【资料名称】：GSM supplementary
【资料作者】：无
GSM supplementary.ppt (1.15 MB)
【资料日期】：2012.12.12

【资料语言】：中文

【资料格式】：DOC

【资料目录和简介】：

hidden and exposed terminals
Hidden terminals (Cause collisions)
A sends to B, C cannot receive A
C wants to send to B, C senses a “free” medium (CS fails)
collision at B, A cannot receive the collision (CD fails)
A is “hidden” for C



Exposed terminals(Cause unnecessary delay)
B sends to A, C wants to send to another terminal (not A or B)
C has to wait, CS signals a medium in use
but A is outside the radio range of C, therefore waiting is not necessary
C is “exposed” to B
near and far terminals (P458)
Terminals A and B send, C receives
signal strength decreases proportional to the square of the distance
the signal of terminal B therefore drowns out A’s signal
C cannot receive A






If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer
Also severe problem for CDMA-networks - precise power control needed!
Access methods SDMA/FDMA/TDMA
SDMA (Space Division Multiple Access)
segment space into sectors, use directed antennas
cell structure
FDMA (Frequency Division Multiple Access)
assign a certain frequency to a transmission channel between a sender and a receiver
permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum)
TDMA (Time Division Multiple Access)
assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time

The multiplexing schemes are now used to control medium access!
FDD/FDMA - general scheme, example GSM 900MHz

Downlink: From Base Station to Mobile Station
(forward link)
Uplink: From Mobile Station to Base Station
(reverse link)