Transcript File

Cellular Networks
Why use cellular networks? What mobile radio services where
provided before cellular?
•Use multiple low-power transmitters (100 W or less), because the
range of such transmitter is small, an area divided into cells
1.
each served by its own antenna
2.
Served by base station consisting of transmitter, receiver, and
control unit
3.
Band of frequencies allocated
4.
Cells set up such that antennas of all neighbors are equidistant
(hexagonal pattern)
Frequency Reuse
Adjacent cells assigned different frequencies to avoid
interference or crosstalk
2.
Objective is to reuse frequency in nearby cells
 Minimum distance between co-channel cells? (minimum
distance between centers of cells that use the same band of
frequencies(co-channels))
 Number of frequencies assigned to each cell? (10 to 50)
 Transmission power control needed (to allow communication
within a cell using a given frequency while limiting the power
at that frequency that escapes the cells into the adjacent
ones. )
 Reuse factor? (N, number of cells in the repetition pattern)
1.
Ways to increase the capacity
 Add
new channels (if such exists…)
 Frequency borrowing from adjacent cells
(dynamically or permanent)
 Cell splitting –cells in areas of high usage can
be split into smaller cells
 Cell sectoring –cells are divided into a
number of wedge-shaped sectors, each with
their own set of channels
 Microcells–radius less than 1 km. Used at
crowded streets, inside buildings, along
highways,…
Cellular System Overview
Cellular Systems Terms
 Base
Station (BS) –includes an antenna, a
controller, and a number of receivers
 Mobile Telecommunications Switching
Office MTSO (or Mobile Switching Center
MSC) –connects calls between mobile units
 Two types of channels available between
mobile unit and BS
1.
Control channels –used to exchange
information having to do with setting up
and maintaining calls
2.
Traffic channels –carry voice or data
connection between users
Typical call
 Monitor for strongest signal
 Request for connection
 Paging
 Call accepted
 Ongoing call
 Handover (handoff)
or termination
 Also: call blocking,
call drop,
call to fixed
phone network
Mobile Radio Propagation Effects
 Take into consideration in design:
1. Signal strength
2. Must be strong enough (signal quality at the
receiver)
3. Must not be too strong (co-channel
interference)
4. Fading
5. Cell size
6. Carrier freq., antenna positions, …
7. Differ between urban, suburban and open
areas
Handover
 Also
called Handoff (mainly in US)
 Network initiated or mobile-assisted
 Performance metrics that may be used in
decision:
1. Cell blocking probability (new call being
blocked)
2. Call dropping probability (termination of call
due to handoff)
3. Call completion probability (admitted call is not
dropped before it terminates)
4. Probability of unsuccessful handover (handoff
is executed while the reception conditions are
inadequate)
5: Handover blocking probability (handoff
cannot be successfully completed)
6: Handover probability (handoff occurs before
call termination)
7: Rate of handover (number of handovers per
unit time)
8: Interruption duration (the interruption of
time during the handoff in which the mobile
is not connected to either BS )
9: Handover delay (the mobile moves from one
point to another where the other handoff
should occur but it delays)
Handover
Strategies
Used
to
Determine
Instant of Handover
Relative
signal strength
Because signal strength fluctuates due to the
multipath effects, even with power averaging, this
approach can lead to a ping pong effect in which a
mobile unit is repeatedly passed back and forth
between two base stations.
Relative signal strength with threshold
Handoff occurs if
1. Signal at a current BS is sufficiently weak (less then a
predefined threshold)
2. The other signal is stronger of the two.
3. If the signal at a current BS is sufficiently strong, handoff is
unnecessary.
4. There are three threshold schemes (th1,th2 & th3)
5. For th1, no need for handoff, for th2 handoff will occur at
L2, for th3, handoff will occur at L4. this reduces the quality
of communication and probability of call dropping increased.
6. It is because of the interference between powers of to base
stations.
Relative signal strength with hysteresis
1. The power of the BS 2 is sufficiently stronger, and the
power of the BS 1 weakens gradually is the case of perfect
handoff (by a margin H).
2. In this case handoff occurs at L3.
3. This scheme prevents ping pong effect, because when
handoff occurs, the effect of margin H reversed.
4. Hysteresis (H) is known as relay hysteresis.
5. Handoff from BS 1 to BS 2 occurs when signal strength
reaches or exceeds H and vice versa.
Relative
signal strength with hysteresis
and threshold
Handoff occurs when,
1. Current signal level drops below the threshold
2. Target BS is stronger then the current one by a hysteresis
margin H.
3. In our case it is at th3
Prediction techniques
The handoff decision is based on the expected future value of
the received signal strength.
Power Control
 Why
is it desirable to include dynamic power
control in a cellular system?
Received power must be sufficiently above the background
noise for effective communication
2. Desirable to minimize power in the transmitted signal from
the mobile
 Co-channel interference
 Health concerns
 Battery power
1.
 In
spread spectrum systems using CDMA it is
crucial for the decoding to equalize the
received power level from all mobile units at
the BS
Types of Power Control

