Wireless Communications: System Design. Dr. Mustafa Shakir. Issues in cell to cell moving. What are different levels of handoff. (1) Intra Cell (2) Inter cell (3) Inter system Importance of handoff.
Dr. Mustafa Shakir
--“Probability of forced termination” decreases at the cost of reduced Total Carried Traffic.
-- Queuing is possible because of the time available between the Threshold power level and the Hand off power level.
1) Co-Channel Interference and System Capacity
The yellow cells use the same set of frequency channels, and hence, interfere with each other.
In case of N=7, there are 6 first-layer co-channels.
Worst case carrier-to-interference ratio
Let n = 4 and D/R = q,
Let reuse N = 7, then
Compute C/I and get C/I = 17.3 dB
(a) n = 4, N = 7
N =7 can be used
(b) n = 3, N = 7
Strong “bleed over”
BS as Tx
Mobile User Rx
The Mobile receiver is captured by the unintended, unknown transmitter, instead of the desired base station
BS as Rx
Strong “bleed over”
Desired Mobile Tx
Adjacent Channel Mobile Tx
The Base Station faces difficulty in recognizing the actual mobile user, when the adjacent channel bleed over is too high.
TYPES OF POWER CONTROL
It depends solely on mobile unit, not as accurate as closed loop, but can react quicker to fluctuation in signal strength. In this there is no feed back from BS.
In this BS makes power adjustment decisions and
communicates to mobile on control channels
Cost of a cellular network is proportional to the number of Base Stations. The income is proportional to the number of users.
Ways to increase capacity:
New spectrum – expensive. PCS bands were sold for $20B.
Architectural approaches: cell splitting, cell sectoring, microcell zones.
Dynamic allocation of channels according to load in the cell (non-uniform distribution of channels).
Improve access technologies.
Cell Splitting is the process of subdividing the congested cell into smaller cells (microcells), Each with its own base station and a corresponding reduction in antenna height and transmitter power.
Cell Splitting increases the capacity since number of clusters over coverage region would be increased thus increasing the number of channels.
New cells added having smaller radius than original cells and by installing these smaller cells (called microcells ) between existing cells , capacity increases due to additional number of channels per unit area.
The area covered by a circle with radius R is four times the area covered by the circle with radius R/2 The number of cells is increased four times
The number of clusters the number of channels and the capacity in the coverage area are increased Cell Splitting does not change the co-channel re-use ratio Q =D/R
New cells are smaller, so the transmit power of the new cells must be reduced
How to determine the transmit power?
The transmit power of the new cells can be found by examining the received power at the new and old cell boundaries and setting them equal
Pr(at the old cell boundary) is proportional to
Pt1 * R-n
Pr(at the new cell boundary) is proportional to
Pt2 * (R/2)-n
Take n=4, we get
Pt2 = Pt1/16
We find that the transmit power must be reduced by 16 times or 12 dB in order to use the microcells to cover the original area. While maintaining the same S/I.
When there are two cell sizes one cant simply use the same transmit power for all cells. If larger transmit power used for all cells some smaller cells would not be sufficiently separated from co channel cells. Using smaller Pt the larger cells might be left unserved.
So old channel broken to two channel groups corresponding to smaller and larger cell reuse.
Larger cell for less frequent hand off.
Antenna down tilting focusing radiated energy from base station to the ground to limit radio coverage of newly formed cells.
Co channel interference may be reduced by replacing omni directional antenna by several directional antennas.
Given cell will receive interference and would transmit with fraction of available co channel cells.
Each sector uses directional antenna at the B.S and assigned a set of channels.
Partitioning into three 120 deg. sectors or six 60 deg. sectors.
Amount of CCI reduced by number of sectors.
Reduced Tx Power…
Reduction in interference offered by sectoring would enable to reduce the cluster size N and additional degree of freedom in channel assignment.
Increased number of antennas with shrinking cluster size and decrease in trunking efficiency due to channel sectoring at base station.
Since sectoring reduces the coverage area of a particular group of channels the number of handoffs increases
Available channels subdivided and dedicated to a specific antenna thus making up of several smaller pools contributing to decrease in trunking efficiency.
To provide dedicated coverage for hard to reach areas
Radio retransmitters for range extension.
Upon receiving signals from base station forward link the repeater amplifies and reradiates the base station signals to specific coverage region.
In building wireless coverage by installing Distributed Antenna Systems.
Repeaters must be provisioned to match the available capacity from the serving base station.
The increased number of handoff as a result of sectoring would result in an increased load on switching and control link elements of the mobile system.
Division into microcell zones and each of the three are connected to a single base station and share the same radio equipment.
Zones connected by a coaxial cable, fiber optic cable or microwave link to the base station.
Handoff not required while mobile travels between zones within cell.
Channel switching and a channel active only within zone of travelling.
In Micro cell zone scenario each hexagon represents a zone while the group of three hexagons represent a cell.
Zone Radius Rz is one hexagon radius.
Capacity of Microcell is directly related to distance betw. Cochannel cells and not zones.
No handoffs is required at the MSC.
The base station radiation is localized and interference is reduced
When a circuit is busy for one hour it handled a traffic of one erlang.
probability that a call is blocked (or delayed).
Traffic intensity is measured in Erlangs:
Call that cannot be completed at time of request due to congestion. Also referred to as Lost Call.
Average duration of typical call.
Traffic intensity across the whole system.
Average number of call requests per unit time.