Docstoc

cellular technology

Document Sample
cellular technology Powered By Docstoc
					1.1- Beginning of wireless communication
•1897- Marconi first demonstrated radio’s ability to provide continuous contact with ships sailing the English Channel. •In 1946- the first public mobile telephone service was introduced in twenty-five major American cities.
Each system used a single, high-powered transmitter and large tower in order to cover the distances over the 50KM in a particular market.

• DUPLEX, HALF-DUPLEX (only one person on the telephone call could talk at a time). • MOBILE: the term mobile has historically been used to classify any radio terminal that could be moved during operation. • Portable: (used by some one at walking speed)
• Subscriber: it is used to describe a mobile user or portable user because in most mobile communication systems each user pays a subscription fees to use the system and each user’s communication device is called a subscriber unit. • Table 1.4 on page 10

• GSM (Global System Mobile)

• Early mobile radio: (to achieve a large coverage area by a mobile user) – High powered transmitters – Antenna mounted over tall tower – But impossible to reuse the same frequency in that coverage area (because any attempt to reuse frequency will cause interference) – E.g. Bell mobiles in 1970s (Max. of 12 calls)

• Government regulation authority

Base Station: • A fixed station in a mobile radio system used for radio communication with mobile station. Base station are located at the center or on the edge of a coverage region and consists of radio channels and transmitter and receiver antennas mounted on a tower. • Base station antennas are designed to achieve the desired coverage between the particular cell. Cell: Geographic area covered by one base station.

1.2- Concept of Frequency Reuse
• To distribute the available channels throughout the geographic region without interference. • The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called FREQUENCY REUSE OR FREQUENCY PLANNING.

• System design: (The Cellular Concept is a system,-level
idea which calls for replacing a single high power transmitter (large cell) with many low power transmitter (small cells), each providing service to only a small portion of the service area.

–Circular form –A Square –An Equilateral Triangular –Hexagonal shape (conceptual & Simplistic (for future growth)) –Footprint

A cell must be designed to serve the weakest mobiles within the footprint, and these are typically located at the edge of the cell.

1.3- Channel Assignment
• Each base station is allocated a portion of the total number of channels available to the entire system. And nearby base station are assigned different groups of channels so that the interference between the stations is minimized. • By limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from one another by distances large enough to keep interference levels within tolerable limits.

1.3.1- Mathematics in Channel Assignment
Total no. of channels available = S (Duplex)

No. of channels allocated to each cell = k N = Total number of cells among which total available channels are divided with reusing any (channels are equally divided)

S=KN

Cluster:
The N cells which collectively use the complete set of available frequencies (channels) is called a Cluster. Or we can say that the factor N is called the cluster size.

If a cluster is replicated M times within the system, the total number of duplex channels (C) can be used as a measure of capacity and is given by

C = MkN = MS

1.4- Frequency Reuse Factor
• The frequency reuse factor of cellular system is given by 1/N, since each cell within a cluster is only assigned the 1/N of the total available channels in the system.

1.4.1- Calculating the Cluster Size
• The cluster size is typically equal to 4, 5 or 12 or etc. • If a cluster size N is reduced while the cell size is kept constant more clusters are required to cover a given area, and hence more capacity is achieved (a larger value of C). • The value for N is a function of how much interference a mobile or base station can tolerate while maintaining a sufficient quality of communications.

.
B G C

A
F B G

D
C

jD o
E

D

B G A F E D C

A
F E

iDo
D

o 120

D:

centre-to-centre distance between reused cells

Do: centre-to-centre distance between adjacent cells N: cluster size N = D2/Do2

Proof
D2 = (iDo)2 + (jDo)2 – 2(iDo)(jDo)cos120o

D2 /Do2 = i2 + j2 + i j and

N = D2/Do2
Where N is the cluster size

1.5- Channel Assignment
• Channel Assignment strategies can be classified as either
1- Fixed 2- Dynamic

• The choice of channel assignment strategy impacts the performance of the system, particularly as to how calls are managed when a mobile user is handed off from one cell to another.

