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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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