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Document Sample
PERSONAL COMMUNICATION
SYSTEMS: SECOND GENERATION
Ian F. Akyildiz
Broadband & Wireless Networking Laboratory
School of Electrical and Computer Engineering
Georgia Institute of Technology
Tel: 404-894-5141; Fax: 404-894-7883
Email: ian@ece.gatech.edu
Web: http://www.ece.gatech.edu/research/labs/bwn
Differences between AMPS and GSM
AMPS uses analog technology
GSM uses digital technology
AMPS has poor performance for data transfer
Both systems use control channels to initiate calls.
AMPS uses 21 control channels w hile GSM uses 3.
AMPS is less secure than GSM
GSM has the SIM smart card w hich holds the user’s
personal information and phone settings.
In AMPS only the HLR has that data.
Both systems support ROAMING but GSM allow s more
compatibility.
IFA’2004 2
Differences between AMPS and GSM
AMPS requires less power at the MS and at the BS.
GSM’s power control attempts to minimize radio
transmission power of the MS and BTS, thus,
minimizes the amount of co-channel interference.
AMPS instead relies on the digital color code (DCC)
for that.
AMPS has a cell radius 1.5km - 25km
GSM is more flexible with cell sizes.
IFA’2004 3
EXAMPLES
IS-41 or IS-136 (D-AMPS) uses TDMA scheme;
(AT&T Wireless, BellSouth, Southwestern Bell)
IS-95 uses CDMA (Bell Atlantic/NYNEX, Verizon,
Sprint PCS)
GSM uses TDMA (used worldwide; here Cingular; T-
Mobile).
IFA’2004 4
INTRODUCTION:
USA: 2G Systems
1G -> AMPS -> problems to serve large number of users.
2G Systems w ith Digital Modulation techniques (called
Digital Cellular) achieved large improvements.
Late 80ies, USDC (US Digital cellular system) started to
support more users in a fixed spectrum allocation.
US Digital Cellular System (D-AMPS:
Digital Advanced Mobile Phone System)
/ \
/ \
Air Interface Mobility Management
IS-54 IS-41 (new version IS-136)
IFA’2004 5
Reference Model for
North American Systems
The TIA Committees TR-45 and
TR-46 develop performance,
compatibility, interoperability,
and service standards.
– TR-45.3 TDMA
– TR-45.5 CDMA
New interfaces when compared
to GSM Model:
– DMH: data message handler
– collects billing information
– IWF: interworking function
– allows an MSC to connect
to other networks
– AUX: auxiliary equipment –
can connect to an MSC
Messaging is carried out by
protocols very similar to SS-7
IFA’2004 6
IS-54
IS-54 Architecture similar to AMPS, GSM
in terms of MSC, BS, Mobile Stations, HLR, VLRs
IS-54 standardized in 1990 (Interim Standard-IS)
IS-54 shares same frequencies, frequency reuse plan and
base stations as AMPS so that base stations and subscriber
units could be equipped w ith both AMPS and IS 54
channels w ithin the same piece of equipment.
Both AMPS and IS-54 cellular carriers provide new
customers w ith IS-54 phones and may gradually replace
AMPS base stations w ith IS-54 BSs channel by channel over
time know n as D-AMPS
AMPS - D-AMPS (IS-54)
IFA’2004 7
IS-54
IS-54 uses TDMA supporting 3 full rate users or 6 half rate users on each
AMPS channel.
6 times more capacity than AMPS
IS-54 uses the same 45 MHz FDD scheme as AMPS
REMARK:
* Change from Analog to Digital
* Temporary increase in Interference and dropped calls
in AMPS; since each BS changed over to digital; the number of
analog channels in geographic area is decreased.
Compatibility with AMPS:
* IS 54 Forward/Reverse control channels use exactly the same
signaling technique as AMPS.
* Voice Channels are 4–ary pi/4 DQPSK modulation with a
channel rate 48.6 kbps.
IFA’2004 8
IS-54
Forward and Reverse control channels use the same
10 kbps FSK signaling scheme as in AMPS
REMARK:
* IS 136 (formerly IS-54-C) includes pi/4 DQPSK
modulation for control channels.
* IS-54-C provides 4-ary keying instead of FSK on
control channels
REASONS: Increase control channel data rate also
provide special services like paging etc..
