Chapter 10 Flashcards
What are the 4 basic components of cellular system
Mobile station: handsets used by users to comm with other users
Base station: each cell contains an antenna that is controlled by a small office
Mobile switching center: each base station is controlled by a switching office and it is the MSC
Public switched telephone network: the network of the world’s public circuit switched telephone networks
What are the 4 mobile switching station databases
- Home location register database (HLR) - stores information about each subscriber that belongs to it
- Visitor location register database (VLR) - maintains information about currently physically in the region
- Authentication center database (AC) - used for authentication activities, holds encryption keys
- Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station
How are MS, BS, MSC, PSTN connected
Coverage area of cell network is divided into smaller areas known as cells each of which is controlled by a BS or node B
BS is fixed and is connected to MSC which is in charge of a cluster of BS
MSC connects to PSTN
MS can communicate with wireline phones through PSTN
Two types of channels are available between mobile station and base
station
Control channels : used to exchange information having to do with setting up
and maintaining calls
Traffic channels : carry voice or data connection between users
A forward (downlink) channel is used to carry traffic from the BS to the MS.
A reverse (uplink) channel is used to carry traffic from the MS to the BS.
Process of making phone call
At transmitting end
* A caller enters a 10-digit code (phone number) and presses the
call button.
* The MS scans the band to select a free channel and sends a strong
signal to send the number entered to BS.
* The BS relays the number to the MSC.
* The MSC in turn dispatches the request to all the base stations in
the cellular system.
* The Mobile Identification Number (MIN) is then broadcast over all
the forward control channels throughout the cellular system. It is
known as paging.
* The MS responds by identifying itself over the reverse control
channel.
* The BS relays the acknowledgement sent by the mobile and informs
the MSC about the handshake.
* The MSC assigns an unused voice channel to the call and call is
established.
At receiving end
* All the idle mobile stations continuously listens to the paging
signal to detect messages directed at them.
* When a call is placed to a mobile station, a packet is sent to
the receiver’s home MSC to find out where it is.
* A packet is sent to the base station in its current cell, which
then sends a broadcast on the paging channel.
* The receiver MS responds on the control channel.
* In response, a voice channel is assigned and ringing starts at
the MS.
What is the path loss model formula and what affects it
Pt/Pr = Ad^a
Pr is power received
Pt is power transmitted
A is coefficient
d is distance
a is path loss exponent
a is affected by obstructed, urban vs suburban, medium vs large city and is between 2 and 4
What is co channel interference
- cells using same frequency interfere with each other
- CCI increases as cluster size N decreases
- Important factor for signal quality is Carrier to interference ratio
- Most interference comes from first channel of co channel cells
Frequency reuse distance and factor
closest distance between center of two cells
D = R (3N)^0.5
R is cell radius and N is number of cells in cluster
1/N is reuse factor
Approaches to cope up with increasing demand
I. Adding new channels
II. Frequency borrowing – frequencies
are taken from adjacent cells by
congested cells
III. Cell splitting – cells in areas of high
usage can be split into smaller cells
IV. Cell sectoring – cells are divided
into a number of wedge-shaped
sectors, each with their own set of
Channels
V. Microcells – antennas move to
buildings, hills and lamp posts
What are the two cases of handover failure
Case 1: UE sent measurement report to Serving-BS, but Serving-BS did not receive it
due to channel state (in cell edge).
Case 2: The device sent a measurement report, and the S-BS received it successfully, but the handover command was not delivered properly to the device for reasons such as poor channel condition or the device moved too fast.
What is the procedure of handover
HO Event: it’s defined to report only when a
particular event occurs.
Measurement Report: UE receives the RS from the base station and measures the strength of this signal, and reports it back to the base station.
Bearer with Core Network: A path through which packet data is passed from the core network to the base station
What is handover
transfer of user connection from one
radio channel to other.
The main purpose of handover is to maintain an ongoing call.
frequency reuse
Cellular telephone systems rely on an intelligent allocation
and reuse of channels.
Each base station is given a group of radio channels to be
used within a cell.
Base stations in neighbouring cells are assigned completely
different set of channel frequencies.
By limiting the coverage areas, called footprints, within cell
boundaries, the same set of channels may be used to cover
different cells separated from one another by a distance large
enough to keep interference level within tolerable limits
