The Internet Encyclopedia (Volume 3)

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CELLULARCOMMUNICATION 187

or the channel bandwidth. Regardless of the type of di-

versity used, the signals must be combined and detected

at the receiver. A proper combination of the signal from

various branches results in improved performance. The

method of combining chosen will affect the receiver per-

formance and complexity. The common combining tech-

niques in wireless communication are maximal ratio com-

bining (MRC), equal gain combining (EGC), and selection

diversity (SD). In MRC, the received signals from individ-

ual paths are weighted and added in such a way as to

emphasize more credible signals and suppress less credi-

ble ones (Yacoub, 1993). In EGC, the received signals are

equally weighted and then combined without regard to

the individual signal strength. In SD, the branch with the

best or most desirable signal is selected and the weaker

ones are ignored.

Multiple Access

Because the RF spectrum is finite and a limited resource,

it is necessary to share the available resources between

users. Multiple access techniques are the primary means

of sharing the resources in wireless systems. These tech-

niques are multiplexing protocols that allow more than a

pair of transceivers to share a common medium, which

can be achieved through frequency, time, or code, giving

rise to three popular techniques known as frequency di-

vision multiple access (FDMA), time division multiple ac-

cess (TDMA), and code division multiple access (CDMA).

In FDMA, the whole spectrum is divided into subbands

and the subbands are assigned to individual users on

demand. The users use the entire channel for the en-

tire duration of their transmissions. If the transmission

path deteriorates, the user is switched to another chan-

nel. This access technique is widely used in wireless mul-

tiuser systems. Instead of dividing the available frequency

as in FDMA, the available time is divided into frames of

equal duration in the case of TDMA. Only one user is al-

lowed to either transmit or receive in each time frame. The

transmissions from various users are interlaced into cyclic

time structure. Instead of using frequencies or time slots,

CDMA techniques distinguish between multiple users us-

ing digital codes. Each user is assigned a unique PN code

sequence, which is uncorrelated with the data. Because

the signals are distinguished by codes, many users can

share the same bandwidth simultaneously; i.e., signals are

transmitted in the same frequency at the same time.

CELLULAR COMMUNICATION

Currently, cellular mobile communication is undoubtedly

the most popular RF wireless communication system. In

cellular systems, instead of using a single large coverage

area with one high-power transceiver (used in traditional

mobile systems), the coverage area is divided into small,

localized coverage areas called cells. Figure 7 compares

the traditional mobile telephone with the cellular tele-

phone structures. Each cell has a base station (BS) or cell

site, which in comparison uses much less power. The BS

can communicate with mobiles as long as they are within

range. To prevent interference, adjacent cells are assigned

different portions of the available frequencies. With a cer-

tain distance between two cells, the assigned spectrum of

a given cell can be reused.

To explain the concept of cellular mobile communica-

tion, a summary of the major concepts and techniques is

presented below.

Cells

A cell is the basic geographic unit of a cellular system,

commonly represented as a hexagon. The term cellular

comes from this hexagonal or honeycomb shape of the

coverage area. Each cell has a BS transmitting over a

cell. Because of constraints imposed by natural terrain

and manmade structures, the true shapes of cells are not

Cellular Cluster

Radio tower

Original Coverage

1. Large coverage area


2. Single high power transceiver


3. Poor transmission quality


4. Excessive delays in call setup


5. Limited frequency reuse


6. Demand in excess of capacity


7. Limited service areas
   
   
   1. Small coverage area
   
   
   2. Multiple low power transceiver
   
   
   3. Good transmission quality
   
   
   4. Acceptable delays in call setup
   
   
   5. Good frequency reuse
   
   
   6. Demand proportional to capacity
   
   
   7. Extended service areas
   
   
   
   

D

R 1

2

5

3

4

6

7

60 o

cell sector

120 o

cell sector

61

54 3

2

3

1

2

Pico Cells

Small Cell

Large Cell

1

2

5

3

4

6

7 1

2

5

3

4

6

7

Figure 7: Traditional and cellular mobile radio structure showing frequency reuse, cell splitting and cell

sectoring (R=cell radius, D=frequency reuse distance).