• 1- Fixed Channel Assignment Strategy: • In fixed channel assignment strategy, each cell is allocated a predetermined set of channels. Any call attempt within a cell can only be served by the unused channel in that particular cell. • If all the channel in that particular cell are occupied, the call is blocked and the subscriber does not receive the service.

Variations in the Fixed Assignment Strategy
• Borrowing Strategy: • In this strategy, a cell is allowed to borrow channels from a neighboring cell if all of its own channels are being occupied. • The mobile switching center (MSC) supervises such borrowing procedures and ensures that the borrowing of a channel does not disrupt (To throw into confusion or disorder ) or interfere with any of the calls in progress in the donor cell.

• 2- Dynamic Channel Assignment Strategy: • In a dynamic channel assignment strategy, the channels are not allocated to different cells permanently. • Each time when a call request is made, the serving base station requests a channel from the MSC. • The switch then allocates the channel to the requested cell following an algorithm takes into account the following things
1- the likelihood of future blocking within the cell 2- the frequency of use of the candidate channel 3- the reuse distance of the channel

1.6- Handoff
• When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. • Many handoff strategies prioritize handoff request over call initiation request when allocating new channel in a cell site. • Handoff must be performed successfully and as infrequent as possible and be imperceptible to the user. • In order to meet these requirements, system designers must specify an optimum signal level at which to initiate a handoff.

.


Where

= Pr

handoff

- Pr minimum usable



may not be too large or too small

?

.
Received signal power Level at point A

Handoff Threshold Minimum acceptable signal to maintain a call

Level at call B (call is terminated)

Time
A B

• In deciding when to handoff, it is important to ensure that the drop in the measured signal level is not due to momentary fading and that the mobile is actually moving away from the serving base station. • In order to insure this, the base station monitors the signal level for a certain period of time before a handoff is initiated. • The length of time needed to decide if the handoff is necessary, depends on the speed at which the vehicle is moving. Is the slope of the short-term average received signal level in a given time interval is steep, the handoff should be made quickly.

Dwell Time
• The time over which the call may be maintained with a cell, is called the dwell time. • Dwell time of a particular user is governed by a number of factors including propagation, interference, distance between the subscriber and the base station, and other time varying effects.

Ist Generation Handoffs

Mobile Assisted Handoffs (MAHO)

Intersystem Handoff

Prioritizing Handoffs
1. Guard Channel Concept: 2. Queuing the Handoff request: (to decrease the probability of forced termination of calls due to lack of available channels.

Umbrella Cell Approach
• By using the different antenna heights (often on the same building or tower) and different power levels, it is possible to provide coverage to “large” and “small” cells which are co-located at a single location. • Large area coverage to high speed users and small area coverage to users traveling at slow speed.

1.7- Interference and the System Capacity
• Interference is the major limiting factor in the performance of the cellular radio system. Source of Interference:
1. 2. 3. 4. Another mobile in the same cell A call in progress in the neighboring cell Other base station operating in the same frequency Any noncellular system inadvertently leaks energy into the cellular frequency band

•

• Interference on voice channel causes cross talks, where the subscriber hears the interference in the background due to an undesired transmission. • On control channels, interference leads to missed and blocked calls due to errors in the digital signaling. • Interference is more severe in the urban areas, due to the greater RF noise floor and the large number of base stations and mobiles. • Interference has been recognized as a major bottleneck in increasing capacity and is often responsible for dropped calls.

1.7.1- Types of interference
• There are two major types of interference 1. Adjacent Channel Interference
2. Co-Channel Interference

Adjacent Channel Interference
• Interference resulting from the channels which are adjacent in frequency to the desired signal is called adjacent channel interference. • Adjacent channel interference results from imperfect filters which allow nearby frequencies to leak into the passband.

Near Far Effect
• This happens in adjacent channel interference.

How ?