IFA’2004 9
RADIO INTERFACE (IS-54)
GOAL: Smooth transition from AMPS IS-54
IS 54 designed to operate using both AMPS and IS 54 standards.
Multiple Access TDMA/FDD
Modulation pi/4 DQPSK
Channel Bandw idth 30 kHz (sam as in AMPS)
Reverse Channel Freq. BW 824-894 MHz (same as in AMPS)
Forw ard Channel Freq. BW 869-894 MHz (same as in AMPS)
FW and Reverse Channel Data Rates 48.6 kbps
Channel Coding 7 bit CRC and ½ rate convolutional coding
of constraint length 6.
Users per channels 3 (full rate speech coder of 7.95
kbps/user) 6 (half rate speech coder of 3.975 kbps/user).
IFA’2004 10
CHANNELS (IS-54)
Control Channels AMPS Control Channels
42 Primary AMPS Control Channels Primary
+ IS 54 (42 additional control channels) dedicated for IS
54 use only (Secondary Control Channels)
IFA’2004 11
DATA CHANNELS (IS-54)
Digital Traffic Channel (DTC) carries user information
(speech or user data)
Reverse DTC: carries speech from mobile to BS.
Forward DTC: carries speech from base to mobile
IFA’2004 12
DATA CHANNELS (IS-54)
Coded Digital Verification Color Code Channel
(CDVCC)
Slow Associated Control Channel (SACCH)
Fast Associated Control Channel (FACCH)
IFA’2004 13
DATA CHANNELS (IS-54)
Coded Digital Verification Color Code Channel (CDVCC)
Is a 12 bit message sent in every time slot.
(Function similar to SAT (Supervisory Audio Tone) as in AMPS
allow s each BS and its mobiles to confirm that they are
properly connected during a call.
It is an 8-bit number ranging from 1 and 255 protected w ith 4
additional channel coding bits (12,8) Hamming code.
BS CDVCC values on Forw ard Voice Channel
Each subscriber using TDMA channel must receive, decode
and retransmit the same CDVCC value to BS on the reverse
voice channel.
Handshake: If not, then time slot w ill be relinguished for other
users.
IFA’2004 14
DATA CHANNELS (IS-54)
Slow Associated Control Channel (SACCH)
Sent in every time slot.
Carries various control and supervisory message
between mobile and BS, e.g., power level changes
and handoff requests.
Also used by mobile to report results of signal
strength measurements of neighboring base stations
to help BS to do MAHO (Mobile Assisted Handoff).
IFA’2004 15
DATA CHANNELS (IS 54)
Fast Associated Control Channel (FACCH)
(Signaling Channel)
Important control or specialized traffic data between
BS and mobiles;
e.g., call release instructions, MAHO, mobile status
requests.
The FACCH data when transmitted takes the place of
user info data within a frame.
IFA’2004 16
Frame Structure for IS-54
(Mobile to BS)
1 Frame=1944 bits (97 symbols)= 40 ms
1 2 3 4 5 6 25 frames/sec
6.67msec 324 bits (260 bits
G R Data SYN DATA S CD DATA
user data)
G Guard Time (6 bits) (when no signal is transmitted)
R Ramp Time (6 bits) (to allow transmitter to reach its full output power
level)
Data 16 bits
SYN 28 bits
DATA 122 bits
SACCH 12 bits
CDVCC 12 bits (helps to identify the frequency
channel to which mobile is tuned)
DATA 122 bits
IFA’2004 17
Mobility Management
IS-41 (new version IS-136)
IS 41 <-> GSM
Procedure for delivering calls to mobile users in
GSM very similar to IS 41.
Note that names, contents, lengths of messages
may be different.
IFA’2004 18
IS-41 Standard for MSC-
MSC interface
IS-41 is primarily used in the core network
to provide services such as automatic roaming,
authentication, intersystem handoff, short
message service, etc.
– All wireless network elements such as the
MSC, HLR, VLR, EIR, and AUC, use this
messaging protocol to communicate among
themselves
– Signaling protocol stack very similar to SS-7
– Intersystem handoff: handoff involving two
BSSs controlled by different MSCs.
