04-12-2012, 02:12 PM
Frequency division multiple access FDMA
Frequency division.pptx (Size: 2.59 MB / Downloads: 152)
one phone circuit per channel
idle time causes wasting of resources
Simultaneously and continuously transmitting
usually implemented in narrowband systems
for example: in AMPS is a FDMA bandwidth of 30 kHz implemented
FDMA compared to TDMA
fewer bits for synchronization
fewer bits for framing
higher cell site system costs
higher costs for duplexer used in base station and subscriber units
FDMA requires RF filtering to minimize adjacent channel interference
Nonlinear Effects in FDMA
many channels - same antenna
for maximum power efficiency operate near saturation
near saturation power amplifiers are nonlinear
nonlinearities causes signal spreading
intermodulation frequencies
IM are undesired harmonics
interference with other channels in the FDMA system
decreases user C/I - decreases performance
interference outside the mobile radio band: adjacent-channel interference
RF filters needed - higher costs
Features of TDMA
a single carrier frequency for several users
transmission in bursts
low battery consumption
handoff process much simpler
FDD : switch instead of duplexer
very high transmission rate
high synchronization overhead
guard slots necessary
CDMA
Solution: power control
Power control is provided by each base station in a cellular system and assures that each mobile within the base station coverage area provides the same signal level to the base station receiver. This solves the problem of a near by subscriber over powering the base station
Power control is implemented at the base station by rapidly sampling the radio signal strength indicator levels of each mobile and then sending a power change command over the forward radio link.