EPL 476Fundamental Concepts in Wireless Networks

Cellular Networks

Prior Cellular Networks Telephone service provided by high power transmitter

Typical system 25 channels with an effective radius 80km

Cellular Networks Lower power transmitter of 100w

Due to low coverage of the transmitter the area can divided into multiple cells

Each cell is served by Base Station Transmitter, receiver, and control unit

What is a Cell?

Cell is the Basic Union in The System defined as the area where radio coverage is given by one

base station. A cell has one or several frequencies, depending on

traffic load. Fundamental idea: Frequencies are reused, but not in

neighboring cells due to interference.

Shape of Cell?

Design Decision Shape of Cell to cover the area

Square Hexagonal,

Frequency Reuse Transmission Power

Carefully controlled to allow communication within the cell using a given frequency band while limiting the power at that frequency that escapes the cell into adjacent cells.

The same frequency is not used in the adjacent cells

The objective is to use the same frequency band in multiple cells at some distance from one another.

At the same cell multiple frequency bands are assigned, the number of bands depending on the traffic expected.

Frequency Reuse A key design

Determine the minimum separation between two cells using the same frequency band so that the two cells do not interfere with each other.

Frequency Reuse A key design

D=minimum distance between centers of cells that use the same frequency band

R=radius of cell d= distance between centers of adjacent cells

N=number of cells in repetitious pattern K= total number of frequency allotted for the

system. Frequencies for each cell = K / N

Frequency Reuse In hexagonal cell pattern the following

values of N are possible:

N=1,3,4,7,9,12,13,16,19,21…..

Problem Assume a system of 32 cells with a cell

radius of 1.6km, a total of 32 cells, a total frequency bandwidth that supports 336 traffic channels, and a reuse factor of N=7.

What is the geographic area covered? How many channels are per cell? What is the total number of concurrent calls

that can be handled? Repeat with 0.8km and 128 cells.

Solution What is the geographic area covered? Find the area for each cell

area of hexagonal=6.65ktotal geographic area= 6.65*32=213k

How many channels are per cell? For N=7 the number of channels per cell is

336/7=48. 48 channels per cell.

Solution What is the total number of concurrent calls

that can be handled? 48 channels per cell* 32 cells=1536 channels

Mobile Radio Propagation Effects Signal Strength

The strength of the signal between the base station and the mobile unit must be strong enough to maintain signal quality at the receiver but not so strong to create too much cochannel interference with channels in another cell using the same frequency band.

Fading Even if the signal is within an effective range, signal

propagation effects may disrupt the signal and cause errors

Mobile Radio Propagation Effects Path loss information HATA

Mobile Radio Propagation Effects Path loss information HATA

Path Loss Problem Let fc = 900MhHz, ht =40m, hr= 5m and d =

10km. Estimate the path loss for a medium-size city.

Traffic Engineering Consider a cell that has L potential of

customers (L mobile units) and that is able to handle N simultaneous users

If L <= N No-blocking system

If L >= N Blocking system

Traffic Engineering Blocking System

What is the degree of blocking? What is the probability the call is

blocked? What is the capacity (N) is needed to

achieve a certain upper bound on the probability of blocking

A=λhA=traffic intensityλ=the mean rate of calls attempt per unit timeh=the mean holding time per successful call

Traffic Engineering

A=λh λh=ρN

A= ρN

A= is the average number of channels requiredλ=the mean rate of calls attempt per unit timeh=the mean holding time per successful callρ=is server utilization