IFA’2004 19
IS-41: Intersystem Handoff
The current MSC will request a RSS measurement from the
candidate MSC.
Once RSS measurements indicate the candidate MSC as suitable
for handoff, the two MSCs will complete the intersystem
handoff.
Three types of handoff:
– Handoff forward: transfer from one MSC to another MSC of a
new system
– Handoff backward: transfer from the new MSC back to the old
MSC
– Handoff third: transfer from an MSC in a second system to a
MSC of a third system
During handoff, IS-41 signaling messages will carry terminal
information, call information, and air-interface information
(serving and destination cells and channels). It also performs
authentication procedures between the two systems.
IFA’2004 20
Differences IS-41 - GSM
When the new VLR receives the registration affirmation
(REGNOT IS-41) from HLR, it assigns a new TMSI to the
terminal for the new RA.
HLR also provides new VLR w ith all relevant subscriber
profile information required for call handling (e.g., call
screening lists, etc.) as part of affirmation message.
Thus, in contrast to IS-41, authentication and subscriber
profile information are obtained from both HLR and VLR
and not just the HLR.
IFA’2004 21
OVERVIEW
IS-54 GSM IS-95
Access Technology TDMA/FDD TDMA/FDD CDMA (DS)/FDD
Freq Band:
BS 869-894 935-960 (1805-1880) 869-894
Mobile 824-849 890-915 (1710-1785) 824-849
Channel Spacing 30kHz 200kHz 1250kHz
Modulation pi/4 DQPSK GMSK BPSK/QPSK
Power Max/
Average Milliwatts 600/200 1000/125 600
Speech Rate (kbps 7.95 13 8 (variable rate)
Frame Duration (ms) 40 4.615 20
Coding ½ rate ½ rate ½ rate conv for Forward
convolutional conv. 1/3 rate conv. for Reverse; CRC
DQPSK: Differential Quadratic Phase Shift Keying; QPSK: Quadrature Phase Shift Keying; GFSK: Gaussian
Freq. Shift Keying; BPSK: Binary Phase Shift Keying; GMSK: Gaussian Minimum Shift Keying
IFA’2004 22
IS-95
Digital Cellular:
– Two different directions for the air-
interface:
IS-136 standard based on TDMA
IS-95 standard based on CDMA
– Interoperability was only possible via dual
mode telephones
– IS-41 standard has now evolved to
support both IS-136 and IS-95
IFA’2004 23
IS-95
IS-95 is the North American digital cellular
standard that employs CDMA as the
Access Method as well as the Air-Interface.
It was developed by Qualcomm around 1990.
CDMA/AMPS dual mode phones were produced by Qualcomm in 1994.
Also called cdmaOne.
In Dec. 1993, the TIA published Qualcomm’s air-
interface specifications as the interim standard IS-95.
Formally adopted in July 1993 and revised in May 1995.
IFA’2004 24
IS-95 CDMA
Digital AMPS increased capacity of AMPS by factor 3
First Code Division Multiple Access (CDMA)
cellular system was developed in 1990, claimed
to increase capacity of AMPS by factor 20.
IFA’2004 25
IS-95 CDMA
GOAL:
Low Cost
Light-weight hand-held portable units
Battery Life
Spectrally efficient
Low link budgets
Minimum number of base stations
Excellent grade of service
Excellent scalability
Reduction of dropped calls
Reduction of fading and poor voice quality
IFA’2004 26
CDMA
CDMA is both an Access Method and an Air-Interface.
Similarities:
Core fixed network infrastructure GSM core network structure
Radio resource management, mobility management, and security
same as in TDMA (D-AMPS) systems.
There are differences in terms of
* Handling thepower control
* Employing soft handoff
IFA’2004 27
Access Method CDMA
CDMA (Code Division Multiple Access)
– All terminals send on the same frequency
probably at the same time and can use the
whole bandwidth of the transmission channel
– Each sender has a unique random number,
the sender XORs the signal with this random
number
– The receiver can “tune” into this signal if it
knows the pseudo random number; tuning is
done via a correlation function
IFA’2004 28
Access Method CDMA
CDMA (Code Division Multiple Access)
User data stream encoded with ½ convolutional
code rate, interleaved and spread by one of 64
orthogonal spreading sequences (Walsh functions).
