達理想所得多少百分比。 練習題 2-1:(Prob. 4.4, p. 301, Haykin) If the ideal integrator is replaced by the simple resistance-capacitance (RC) low-pass
filter, determine the output SNR as a function of the time constant RC. Determine the value of RC that maximizes the output SNR. The frequency response of this filter is
RCfwhere
ffj
fHπ21
1
1)( 0
0
=+
=
The requirement is to optimize the selection of the 3-dB cutoff frequency 0f of the filter so that the peak pulse signal-to-noise ratio at the filter output is maximized. With this objective in mind, show that the optimum value of 0f is 0.2/T, for which the loss in signal-to-noise ratio compared to the matched filter is about 1 dB. 另見練習題 3.1-2-2 (Prob. 7.17, p. 459, Proakis) 練習題 2-2:(Prob. 7.31, p. 463, Proakis) In the case when n is a power of 2, an nn × Hadamard matrix is constructed by means of the recursion
−
=1111
2H
−
=nn
nnnH
HHHH
2
通訊系統(二)第六單元
6-11
(a) Let Ci denote the ith row of an nn × Hadamard matrix as defined above. Show that the waveforms constructed as
nikTtpctsn
kciki ,,2,1,)()(
1K=−= ∑
=
are orthogonal, where p(t) is an arbitrary pulse confined to the time interval cTt ≤≤0 .
(b) Show that the matched filters (or crosscorrelators) for the n waveforms {si(t)} can be realized by a single filter (or correlator) matched to the pulse p(t) followed by a set of n crosscorrelators using the code words {Ci}.
練習題 2-3:(D-6-4)
For the pairs of binary signaling waveforms sketched below, determine whether they are antipodal, orthogonal, and determine the matched filter for each.
例題 3-1:(C-5-3, C-6-1, C-6-2) Three pulses sketched below are used to communicate three messages in a digital communication system. (a) Determine an orthonormal basis which spans this signal set, and determine the
orthonormal series expansion for each signal. (b) Determine the norm for each signal. (c) Determine the distance between each pair of signals. (d) Determine the cross-correlation and correlation coefficient between each pair of
signals. (e) If only two messages are to be communicated, which pair of signals is the best
choice? (f) For each of these signals, determine a signal which is antipodal and a signal which
is orthogonal. (g) Determine the impulse response h(t) for the filter matched to the pulse if the
sampling time is 0.75 sec, 1.0 sec, or 1.5 sec. (h) For your answer in Part (g), sketch the output of the matched filter for all time
when the filter’s input signal is the signal to which the filter is matched.
通訊系統(二)第六單元
6-16
(i) Sketch the response for all time of the filters matched to signal #2 and #3 designed for a 1 second sampling time when signal #1 is used as input.
(j) Design matched filters with a one-second sampling time for signals which are orthogonal and antipodal to signal #1 determined in Part (f). Sketch their output for all time when signal #1 is used as the input. How do your sketches compare at the sampling time to your results in Part (i)? Why?
(k) Determine the bit error probability if the signals are #1 and #2. (l) Determine the bit error probability if the signals are #1 and #3. (m) Determine the bit error probability if the signals are #2 and #3. (n) Determine the bit error probability if the signals are #1 and its orthogonal signal. (o) Determine the bit error probability if the signals are #1 and its antipodal signal. #1 )(1 tS #2 )(2 tS #3 )(3 tS 2 1 1.58 1 1 0 0 0.5 1 0 0.5 0.5 1 -1.58 -2 圖中三個脈波用來傳輸在數位通訊系統中的三個訊息(message) 1. 信號空間計算(Signal space computations) (a)求生成(span)此信號集的正規化基底(orthonormal basis),並求每個信號之正交級數展開式(orthonormal series expansion)。
注意:正相反信號對(antipodal signal pair)有最低的 p[error]位元錯誤機率因為此兩個信號間的距離最大。而其中 Q(x)的函數值亦可經由查表方式求得。 練習題 3-1:(Prob. 4, mid1989, Pawlowski)
A baseband digital communication system uses the two symbols sketched below to transmit information:
通訊系統(二)第六單元
6-24
1) Sketch the impulse response )(0 th and )(1 th , each with a sampling time of
1 µsec, for filters matched to )(0 ts and )(1 ts respectively. 2) Sketch the optimum receiver for this system. Clearly label and define each
function, and specify all parameter values 3) .Assume E = 19.22, and that zero-mean, WSS, AWGN with N0 = -60dBW/Hz
is added to the transmitted signal. Determine the symbol error probability at the receiver output.
4) If the system organizes its information in words which are eight symbols in length, determine the word error probability at the receiver output.
5) Is the signal s1(t) the best possible choice when s0(t) is used for binary baseband signaling? If your answer is yes, explain why. If your answer is no, also explain why not and sketch a better choice of s1(t).
