7. What are GPRS services?
GPRS services are defined to fall in one of the two categories :
- PTP ( Point to point)
- PTM ( Point to Multipoint)
Some of the GPRS services are not likely to be provided by network operators
during early deployment of GPRS due in part to the phased development of
standard. Market demand is another factor affecting the decision of operators
regarding which services to offer first.
8. What are the advantages of CDMA?
Advantages of CDMa are as follows :
1. Frequency diversity : Transmission is spread out over a large bandwidth due
to that less affected by noise. If bandwidth is increased S/N ratio increases,
which means noise will be reduced.
2. Multiplication Resistance : Chipping codes used for CDMA not only exhibit
low correlation but also low autocorrelation. Hence a version of the signal
that is delayed by more than one chip interval does not interfere with dominant
signal as in other multipath environments.
3. Privacy : Due to spread spectrum is obtained by the use of noise like
signals, where each user has a unique code, so privacy is inherent.
4. Graceful Degradation. In CDMA, more users access the system simultaneously
as compared to FDMa, TDMA.
9. What are the advantages of spread spectrum?
SPread spectrum has the following advantages :
1. No crosstalk interference.
2. Better voice quality/data integrity and less static noise.
3. Lowered susceptibility to multipath fading.
4. Inherent security.
6. Longer operating distances.
7. Hard to detect.
8. Hard to intercept or demodulate.
9. Harder to jam than narrow bands.
10. Use of ranging and radar.
10. Explain the steps involved in demodulating a signal.
Once the signal is coded, modulated and then sent, the receiver must demodulate
the signal. This is usually done in two steps :
1. Spectrum spreading (e.g., direct sequence or frequency hopping) modulation
2. The remaining information bearing signal is demodulated by multiplying with
a local reference identical in structure and synchronised with received signal.
11. How can a Pseudo Random Noise COde be usable?
To be usable for direct sequence spreading, a PN code must meet the following
1. Sequence must be built from 2 levelled numbers.
2. The codes must have sharp autocorrelation peak to enable code
3. Codes must have a low cross-correlation value, the lower it is, more are the
number of users which can be allowed in the system.
4. The codes should be “balanced” i.e. the difference between ones and zeros in
code may only be 1.