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Ch10. Structure of a W.

Communication Link

Instructor:

• Mohammed Taha O. El Astal

1.Information Source

Real time voice.

oNeed to arrive to Rx. in same order.

Real time video

oNeed to arrive to Rx. in same order.

Data (voice file/video file/image/…etc)

oNeed not to arrive in same order.

Information Source

2.ADC

Any analog source must feed into ADC.

ADC process done through :

1. Sampling.

2. Quantization.

3. Encoding.

Imp. factors :

1. Sample rate (Sampling frequency).

2. Number of Quantization Levels

ADC

CONT.

3.Source Coder

Uses a priori information on the properties of the source data in order to

reduce redundancy in the source signal.

This lead to reduce required time and/or BW.

i.e.:

1. In Fax (factor of 10)

2. In GSM (64kb/s to 13kb/s).

Source Coder

3.Channel Coder

Adds redundancy in order to protect data against transmission errors.

o i.e.: In GSM (13kb/s to 22.8kb/s).

It will increased the required data rate at the output.

Reed-Solomon/BCH/and Hamming codes.

Simplest form: parity checker.

Channel Coder

4.Multiplexer

It combines user data and signaling info.,

and combines the data from multiple users.

Signaling Information : it is control

information for the establishing and ending

of connections, synchronization, etc.

In GSM, 22.8 to 182.4 Kbit/s (8 · 22.8)

271 Kbits/sec

MUX

Signaling

8 timeslots of 577 μs in a 4.615 ms

114 of user data and 35 of control data

5. Baseband Modulator

Assigns the gross data bits (user data and signaling) to complex transmit

symbols in the baseband.

The OP. is TX. symbols in oversampled form, discrete in both of time and

amplitude

Baseband Modulator

6. The TX. DAC / 7.LP Filter

Generates a pair of analog, discrete amplitude

voltages corresponding to the real and imaginary part

of the transmit symbols, respectively.

Due to the imperfections of :

oBaseband modulator.

oDAC.

a spurious emissions have raised.

The analog low-pass filter will eliminates the

(inevitable) spectral components outside the desired

transmission bandwidth.

TX. DAC LP Filter

8. TX Local Oscillator:

Up Converter

TX. LO

Provides an unmodulated sinusoidal

signal with specific carrier frequency.

Up conversion can occur in a single

step, or in several steps.

9. RF. TX. Filter:

RF. TX. Filter

Even if the low-pass filter succeeded

in eliminating all out-of-band

emissions, up conversion can lead to

the creation of additional out-of-band

components..

10. The propagation channel:

Channel

Attenuation

Delay and Frequency Dispersion

AWGN.

Co-channel interference.

11. RF. Filter:

Rx. Filter

Performs a rough selection of the received band of the service.

12. The low-noise amplifier :

L.N. Amplifier

It amplifies the signal.

Why? To let the noise which will be added by later components of the RX

chain has less effect on the (SNR)

LNA LNB

13. RX LO / 14. Down converter:

Down Converter

RX. LO

Down conversion can occur in a single step, or in several steps.

The RX LO : provides sinusoidal signals corresponding to that at the TX LO.

The frequency of the LO can be fine-tuned by a carrier recovery algorithm

to make sure that the LOs at the TX and the RX produce oscillations with the

same frequency and phase.

Carrier recovery determines the frequency and phase of the carrier of the

received signal, and uses it to adjust the RX LO.

Carrier

Recovery

15. RX. Baseband Filter:

Rx. Baseband Filter

Provides a selection of desired frequency

bands for one specific user .

It eliminates adjacent channel interference

as well as noise.

16. AGC / 17.ADC / 18. Demod. /19.Decoder :

AGC

Automatic Gain Control (AGC): Amplifies the signal such that its level is

well adjusted to the quantization at the subsequent ADC.

The Rx. ADC : converts the analog signal into values that are discrete in time

and amplitude.

The baseband demodulator: obtains soft-decision data from digitized

baseband data, and hands them over to the decoder.

The decoder: uses soft estimates from the demodulator to find the original

(digital) source data.

Rx. ADCBaseband

Demod.Decoder

20. DeMux / 21. Ch. Decoder /22. S. Decoder:

DeMux

The demultiplexer separates the user data and signaling information and

reverses possible time compression of the TX multiplexer.

The Ch. Decoder : reverse operation of channel encoder.

The Source Decoder: reverse operation of source encoder.

The DAC: if voice and video , reconstruct the analog signal from the digital

data.

Ch. Decoder S. Decoder DAC

Signaling

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