TRUYỀN THÔNG SỐ DIGITAL COMMUNICATION ppt

– Power and energy spectral densities– Noise in communication systems – Signal transmission through linear systems – Bandwidth of signal 6... Classification of signals• Deterministic and

TRUYỀN THÔNG SỐ DIGITAL COMMUNICATION

30 tiết, 2 tín chỉ

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Mục tiêu:

• Học phần này, tiếp theo học phần Các hệ thống truyền

thông, có mục tiêu cung cấp cơ sở cho các học phần

Truyền thông không dây, Truyền thông di động

Đánh giá kết quả học tập:

• Bài tập ở nhà nộp cho giảng viên: 30% tổng điểm

• Thi hết môn học: 70%

Tài liệu tham khảo:

– Bài giảng (Dựa trên bài giảng của Uppsala University và cuốn 1)

– 1 Digital Communications Fundamentals and Applications, Bernard Sklar

– 2 “Digital Communications”, Jonh G Proakis, McGraw-Hill, 2001 – Internet

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What is Digital Communication?

• Digital Communication is any message passed through digital devices

• Digital Communication can be easy and quick to use

• E.g of digital communication are:

Nội dung môn học Contents of the course

1 Source coding / Mã hóa nguồn

2 Channel coding / Mã hóa kênh

3 Modulation (Baseband and passband

signaling) / Điều biến

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– Power and energy spectral densities

– Noise in communication systems

– Signal transmission through linear systems

– Bandwidth of signal

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Key blocks:

• Formatting: transforms the source information into bits.

• Modulation: the process by which message symbols or channel

symbols are converted to waveforms that are compatible with the requirements imposed by the transmission channel

• Pulse modulation: transform each symbol from a binary

representation to a baseband waveform

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Key blocks:

• Bandpass modulation:

basepand waveform g i (t) is frequency translated by a carrier wave to

a frequency that is much larger than g i (t), called bandpass waveform

s i (t), i = 1, … , M (M-ary pulse waveform types).

• Source coding: converts A/D (for analog souces) and removes

redundant information  formatting transformation (for digizing)

• Channel coding, for a given data rate, can deduce the probability of

error, P E, at the expense of transmission BW or decoder complexity

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Classification of signals

• Deterministic and random signals

– Deterministic signals: whose values are completely

specified for any given time Thus, a deterministic signal can be modeled by a known function of time

• E.g x(t) = 5.cos10t

– Random signals: also called non deterministic signals

are those signals that take random values at any given

time and must be characterized statistically

• E.g Noise in electronic circuits

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Deterministic signals Random signal

Classification of signals …

• Periodic and non-periodic signals

• Analog and discrete signals

A discrete signal Analog signals

A non-periodic signal

A periodic signal

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Classification of signals

• Energy and power signals: as ways to measure a signal.

– A signal is an energy signal (tín hiệu năng lượng) if, and only if, it has

nonzero but finite energy for all time:

– A signal is a power signal (tín hiệu công suất) if, and only if, it has finite

but nonzero power for all time:

– General rule: Periodic and random signals are power signals Signals that are

both deterministic and non-periodic are energy signals.

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The energy of this signal is the shaded region

A simple, common signal with infinite energy

Bài tập

Phân loại tín hiệu: energy signals hay power signals

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• Correlation = a matching process.

• Autocorrelation = matching of a signal with a delayed version of itself

• Autocorrelation function of a real-valued energy signal x(t):

• The autocorrelation function provides a measure of how closely the signal matches a copy of itself as the copy is shifted τ units in time

• Autocorrelation of a power signal

– For a periodic signal:

• Autocorrelation of a random signal

– For a WSS process:

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Remind: Cross-correlation

• The cross-correlation function describes the

general dependency of x(t) with another

random process y(t+ τ ), delayed by a time delay,

τ

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Random process

1 The collection of random signals is called a random process.

2 Each signal in this collection is referred to a realization or sample

function of the process.

Sample functions

or realizations (deterministic function)

Random variables

Random process …

• Wide sense stationary (WSS): If the mean and autocorrelation function do not

change with a shift in the origin time

• Ergodic process: A random process is ergodic in mean and autocorrelation, if

and

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n 2

T

2

T -

2

0

0 0

Fig Autocorrelation and Power Spectral Density

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Bài tập

Tìm Ex hay Px tương ứng

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Noise in communication systems

 Thermal noise is described by a zero-mean Gaussian random process, n(t).

 Its PSD is flat, hence, it is called white noise.

[w/Hz]

Probability density function

Power spectral density of white noise

Autocorrelation function of white noise

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 Thermal noise is present in all comm systems  The thermal noise characteristics – additive white Gaussian noise AWGN – are most often used to model the noise in comm systems.

• Input - output relationships :

Relation between spectral density of output and spectral density of input :

|H(f)| 2 is a transfer function, frequency response function

(f)S

.H(f)

A (f)

Signal transmission through linear systems

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• Recall: In Fourier Analysis, signals cannot be

limited both time and frequency(band)

• For real-world (time-limited) signals this

means BW can be difficult to define

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Bandwidth of signal …

• Different definition of bandwidth:

(a) Half-power bandwidth

(b) Noise equivalent bandwidth

(c) Null-to-null bandwidth

(d) Fractional power containment bandwidth

(e) Bounded power spectral density

(f) Absolute bandwidth

(a) (b) (c) (d)

• Half-power BW: Gx(f) drop to half-power or -3 dB

over all freq.

• Null-to-null BW: the most popular measure of BW for

digital communications is the width of the main spectral

lobe, where most of the power is contained.

• Fractional power containment BW: the band leaves 0.5% of the signal power above the upper band limit and 0.5% of the signal power below the lower band limit.

• Bounded power spectral density: the band leaves a certain stated level Typical attenuation levels = 35 or 50 dB.

• Absolute bandwidth = infinite.

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• Important features of digital communication systems

• Some basic concepts and definitions as

signal classification, spectral density,

random process, linear systems and signal

bandwidth.

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