Each MS in a given cell is assigned a different
spreading sequence, providing perfect separation
among signals from different users.
IFA’2004 29
Access Method CDMA
CDMA (Code Division Multiple Access)
To reduce interference between MSs which
use same spreading sequence in different
cells and to provide the desired spectral
characteristic, all signals in a cell are
SCRAMBLED using a pseudo-random
sequence of length 2^15 chips.
IFA’2004 30
CDMA (Code Division Multiple Access )
Frequency
User 1
User 2
...
User n
Time
Code
IFA’2004 31
CDMA (Advantages and Disadvantages)
Advantages:
– All terminals can use the same frequency, no planning needed
– Huge code space compared to frequency space
– Interference (e.g. white noise) is not coded
– Forward error correction and encryption can be easily integrated
Disadvantages:
– Higher complexity of a receiver (receiver cannot just listen into
the medium and start receiving if there is a signal)
– All signals should have the same strength at a receiver
IFA’2004 32
CDMA: Further Advantages
and Disadvantages
Advantages:
– CDMA provides an increase in system capacity when
compared with the analog and TDMA systems.
– CDMA improves quality of voice by using a better voice
coder.
– CDMA has resistance to multipath and fading.
– CDMA implements soft handoffs.
– CDMA has less power consumption (about 10% of analog
or TDMA phones) because of implementation of power
control.
– CDMA does not require frequency planning because all
cells employ the same frequency at the same time.
Disadvantage:
– Necessity for power control and complexity.
IFA’2004 33
Comparison
TDMA/FDMA/CDMA
Approach TDMA FDMA CDMA
Idea segment sending segment the spread the spectrum
time into disjoint frequency band into using orthogonal codes
time-slots, demand disjoint sub-bands
driven or fixed patterns
Terminals all terminals are every terminal has its all terminals can be active
active for short own frequency, at the same place at the
periods of time on uninterrupted same moment,
the same frequency uninterrupted
Signal synchronization in filtering in the code plus special
separation the time domain frequency domain receivers
Advantages established, fully simple, established, flexible, less frequency
digital, flexible robust planning needed, soft
handover
Dis- guard space inflexible, complex receivers, needs
needed (multipath frequencies are a more complicated power
advantages propagation), scarce resource control for senders
synchronization difficult
Comment standard in fixed typically combined still faces some problems,
networks, together with TDMA higher complexity,
with FDMA/SDMA (frequency hopping lowered expectations; will
used in many patterns) and SDMA be integrated with
mobile networks (frequency reuse) TDMA/FDMA
IFA’2004 34
Air-Interface CDMA
The air-interface in CDMA is not
symmetrical on the forward and reverse
channels (separated by 45MHz)
– One Forward Channel (1.25 MHz in 824-
849 MHz bands): transmissions originate
at a single transmitter (BS) and
transmissions for all users are
synchronized.
– One Reverse Channel (1.25 MHz in 869-
894 MHz bands): mobile terminals
transmit whenever they have to.
IFA’2004 35
Air-Interface CDMA
Forward Channels (BS MOBILE):
– Pilot (1 Channel):
Provides a reference signal to all MSs within a cell
for demodulation. It is also used for signal
strength comparison. Determines when to handoff.
– Synchronization (1 Channel):
Used to acquire initial time synchronization. The
sync message includes the system and network
identification, coding information, and the paging
channel data rate. Operates at 1200kbps.
IFA’2004 36
Air-Interface CDMA
Forward Channels:
– Paging (7 channels)
As in GSM, used to page the MS when there is an
incoming call, and to carry the control messages for call
setup. Operates at 9600,4800, 2400 bps)
– Forward Traffic (63 channels):
Carries the actual user information.
Two possible rate sets, RS1 and RS2.
RS1 supports date rates of 9.6, 4.8, 2.4, and 1.2 kbps.
RS2 supports 14.4, 7.2, 3.6, and 1.8 kbps.
Multiplexed with power control information.
IFA’2004 37
Air-Interface CDMA
Reverse Channels (MOBILE BS):
– Access (max. 32 channels):
Used by the MS to transmit information to the BS such as
call origination, response to a page and so on.
Fixed Data Rate 4800 kbps. Random access channel user
uniquely identified by their long codes.