Co-Channel Interference
• From the concept of frequency re-use we see that there are several cells that use the same set of frequencies. • These cells are called co-channel cells and the interference between signals from these cells is called co-channel interference. • Co-channel interference can not be combated by simply increasing the carrier power of a transmitter. This is because an increase in carrier transmit power increases the interference to the neighboring co-channel cells.

• To reduce co-channel interference, cochannel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation.

•

Before doing anything, first have to confirm that 1. Size of each cell must be approximately same 2. Each base-station transmits the same power

• Now we see that when the above said are met, then the co-channel interference ratio becomes independent of the transmit power and becomes function of the radius of the cell and the distance between the centers of the nearest co-channel cells.
– R = radius of the cell
– D = distance between the nearest cochannel cells.

Co-channel reuse ratio
• D is the distance between reuse cells • R is the distance between the BS and the farthest Mobile (which is at the edge of the cell). • Q is the co-channel reuse ratio which is given by

Q = D/R =

3N

Proof
• Do/2 = R cos30o • Do2 = 3R2
Do R

• • • •

N = D2/Do2 N = D2/3R2 D/R = 3N Q = D/R = 3N

Do

• The parameter Q in the above equation is called the co=channel reuse ratio and is related to the cluster size. • So we see that the small value of Q provides larger capacity since the cluster size N is small. • Whereas the larger value of Q improves the transmission quality due to a smaller level of cochannel interference. • So a trade of must be made between these two objectives in actual cellular design. • We can increase or decrease the value of cochannel reuse ratio by increasing or deceasing the value of D or R.

• Let io be the number of co-channel interfering cells. • S is the desired signal power from the desired base station. • Ii is the interference power caused by the ith interfering co-channel cell base station.

• The signal-to-interference-ratio (S/I) which monitors a forward channel can be expressed as

S I



S

I
i 1

6

i

• Propagation measurements in a mobile radio channel show that average received signal strength at any point decays as a power law of the distance of separation between a transmitter and receiver. • The average received power Pr at a distance d from the transmitting antenna is n given by

Pr

d   Po   d   o

• Where n the path loss exponent. • When the transmit power of each base station is same and the path loss component is same throughout the coverage area then the S/I for mobile can be approximated as

S I



R
6 i 1

n n

 D 
i

S I
• --------------- ---------------



R 6 D n

n

• --------------- ---------------

S I
S I




D R 
6

n




3N 6



n

• --------------- ---------------

Where

S I

C I

• The path loss component ranges between 2 and 4 in the urban cellular systems. If we take n=4, we get the following result
S I 



3N 6



4

S I



9N 2 6

N

2 3

S I

1.8- Capacity Equation
• Capacity of the cellular system is given by the following equation

C%  f c

f

 2 PR 3

A C I

Proof
• R = radius of a cell • A = total area of a complete cellular system • P = population in thousands • 10 = trunking gain • nc = number of channels per cell •  R2 = area of a cell • A /  R2 = total number of cells in complete cellular system • f  total frequency or bandwidth available



• f c  bandwidth of one channel

• Capacity is defined as the total number of subscribers in the cellular system which can use the cellular mobile facility. • Capacity = C C = total no. of cells

 subscribers per cell
 A   2   R  10nc

C



• From the above equation we get the following result

10nc A C 2 R

• But the capacity is in percentage is given by the following

10nc A C%  100 2 1000PR nc A C%  2 PR

Because the population is taken in thousands.

eq. A

• We know that

f nc  k f c
A C 2 PR  f   kf c     

• By putting the value of nc in eq.A we get

• By putting the value ok k in the above equation   we get  

A  f C 2  PR 2  f c 3 

 C   I 

1.9- Improving the Coverage and Capacity in Cellular System
• As the demand increases, the number of channels assigned to a cell eventually becomes insufficient to support the required number of users, following techniques are being used to expand the capacity of cellular system:
»Cell Splitting »Sectoring »Microcell Zone

• 1.9.1 Cell Splitting


				
DOCUMENT INFO
Shared By:
Stats:
views:2570
posted:10/10/2009
language:English
pages:54