1.
2.
3.
4.
5.
6.
Open-loop power control
Depends solely on mobile unit with no feed back from BS,
and is used in some SS (spread spectrum) systems
Uses the pilot signal
Not as accurate as closed-loop, but can react quicker to
fluctuations in signal strength (important in CDMA)
Pilot allows the MS to acquire the timing of the forward (BS
to MS)CDMA channel and provides the phase reference for
demodulation.
MS monitors the received power level of pilot and sets the
transmitted power in the reverse (Ms to BS) channel
inversely proportional to it.
That’s why CDMA mobile sets get less battery power then
GMS mobile sets.
Closed-loop power control
1. Adjusts signal strength in reverse channel (MS to BS) based
on metric of performance
2. Which means BS makes the power adjustment decisions,
received SNR, received BER, received signal power level.
3. BS makes power adjustment decision and communicates to
mobile on control channel
4. Closed loop power control is used to adjust a power in the
forward channel.
5. Which means that power for the GSM mobile sets are
adjusted by the BS. So because of dual power control they
take more battery consumption then CDMA mobile sets.
 Example: GSM uses closed-loop with 8 power classes
in BS (2.5-320 W) and 5 in mobile station (0.8-20 W)

Traffic Engineering
 “To
size the network to be able to handle
some expected level of load”
 In practice, not feasible to have capacity
handle all possible load
 For L subscribers and capacity of N
simultaneous users:
1.
2.
L <N–non blocking system
L> N–blocking system
Blocking System Performance Questions
(Assignment)
Probability
that call request is blocked?
What capacity is needed to achieve a
certain upper bound on probability of
blocking?
What is the average delay?
What capacity is needed to achieve a
certain average delay?
Traffic Intensity
 Load presented to a system:
A=λh
λ=
mean rate of calls (requests)
attempted per unit time
h =mean holding time per successful call
A= traffic intensity [erlang]
Factors that Determine the Nature of
the Traffic Model
Manner in which blocked calls are handled
1. Lost calls delayed (LCD) –blocked calls put in a queue
awaiting a free channel
2. Blocked calls rejected and dropped
3. Lost calls cleared (LCC) –user waits a random time interval
before another attempt
4. Lost calls held (LCH) –user repeatedly attempts calling
 Number of traffic sources, L
1. Whether number of users is assumed to be finite or infinite
2. Infinite L easier to analyze (reasonable assumption when L
is at least 5 to 10 times the capacity)

First-Generation Analog (examples)
Advanced Mobile Phone Service (AMPS)
In North America, two 25-MHz bands allocated to AMPS
(to/from base station), below 900 MHz
 Each band split in two to encourage competition
 Channel spacing 30 kHz, which allows 416 channels per
operator from which 21 channels for control and 395
channels to carry calls (Total 832 channels, uplink(MS to
BTS) & downlink(BTS to MS))..See table 10.4 page 318 book.

