28-11-2012, 06:15 PM
Modulation , Channel Coding and Interference issues
Modulation.ppt (Size: 542 KB / Downloads: 21)
Modulation
For efficient and error free transmission, modulation and channel coding are the key considerations
Impact on Interference to another systems and from another system
Modulation that has larger number of bits per symbol
Small bandwidth
High amplifier linearity
High C/N ratio at the receiver
Ease of implementation
VSAT are very cost sensitive
Most common modulation used is QPSK
BPSK when spectrum efficiency is of less important
C/N requirement can be reduced if channel coding is applied
Channel Coding
Block code or convolutional codes
Forward error correcting code – Encoding is done prior to
the signal transmission
Convolution code and block code can be used together in a
channel
Interleaved coding will counter burst errors while
convolutional coding will counter individual bit errors
For eg: Direct broadcast satellite television
VSATS with small traffic streams, excess processing delay
can add significantly to the end-to-end link delay
Processing delay due to the first interleaving a signal and
then de-interleaving adds a fixed amount of overhead and
require buffering at both ends of the transmission links.
RS-codes are not added for signals with information rate
below 256 kb/s
RS codes are the best way to reduce the power
requirements for a given link and BER specification.
Interference
Interference occurs when a new system seeks to operate very close to the existing system, in terms of orbital separation or antenna beam directions
Coordination
Key exercise lies in determining the power radiated by the
interfering station in the direction of the interfered-with station
The output power of the interfering stations transmit amplifier
Transmit gain of the interfering station’s antenna
The receive gain of the interfered-with station’s antenna
The path loss between the two stations