– Reverse Traffic:
Supports voice data at two rate sets: RS1 and RS2.
It is used to send information related to the signal strength
of the pilot and frame error rate statistics to one BS or
multiple BSs.
It is also used to transmit control information to the BS
such as a handoff completion message.
Operates on variable data rate.
IFA’2004 38
IS-95 CDMA Channels
Overview
Types of Channels
Forward Channels Application
Pilot System mon.
Synchronization Sync.
Paging Signaling
Traffic Voice/data
Reverse channels
Access Signaling
Traffic Voice/data
IFA’2004 39
IS-95 CDMA Interesting Features
Multiple users can share same frequency
Soft capacity limit: more users raises noise
floor linearly, no absolute limit on number
of users - performance degrades gradually
for all users
Multipath fading is reduced by signal
spreading
IFA’2004 40
IS-95 CDMA Interesting Features (cont)
Spatial diversity provides soft handoff:
MSC monitors signal of a user from multiple base
stations and chooses best version of signal at any
time
Self-Jamming is a problem:
Because spreading sequences of different users
are not exactly orthogonal
– When despreading, other users can
contribute significantly to receiver decision
statistic
IFA’2004 41
IS-95 CDMA Interesting Features (cont)
Near-Far Problem:
If power of multiple users are unequal, the
strongest received mobile signal will
capture demodulator at the base station
–Base stations must implement power
control to ensure that each mobile
within coverage area provides same
signal level to base station receiver
IFA’2004 42
IS-95 CDMA Interesting Features (cont)
The system can move a call from digital to
analog when the call enters the coverage
area of a cell that does not have CDMA
capability.
The opposite does not work.
IFA’2004 43
Mobility and Radio Resource
Management in IS-95
Soft Handoff
– Note: Unlike channelized wireless systems that assign
different radio channels during a handoff (called a
hard handoff), spread spectrum mobiles share the
same channel in every cell, thus the term handoff
does not mean a physical change in the assigned
channel, but rather that a different base station
handles the radio communication task.
– Refers to the process by which an MS is in
communication with multiple candidate BSs before
finally deciding to communicate its traffic through one
of them.
IFA’2004 44
Mobility and Radio Resource
Management in IS-95
Soft Handoff
– Reason to implement it is based on the near-far problem and
the associated power control mechanism:
If an MS moves far away from a BS and continues to
increase power to compensate for the near-far problem,
it might cause a lot of interference to MSs in neighboring
cells.
To avoid the above situation and to ensure that an MS is
connected to the BS with the largest RSS (Received
Signal Strength), a soft handoff strategy is implemented.
IFA’2004 45
Soft Handoff
IS-95 defines three types of soft handoff:
(a) Softer Handoff: between two sectors of the same cell
(b) Soft Handoff: between two sectors of different cells
(c) Soft-Softer Handoff:
includes two sectors from the same cell and
a third sector from a different cell.
IFA’2004 46
Soft Handoff
A controlling BS
coordinates the addition or
deletion of other BSs
The primary BS uses a
handoff direction message
(HDM) to indicate the pilot
channels to be used or
removed.
At some point, the primary
BS is also changed after
handoff.
The signals from multiple
BSs are combined in the
BSC or MSC and processed
as a single call. This is
achieved by using a frame
selector join message.
A frame selector remove
message is used to remove
the old BS.
IFA’2004 47
Soft Handoff
The pilot channels of each cell are involved in the handoff
mechanism.
– Only channel not subject to power control and providing a measure of
the RSS.
The MS maintains a list of pilot channels that it can hear and
classifies them into:
– Active Set: pilots continuously monitored or used by the MS
(MS has three RAKE fingers that allows it to monitor or use
up to three pilots).
– Candidate Set: can have at most six pilots, not in the active
set but with sufficient RSS to be used.
– Neighbor Set: contains pilots that belong to neighboring cells
and are intimated to the MS by a message on the paging
channel.
– Remaining Set: all other possible pilots in the system.
IFA’2004 48
Soft Handoffs and Thresholds
Whenever the strength of a pilot falls
below a threshold, the MS starts a dwell
timer.
Unless the pilot strength goes back above
the threshold before the timer expires,
the MS will drop it from a given set.
