Traffic channels: different users are assigned unique code and transmitted over the same frequency band, for example, WCDMA and CDMA2000Traffic channels: different frequency bands are allocated to different users, for example, AMPSTraffic channels: different time slots are allocated to different users, for example, GSMFDMATDMACDMAUser User User User User User Introduction
Time slots and frame
Capacity of cellular CDMA:N: the number of usersS: the signal power of each userR: baseband information bit rateW: total RF bandwidth: background thermal noise in the spread bandwidthAssume perfect power control
The number of users that can access the system is thus given asWhere W/R is called the processing gain
The principle behind the spreading of a signal is explained by the Shannon channel capacity formula:B = bandwidth in HertzC = channel capacity in bits/secondS = signal powerN = noise power
The received energy per bit is the received signal power divided by the data rate R (bits/s), Eb = Ps/R
1-Direct sequence Spread spectrum (DSSS). 2- Frequency hopping spread spectrum (FHSS). 3-Time hopping spread spectrum (THSS). 4-Hybrids Spread spectrum (HSS).
W - Spread bandwidth in HzR = 1/Tb = Date RateS - Received power of the desired signal in WJ - Received power for undesired signals like multiple access users, multipath, jammers etc in WEb - received energy per bit for the desired signal in W N0 - equivalent noise spectral density in W/Hz
Are portion of a physical communications channel that is used to for a particular communications purpose. There are two groups of channels used in the CDMA system; control channels and traffic channels. Control channels are used to setup, manage, and terminate communication sessions. Traffic channels are primarily used to transfer user data but can also transfer some control information.
Figure 2.9: CDMA Forward and Reverse Channels
Cells Different Frequencies or CodesBase Station Fixed transceiverMobile Station Distributed transceiversDownlinkUplinkHandoffMultiple Access
CDMAmake before break---soft handoffOther systems: make after break---hard handoff
Use soft handoff, decrease drop-call rate
Definition of Coverage Areas
Location areaMSC areaPLMN areaService areaSectorareaCDMA Number Planning
Rate of reception level variation is known as the fading rate or the Doppler rate, fd, and depends on the velocity of movement V and carrier wavelength. It can be expressed as:
W1=0W2=0001W4=Walsh codeWalsh Code is one kind of orthogonal code.1 11 0
Orthogonal codes are easily generated by starting with a seed of 0, repeating the 0 horizontally and vertically, and then complementing the 1 diagonally
suppose we have four stations each has a sequence of chips which we designate as (A,B,C) and (D)(see the figure). Each station is assigned a code which is sequence of number called chips.