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# describe the two principles and explain where appropriate to use and principle. What types of media do the different principles fit? Motivate!

INSTRUCTIONS TO CANDIDATES

The theoretically part

There are two principles for transmitting digital information over a channel, depending on the channel

properties: either send the data signal “direct” out on the channel (baseband transmission) or

this is how carrier modulation is used.

(a) describe the two principles and explain where appropriate to use and principle. What types of media do the different principles fit? Motivate!

b) describe the difference in information speed (=transfer rate) and symbol rate.

c) which two factors directly determine the transfer speed of a channel? Why?

(D) explain how to increase the speed of information on a channel without having to increase

the symbol rate. Describe any appropriate modulation method. What limitations

does it exist?

Application

ADSL is a technology for increasing the transmission speed of copper cables. They use it

asymmetric transmission, i.e. the transmission capacity is different depending on whether one is transmitting upstream or downstream. The requirement for the ADSL modem is to the standard telephone should be capable of being used simultaneously with the ADSL modem. This means that you must use carrier modems and in this case with different frequency ranges for upstream and

downstream. (see fig.)

The downstream frequency range is divided into several subchannels of 4 kHz, where each subchannel can transmit 4000 symbols/second. Each subchannel is then modulated using the appropriate modulation method (the same modulation method is always used in all subchannels). The transmission power in each subchannel is 0,1 mW and received noise can be estimated at -100 dBm I

each subchannel. One also needs a given signal-to-noise ratio (SNR) depending on the chosen modulation method the damping of the copper cable is frequency dependent and can be described in our example:

Attenuation/km L=10+8,0*f [dB/km] f= frequency in MHz

a) at one point, 256-QAM downstream is used. 256-QAM requires SNR of 30 dB. Sketch in a graph maximum range at 256 kbps, 512 kbps, 2 Mbps, 8 Mbps and 24 Mbps speeds.

b) what happens to the range if you use 1024-QAM instead (which requires a SNR of 36 dB)? Sketch the result in the same graph as in a) and comment on.

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