Nordic Mobile Telephone System (NMT)
Mainly in Scandinavia, but later spread
 Two systems: 450 MHz and 900 MHz
 Channel spacing 12.5 kHz, 1999 channels (NMT900)

Differences Between First and
Second Generation Systems
 Digital
traffic channels in second generation
 Encryption –all second generation systems
provide encryption to prevent eavesdropping
(To listen secretly)
 Error detection and correction in secondgeneration digital traffic
 Channel access –second-generation systems
allow channels to be dynamically shared by a
number of users
 Higher data rates
Mobile Wireless TDMA Design
Considerations
Number
of logical channels: (number of
time slots in TDMA frame)
Maximum cell radius (R): 35 Km to give
a sufficiently high traffic level in rural
areas
Frequency: Region around 900 MHz.
Maximum vehicle speed (Vm): 250
Km/hr or 69.4 m/s to accommodate
mobile units in high speed trains
Maximum
coding delay: Approximately
20 ms to avoid unduly to delays within
fixed networks, which may involve
satellite links
Maximum delay spread (Δm): The
difference in the propagation delay
among different multipath signals
arriving at the same antenna.
Bandwidth: Not to exceed 200 KHz
corresponding to 25 KHz/channel
Global
System
for
Communications, GSM
 Developed
Mobile
to provide a common 2G technology for
Europe
 Now mainly in Europe and Pacific Asia but also in
North America.
 Standardized functional elements (equipment from
different manufacturers can be combined)
 Today probably the most popular standard for new
implementations
 Subscriber Identity Module (SIM) concept of great
importance
Mobile Station
 Mobile
station communicates across air interface
with base station transceiver in same cell as mobile
unit
 Mobile equipment (ME) –physical terminal, such as
a telephone or PCS
 ME includes radio transceiver, digital signal
processors and subscriber identity module (SIM)
 GSM subscriber units are generic until SIM is
inserted
 SIMs roam, not necessarily the subscriber devices
(only the device can be used for emergency services
without SIM)
Base Station Subsystem (BSS)
 BSS
consists of base station controller and
one or more base transceiver stations (BTS)
 Each BTS defines a single cell
 Includes radio antenna, radio transceiver and
a link to a base station controller (BSC)
 BSC reserves radio frequencies, manages
handover of mobile unit from one cell to
another within BSS, and controls paging
Network Subsystem (NS)
 NS
provides link between cellular network
and public switched telecommunications
networks
 Controls handovers between cells in different
BSSs
 Authenticates users and validates accounts
 Enables worldwide roaming of mobile users
 Central element of NS is the mobile switching
center (MSC)
Mobile Switching Center
supported by 4 databases
(MSC)
Home location register (HLR) database –stores
information about each subscriber that belongs to it
 Visitor location register (VLR) database –maintains
information about subscribers currently physically in the
region
 Authentication center database (AuC) –used for
Authentication activities, holds encryption keys
 (note: no encryption in PSTN end)
 Equipment identity register database (EIR) –keeps
track of the type of equipment at the mobile station. Blocking
stolen mobile phones, un-approved phones, …

