plz help me assp

[rahalmmu]

Q1: The modulation is the process of converting a baseband signal to a bandpass signal using a carrier signal. One may ask “why do we need to do modulation?” or “why don’t we send the baseband signal just as it is?” answer these questions based on your understanding of the modulation concepts and radiating/receiving antenna requirements?
Q2: The demodulation is the process of reconstructing the baseband signal from the bandpass signal. By considering the modulated signal g(t) cos ùct, show how the signal g(t) can be recovered by multiplying this modulated signal with 2 cos ùct and passing the product through a low-pass filter of bandwidth B Hz that is the bandwidth of g(t) and B < ùc.

Q3: A newcomer to digital communications may question the usefulness/goodness of the parameter Eb/N0 as a figure of merit instead of S/N used as a useful/good figure of merit for analog communications. In other words, why can’t we continue to use S/N as a figure of merit for digital communications but instead we use Eb/N0?

Q4: The communications resource can be partitioned by the use of a hybrid combination of FDMA and TDMA known as CDMA. One might ask: Don’t the FDMA and TDMA options provide sufficient multiple access flexibility? Of what use is this hybrid technique? Discuss the motivation towards CDMA in terms of privacy, performance in fading channels, jam resistance, and flexibility.

Q5: In the case of baseband signaling, the received waveforms are already in a pulse-like form. Why then, is a demodulator (receiving filter) needed to recover the pulse waveform?

[charan langton]

Aug 25, 2009 12:27:37 GMT -5 rahalmmu said:

Q1: The modulation is the process of converting a baseband signal to a bandpass signal using a carrier signal. One may ask “why do we need to do modulation?” or “why don’t we send the baseband signal just as it is?” answer these questions based on your understanding of the modulation concepts and radiating/receiving antenna requirements?

Baseband frequencies are usualy low. To transmit a certain distance at these frequencies would require very large size antennas. So we use higher frequencies for gain purposes. Also depending on the medium used to transmit, not all media can carry the low frequency signals well.

Q2: The demodulation is the process of reconstructing the baseband signal from the bandpass signal. By considering the modulated signal g(t) cos ùct, show how the signal g(t) can be recovered by multiplying this modulated signal with 2 cos ùct and passing the product through a low-pass filter of bandwidth B Hz that is the bandwidth of g(t) and B < ùc.

Take a phase modulated signal" cost (wt + phi(t)). On the receive side, multiply this with 2 cost wt. Then expand by trignometric identities, and you will get a cos2 wt phi(t) - sin(2wt) phit(t). The second term is at twice the frequency and can be filtered out. The first term has the baseband information which can be extracted. You do the math.


Q3: A newcomer to digital communications may question the usefulness/goodness of the parameter Eb/N0 as a figure of merit instead of S/N used as a useful/good figure of merit for analog communications. In other words, why can’t we continue to use S/N as a figure of merit for digital communications but instead we use Eb/N0?

SNR does not differentiate between a signal that has 2 symbols or a lot more. It is total signal parameter. Digital signal performance is a function of the chosen constellation and coding. So to compare one choice from another, EbN0 is used instead. This parameter accounts only for information bits and is the performance after all parameters such as bit per symbol and code rate are accounted for. For this reason it provides for a truly normalized method of comparing digital system.

Q4: The communications resource can be partitioned by the use of a hybrid combination of FDMA and TDMA known as CDMA. One might ask: Don’t the FDMA and TDMA options provide sufficient multiple access flexibility? Of what use is this hybrid technique? Discuss the motivation towards CDMA in terms of privacy, performance in fading channels, jam resistance, and flexibility.

CDMA spreads the signal over wider bandwidth. Because orthogoal codes are used, it provides privacy and jam resistance. There is no need to channelize the band, all users can use it without being assigned a specific frequency. It offers grace degradartion as users come in and go, larger number of users increase C/I but do not cut off the other users. Fading which maybe frequency selective degrades a CDMA less than a narrower FDMA signal well because the signal bandwidthis so large. etc.

Q5: In the case of baseband signaling, the received waveforms are already in a pulse-like form. Why then, is a demodulator (receiving filter) needed to recover the pulse waveform?

Receive filter to be optimum i.e. one that maximize the SNR must be matched to the transmit pulse. So we filter it with a matched filter for that purpose.

Charan Langton


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