Transmission Techniques Traffic channels: different users are assigned unique code and transmitted over the same frequency band, for example, WCDMA and CDMA2000 Traffic channels: different frequency bands are allocated to different users, for example, AMPS Traffic channels: different time slots are allocated to different users, for example, GSM Frequency Time Power Frequency Time Power Frequency Time Power FDMA TDMA CDMA User Introduction

Time slots and frame

Capacity of cellular CDMA: N: the number of users S: the signal power of each user R: baseband information bit rate W: total RF bandwidth η: background thermal noise in the spread bandwidth Assume perfect power control

The number of users that can access the system is thus given as Where W/R is called the processing gain

The principle behind the spreading of a signal is explained by the Shannon channel capacity formula: Bω = bandwidth in Hertz C = channel capacity in bits/second S = signal power N = noise power

The received energy per bit is the received signal power divided by the data rate R (bits/s), E b = P s /R

1-Direct sequence Spread spectrum (DSSS). 2- Frequency hopping spread spectrum (FHSS). 3-Time hopping spread spectrum (THSS). 4-Hybrids Spread spectrum (HSS).

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W - Spread bandwidth in Hz R = 1/T b = Date Rate S - Received power of the desired signal in W J - Received power for undesired signals like multiple access users, multipath, jammers etc in W E b - received energy per bit for the desired signal in W N 0 - 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.

Sync Pilot FW Traffic (for user #1) Paging FW Traffic (for user #2) FW Traffic (for user #3)

Figure 2.9: CDMA Forward and Reverse Channels

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Cells Different Frequencies or Codes Base Station Fixed transceiver Mobile Station Distributed transceivers Downlink Uplink Handoff Multiple Access

CDMA ： ”make before break”---soft handoff Other systems: “make after break”---hard handoff CDMA ： ”make before break”---soft handoff Other systems: “make after break”---hard handoff Use soft handoff, decrease drop-call rate

Definition of Coverage Areas Location area MSC area PLMN area Service area Sector area CDMA Number Planning Cell area

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:

W 2n = W n W n W n W n W 1 =0 W 2 = W 4 = Walsh code Walsh Code is one kind of orthogonal code 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.