There is a trade-off in setting high and
low values for these thresholds and
timers.
IFA’2004 49
Soft Handoffs and Thresholds
If the strength of a candidate pilot
channel is above the pilot detection
threshold (T_ADD), this pilot must be
added to the active set and the MS enters
a soft handoff region.
– If T_ADD is too small, there may be
false alarms caused by noise or
interfering signals.
– If T_ADD is too large, useful pilot are
not detected, and the call may be
dropped.
IFA’2004 50
Handoff Thresholds in IS-95
IFA’2004 51
Handoff Thresholds in IS-95
1. As soon as the strength of the pilot exceeds
T_ADD, it is transferred to the candidate set, and
the MS sends the new pilot strength measurement
to the BS that is transmitting the current pilot.
2. The BS sends a handoff direction message to the
MS. The pilot is transferred to the active set.
3. The MS acquires a traffic channel and sends a
handoff completion message.
4. After the pilot strength falls below T_DROP, the
handoff timer is started.
IFA’2004 52
Handoff Thresholds in IS-95
If it is still below T_DROP after the timer expires,
the MS sends another pilot strength measurement to
the BS associated with the pilot.
The BS responds with HDM without the pilot in it.
The MS moves the pilot to the neighbor set.
The MS then sends a handoff completion message.
At some point, the BS may send it a neighbor update
list message that no longer contains the pilot and it
is moved into the remaining set.
IFA’2004 53
Power Control in CDMA
Co-channel and adjacent channel interference are not
the major problems in CDMA. Why?
The interference is from other users transmitting in
the frequency band at the same time.
To avoid the near-far problem, it is important to
implement good power control.
Signal strength may be good in CDMA, but frames may
still be received in error because of interference.
Thus, Frame Error Rate (FER) is used for power
control decisions rather than the signal strength used
in other PCS systems.
IFA’2004 54
Open Loop Reverse Link
Power Control in IS-95
RULE: Use a transmit power that is inversely
proportional to the received signal strength of pilots
from all BSs.
– On the access channel, the MS sends a request
using a weak signal if the pilot is strong.
– An ACK may not be received because the
transmit power was low or because of collisions.
– If no ACK is received, a stronger signal is
transmitted.
– This is repeated until a maximum power level is
reached.
– The process is repeated after a back-of delay.
IFA’2004 55
Open Loop Reverse Link
Power Control in IS-95
Up to 15 attempts can be made to obtain a
traffic channel.
Disadvantages: Assumption that forward
and reverse channels are identical; slow
response time; and using the total power
received from all BSs in calculating the
required transmit power.
IFA’2004 56
Closed Loop Reverse Link Power
Control in IS-95
On Downlink channel -> A power control bit is
transmitted every 1.25 ms (800 times per second).
– A “0” indicates the MS should increase the power
and a “1” indicates it should decrease it.
Every 1.25 ms, in the BS, the receiver determines
the received signal to interference ratio by sampling
it 16 times.
– If it is above a preset target, the MS is
instructed to reduce its power by 1 dB.
IFA’2004 57
Closed Loop Reverse Link Power
Control in IS-95
– Inner-Loop Power Control:
If below the target, the MS is instructed to
increase its power by 1 dB.
– The target value controls the long-term frame
error rate.
– Outer-Loop Power Control:
The target is varied over time to reflect
accurate values related to velocity, fading,
environment, and so on. It is reduced (increased)
by x dB every 20 ms if the FER is small (large)
enough.
IFA’2004 58
Closed Loop Reverse Link Power
Control in IS-95
IFA’2004 59
Forward Link Power Control
Employed to reduce inter-cell interference.
Wthin a cell, multiple users employ orthogonal
sequences and the primary source of interference is
from users of other cells or from multi-path.
A mobile assisted power control is employed:
– The MS periodically reports the FER on the forward
link to the BS, which will then adjust its transmit
power accordingly.
A maximum transmit power is defined to avoid
excessive interference.
A minimum transmit power is defined to avoid allowing
voice quality to drop.
IFA’2004 60
Overview of 2G Systems
(S/I)min N
AMPS ~18 dB 7
GSM ~11 dB 4
IS-54 ~16 dB 7
IS-95 ~15 dB 1
IFA’2004 61