4. 基頻數位傳輸
針對基頻數位信號傳輸,可分成三個方面討論,一為二位元信號傳輸,其
次是多振幅信號傳輸,三是多維度信號(multidimensional signal)傳輸。接下來分別對各種傳輸信號經 AWGN(Additive White Gaussian Noise)通道,以理論分析比較其性能(performance),所用之性能指標為錯誤機率(error probability)與信號雜訊比(SNR)間的關係,並且用電腦加以模擬結果,相互比較其特性。 4.1 二位元信號傳輸 二位元信號傳輸之傳輸信號可分為三種,一為正交(orthogonal)信號,二
圖 6 )(0 ts 與 )(1 ts 為正交 練習 4.1-1-3: 我們也可以試著將相關器改成匹配濾波器,再做比較。 練習 4.1-1-4: (Prob.5.5, p. 338, Haykin) An orhtogonal set of signals is characterized by the property that the inner product of any pair of signals in the set is zero. The following figure shows a pair of signals s0(t) and s1(t) that satisfy this condition.
1) Construct the signal constellation for s0(t) and s1(t).
Further problems:
2) Determine the correlator outputs at the sampling instants. 3) Consider the use of matched filters for the demodulation of the signals shown,
determine the outputs. 4) Consider the detector for the signals shown, which are equally probable and
have equal energies. The optimum detector for these signals compares r0 and r1 and decides a 0 was transmitted when r0 > r1 and that a 1 was transmitted when r1 > r0. Determine the probability of error.
練習 4.1-1-5: (Prob. 5.12, p. 340, Haykin)
s0(t) s1(t)A A
0 0Tb
TbTb/2t t
-A
通訊系統(二)第六單元
6-30
Figure P5.12 shows a pair of signals s1(t) and s2(t) that are orthogonal to each other over the observation interval 0 ≤ t ≤ 3T. The received signal is defined by x(t) = sk(t) + w(t), 0 ≤ t ≤ 3T k = 1, 2 where w(t) is white Gaussian noise of zero mean and power spectral density N0/2. 1) Design a receiver that decides in favor of signals s1(t) or s2(t), assuming that
these two signals are equiprobable. 2) Calculate the average probability of symbol error incurred by this receiver for
E/N0 = 4, where E is the signal energy.
FIGURE P5.12 例題 4.1-2: 正反信號(Antipodal Signals)配對
前面我們討論使用正交信號做為二位元信號傳輸的方法,接下來我們要討
論使用正負相反信號(antipodal signal)做為二位元信號傳輸的方法。首先設 0s 與
1s 叫的信號波形正好是正負相反,也就是 )()(0 tsts = 和 )()(1 tsts −= ,如圖 7所示。圖中上方配對即為 Sec. 4.3, pp. 253-258, Haykin所使用。經過 AWGN通道的接收信號為
can be accomplished by use of a single integrator, as shown in the following figure , which is sampled periodically at t= kT, k=0, 1± , 2± , ….. The additive
noise is zero-mean Gaussian with power-spectral density of 2
0N W/Hz.
1) Determine the output SNR of the demodulator at t = T
通訊系統(二)第六單元
6-34
2) If the ideal integrator is replaced by the RC filter shown in the figure, determine the output SNR as a function of the time constant RC.
3) Determine the value of RC that maximizes the output SNR.
練習題 4.1-2-3:(Prob. 7.23, p. 460, Proakis, 7.32)
Consider a signal detector with an input nAr +±= where +A and –A occur with equal probability and the noise variable n is
characterized by the (Laplacian) p.d.f. shown in the following figure . 1 ) Determine the probability of error as a function of the parameters A and σ . 2 ) Determine the “SNR” required to achieve an error probability of 510 − . How dose the SNR compare with the result for a Gaussian p.d.f.?
練習題 4.1-2-4:(Prob. 5.13, pp. 339-340, Haykin , Prob. 7.24, p. 461, Proakis, 7.33)
A Manchester encoder maps an information 1 into 10 and a 0 into 01. If the output of the encoder is transmitted by use of NRZ, the signal waveforms corresponding to the Manchester code are shown in the following figure. Determine the probability of error if the two signals are equally probable.
Assume a typical binary sequence and show that if the corresponding polar NRZ signal and unipolar NRZ signal have the same peak-to-peak amplitude, the polar signal has less power (an advantage) than the unipolar signal. If noise is added to these signals, how do the probabilities of error compare for these two signaling techniques? 例題 4.1-4:信號星座圖 前面所講的那三種二位元信號:正交、antipodal及 on-off信號配對用『信號
Determine the average energy of a set of M PAM signals of the form
TtMmtsts mm ≤≤== 0,,2,1),()( Kψ
where MmAEs mgm ,,2,1, K== . The signals are equally probable with amplitudes that are symmetric about zero and are uniformly spaced with distance
d between adjacent amplitudes as shown in the above figure. The decision is made in favor of the amplitude level that corresponds to the smallest distance. Determine the probability of error for the optimum detector.