Radio Link
2
x 25MHz allocated
Combines FDMA and TDMA
125 channels full duplex,
spaced 200kHz, data rate 271
kbps
TDMA Format Time Slot Fields
 Trail
bits–allow synchronization of
transmissions from mobile units located
at different locations from base station
 Encrypted bits–encrypted data in blocks
by conventional encryption of 114
plaintext bits into 114 cipher text bits.
Encrypted bits are then placed in two 57
bits fields in the time slot
 Stealing
bit-indicates whether block
contains data or is "stolen“ for urgent
control signaling
Training
sequence–used to adapt
parameters of receiver to the
current
path
propagation
characteristics and to select the
strongest signal in case of multi
path propagation.
Timing sequence is also known as bit
pattern that differs for different
adjacent cells.
Guard
bits–used
to
avoid
overlapping with other bursts due
to different path delays
GSM Frame Format
8
slot TDMA frames are typically organized into 26frame multi frame. One of the frame in multi frame
is used for control signaling and the other is
currently unused, leaving 24 frame for data traffic.
 So each traffic channel receives one slot/frame and
24 frames/120ms multi frame, then the resulting
data rate is:
The
GSM specification also allows half
rate traffic channels, with two traffic
channels occupying one time slot in 12 of
the 26 frames. With the use of half rate
speech coders, this effectively doubles
the capacity of the system.
There is also a 51 frame used for control
traffic.
 Frame
structure is the division of defined length
of digital information into different fields
(information parts). A GSM frame is 4.615 m sec
and it is composed of 8 time slots (numbered 0
through 7). During voice communication, one user is
typically assigned to each time slot within a frame.
The GSM system also combines frames to form
Multi frames.
 Multi frames are frames that are grouped or linked
together to perform specific functions. Multi frames
on the GSM system use established schedules for
specific purposes, such as coordinating with
frequency hopping patterns. Multi frames used in
the GSM system include the 26 traffic multi
frame, 51 control multi frame, super frame,
Traffic Multi frame Structures - The 26 traffic multi
frame structure is used to send information on the traffic
channel. The 26 traffic multi frame structure is used to
combine user data (traffic), slow control signaling (SACCH),
and idle time period. The idle time period allows a mobile
device to perform other necessary operations such as
monitoring the radio signal strength level of a beacon
channel from other cells. The time interval of a 26 frame
traffic multi frame is 6 blocks of speech coder data (120 m
sec).
 Control Multi frame Structures - The 51 control multi
frame structure is used to send information on the control
channel. The 51 frame control multi frame is sub divided into
logical channels that include the frequency correction burst,
the synchronization burst, the broadcast channel (BCCH),
the paging and access grant channel (PAGCH), and the
stand-alone dedicated control channel (SDCCH). The PAGCH
is logically sub divided into PCH and AGCH.

Super frame - A super frame is a multi frame sequence that
combines the period of a 51 multi frame with 26 multi frames
(6.12 seconds). The use of the super frame time period allows
all mobile devices to scan all the different time frame types at
least once.
 Hyper frame - A hyper frame is a multi frame sequence
that is composed of 2048 super frames, and is the largest
time interval in the GSM system (3 hours, 28 minutes, 53
seconds). Every time slot during a hyper frame has a
sequential number (represented by an 11 bit counter) that is
composed of a frame number and a time slot number. This
counter allows the hyper frame to synchronize frequency
hopping sequence, encryption processes for voice privacy of
subscribers' conversations.

Speech and Data Encoding
 Speech
1.
coding
RPE-LPE, regular pulse excited -linear
predictive coding (Data from the
previous samples are used to predict
the current sample) (each sample is
then encoded to consist of bits
representing the coefficients of the
linear combination of previous samples
plus an encoded form of the difference
between the predicted and actual
sample)
1.
2.
3.
4.
Bits divided into 3 classes with different
protection
22.8 kbps resulting data rate
spread into different time slots, interleaving
(interleaving is a way to arrange data in a
non-contiguous (referring to two or more
information's of the system which are
not connected) way to increase
performance, e.g ,, In error-correction
coding, particularly within data
transmission, disk storage, and computer
memory.)
Gaussian minimum shift keying is used
Data
1.
2.
3.
encoding
Error correcting code
Interleaving
9.6, 4.8 or 2.4 kbps
Freq.
Hopping
Equalization
and
Delay
Frequency hopping
1. Slow frequency hopping, hop for each TDMA frame
2. Suppresses multipath fading and co-channel interference
 Delay equalization
1. Several users with different distance share same TDMA
frame
2. Base station provides control signal, mobile unit adjusts
timing
3. Tail and guard bits provide margin

Functions Provided by Protocols
 Protocols
above the link layer of the GSM signaling
protocol architecture provide specific functions:
1.
2.
3.
Radio resource management ( controls the
setup, maintenance, termination of radio channels
including handoff)
Mobility management (Manages the location,
updating registration procedures, as well as
security authentications)
Connection management (Handles the setup,
maintenance and the terminations of calls
(connection between end users))
4. Mobile application part (MAP) (handles
Most of the signalling between different
entites in the fixed part pf the network such
as HLR, VLR)
5. BTS management (Performs various
management and administrative functions
at the BTS under the control of the BSC)