Consider an M-ary digital communication system where NM 2= , and N is the dimension of the signal space. Suppose that the M signal vectors lie on the
通訊系統(二)第六單元
6-50
vertices of a hypercube that is centered at the origin, as illustrated in the following figure. Determine the average probability of a symbol error as a function of
0/ NEs where sE is the energy per symbol, 2/0N is the power-spectral density of the AWGN, and all signal points are equally probable.
練習題 4.3-4: (Prob. 7.19, p. 460, Proakis, 7.28) Three equally probable messages 1m , 2m , and 3m are to be transmitted over an
AWGN channel with noise power-spectral density 2
0N . The message are
≤≤
=otherwise
Ttts
001
)(1
≤≤−
≤≤
=−=
otherwise
TT
Tt
tsts
0
02
12
01
)()( 32
1 ) What is the dimensionality of the signal space? 2 ) Find an appropriate basis for the signal space (Hint: You can find the basis
without using the Gram-Schmidt procedure). 3 ) Draw the signal constellation for this problem. 4 ) Derive and sketch the optimal decision regions 1R , 2R , and 3R . 5 ) Which of the three messages is more vulnerable to errors and why? In other
words, which of p(Error im transmitted), i=1, 2, 3 is larger?
1s
3s
2s
4s
)(1 tψ
)(2 tψ
)(1 tψ
)(2 tψ
)(3 tψ1s
2s
3s
4s
5s
8s
7s
6s
N=2
N=3
通訊系統(二)第六單元
6-51
練習題 4.3-5: (Prob. 7.20, p. 460, Proakis, 7.29)
An optimal demodulator can be realized as: A correlation-type demodulator A matched-filter-type demodulator
Where in both cases )(tjψ , Nj ≤≤1 were used for correlating r(t) or designing the match filters. Show that an optimal demodulator for a general M-ary communication system can also be designed based on correlating r(t) with )(tsi ,
Mi ≤≤1 , or designing filters that are matched to stsi )'( , Mi ≤≤1 . Precisely describe the structure of such receivers by giving their block diagram and all relevant design parameters.
練習題 4.3-6: (Prob. 7.22, p. 460, Proakis, 7.31) In an additive white Gaussian noise channel with a noise power-spectral density of
20N , two equiprobable messages are transmitted by
≤≤=
otherwise
TtTAt
ts0
0)(1
≤≤−=
otherwise
TtTAtAts
0
0)(2
1) Determine the structure of the optimal receiver. 2) Determine the probability of error.
通訊系統(二)第六單元
6-52
5. Regenerative repeater (Fig 3.18, p.208, Haykin) 回憶:Repeaters for analog communication systems
值得注意的是,衛星通訊的 On Board Processing(OBP)系統與地面 Wireline Channel 使用之 regenerative repeater意思是一樣的。 Ex. 7 (Ex.7.7.1, p.438, Proakis) A binary digital communication system transmits data over a wireline channel of length 1000km. Repeaters are used every 10km to offset the effect of channel
attenuation. Let us determine the o
b
NE that is required to achieve a probability of a bit
error of 510− if (a) analog repeaters are employed , and (b) regenerative repeaters are employed. The number of repeaters used in the system is K=100. If regenerative
repeaters are used, the o
b
NE obtained from (2.54) is
510− = ⇒)(100 2o
bNEQ 710− = )( 2
o
b
NEQ
which yields approximately 11.3 dB. If analog repeaters are used, the o
b
NE obtained
from (2.53) is 510− = )( 1002
o
b
NEQ , which yields
o
b
NE ≈29.6 dB. Hence, the difference in
the required SNR is about 18.3dB, or approximately 70 times the transmitter power of the digital communication system. 練習題 (Prob. 7.55, p. 471, Proakis, 7.44) Consider a transmission line channel that employs n-1 regenerative repeaters plus the terminal receiver in the transmission of binary information. We assume that the probability of error at the detector of each receiver is p and that errors among repeaters are statistically independent.
(1) Show that the binary error probability at the terminal receiver is
[ ]nn pP )1(1
21
−−=
(2) IF 610−=p and n=100, determine an approximate value of Pn. (Prob. 7.56, p. 471, Proakis, 7.45) A digital communication system consists of a transmission line with 100 digital (regenerative) repeaters. Binary antipodal signals are used for transmitting the information. If the overall end-to-end error probability is 610−=p , determine the
probability of error for each repeater and the required 0/ NEb to achieve this