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CELLULAR MOBILE COMMUNICATION SYSTEMS
May 16, 2006Niclas Wiberg, Ericsson Research
© Ericsson AB 2006 2006-05-152 (33)
Contents
� Introduction to cellular systems� GSM, GPRS (2G)� WCDMA (3G)� System modeling and analysis� High-Speed Downlink Packet Access (HSDPA)
© Ericsson AB 2006 2006-05-153 (33)
History
� Analog systems (e.g NMT)– speech
� Digital systems (e.g GSM)– speech and simple data, improved capacity
� Evolved digital systems (GPRS, EDGE)– improved data services
� Third generation (WCDMA)– high bit rates, improved flexibility, improved capacity
� Fourth generation?
© Ericsson AB 2006 2006-05-154 (33)
Architecture Overview
base stations
mobile stations
base station controller(s)
mobile switch(es)
fixed network
air interface
© Ericsson AB 2006 2006-05-155 (33)
Resource Limits
� attenuation, shadowing, and receiver noise� multipath fading & time dispersion� transmitter power� interference
� coverage vs. capacity limits
© Ericsson AB 2006 2006-05-156 (33)
Multiple Access:Handling Several Users Simultaneously
TDMA: Time Division Multiple Access
t
f
FDMA: Frequency Division Multiple Access
t
f
FDMA+TDMA
t
f
t
f
CDMA: Code Division Multiple Access
© Ericsson AB 2006 2006-05-157 (33)
The GSM Air Interface
� FDMA– 174 frequencies with 200kHz spacing (GSM 900)– separate bands for uplink and downlink– divided between operators
� TDMA– 8 time slots per frequency band (one user per time slot)
� Modulation– 271 kbit/s GMSK
© Ericsson AB 2006 2006-05-158 (33)
The GSM Air Interface, cont.
456 bits
244 bits
interleaving & segmentation
456 bits
244 bits
channel coding
20 ms audio frame 20 ms audio frame
speech coding
TDMA frame with 8 time slots
© Ericsson AB 2006 2006-05-159 (33)
The GSM Air Interface, cont.
TDMA frame ≈ 4.6 ms
3 bits tail
3 bits tail
8 bits guard
26 bits pilot
58 bitsdata
58 bitsdata
timeslot ≈ 0.58 msbitrate 271 kbit/s, GMSK modulation
payload burst
3 bits tail
3 bits tail
68 bitsguard
41 bitspilot
36 bitsdata
access burst
© Ericsson AB 2006 2006-05-1510 (33)
GSM: Frequency Planning
� Reuse factor
� Tighter reuse:– higher capacity– interference between
cells
© Ericsson AB 2006 2006-05-1511 (33)
GSM: Handover
activeradio link
measurement1. mobile measures
other cells
activeradio link
measurement2. better cell detected,
handover initiated
measurement activeradio link
3. handover completed
© Ericsson AB 2006 2006-05-1512 (33)
GSM, cont.
� Power control– maintain adequate quality at minimum power– reduces interference– increases battery life
� Frequency hopping– avoids bad quality due to fading and interference
© Ericsson AB 2006 2006-05-1513 (33)
GPRS:General Packet Radio Services
� Uses GSM modulation� Channel setup and release on demand
– “always connected” at low cost
� Timeslot scheduling– several time slots per user– several users per time slot
� Different coding schemes– bitrate depends on radio conditions
� Retransmission of erroneous packets
© Ericsson AB 2006 2006-05-1514 (33)
EDGE:Enhanced Data rates for GSM Evolution
� Higher-order modulation – 8-ary phase-shift keying– bitrates up to 384 kbps– same spectral properties as GSM & GPRS
� Fast link adaptation– good channel conditions => high bitrates– bad channel conditions => low bitrates, better robustness
© Ericsson AB 2006 2006-05-1515 (33)
WCDMA (UMTS, 3G) Requirements� Speech capacity & coverage better than GSM� Efficient & flexible services:
– realtime– variable bitrate– packet
� 2 Mbit/s in good conditions� 384 kbit/s everywhere
© Ericsson AB 2006 2006-05-1516 (33)
CDMA Principle
Up sampling× SF
Encoded bit sequence (±1)
Pseudo noise spreading
sequence (±1)
ModulationTransmitted signal
Demodulation
Received noisy signalAveraging
÷ SF
Received (less) noisy bit sequence
© Ericsson AB 2006 2006-05-1517 (33)
CDMA Interference Suppression
f
P
fc
original signal
f
P
fc
spread signal
f
P
fc
received signal
f
P
fc
despreadsignal
f
P
fc
received signal after averaging
© Ericsson AB 2006 2006-05-1518 (33)
CDMA Bit Rate Flexibility
Power is the common shared resource
VaryingUser Bit Rate
VaryingUser Bit Rate
Translates intoVarying spreading factorVarying power levelBut, same physical resource
Varying spreading factorVarying power levelBut, same physical resource
Bit rate
Spreading factor
Power level
© Ericsson AB 2006 2006-05-1519 (33)
CDMA Near/Far Effect
Power control needed to keep interference down
© Ericsson AB 2006 2006-05-1520 (33)
WCDMA Air Interface Basics
� 3.84 MHz chip rate� QPSK modulation, 5 MHz bandwidth� Spreading factor from 4 to 256� Bitrates up to
– 2 Mbit/s with multicode– 1 Mbit/s with single code
� 1500 Hz closed-loop power control� Frame length 10-80 ms
© Ericsson AB 2006 2006-05-1521 (33)
WCDMA Physical Channels and Power Control
uplink physical control channel
uplink physical data channel
downlink physical control channel
downlink physical data channel
Physical control channel:• pilot for channel estimation• power control commands• data format information
Physical data channel:• user data
© Ericsson AB 2006 2006-05-1522 (33)
WCDMA Air Interface Example:Speech and Data Multiplexed
244 bit speech frame
320 bit data640 bit data
244 bit speech frame
320 bit data(no data)
coding, multiplexing, and interleaving
1200 bit2400 bit
300 bit 1200 bit
10 ms
slot segmentation
15 slots
© Ericsson AB 2006 2006-05-1523 (33)
WCDMA: Frequency Reuse 1
© Ericsson AB 2006 2006-05-1524 (33)
WCDMA: Soft Handover
� Enhances quality� Reduces interference due to
fast fading� Transmit power determined
by best link
© Ericsson AB 2006 2006-05-1525 (33)
Simple Quality Model (Uplink)
� Received signal power user i:
gi pi
� Total received power, “interference”
I = N0 + g1 p1 + g2 p2 + … + gM pM
� Signal-to-noise ratio user i
γi = gi pi / (I − gi pi) ≈ gi pi / I
p1
p2
pM
…
g1 g2
gM
© Ericsson AB 2006 2006-05-1526 (33)
Simple Capacity Calculation (Uplink)� Power control: assume pi is
controlled such that the signal-to-noise ratio is constant:
gi pi / I = Γ
� Interference
I = N0 + g1 p1 +…+ gM pM = N0 + M I Γ
� Solving for I results in
I = N0 / (1 − M Γ)0 20 40 60 80 100
0
2
4
6
8
10
12
14
16
18
20
Number of users
Noi
se r
ise
[dB
]
© Ericsson AB 2006 2006-05-1527 (33)
High-Speed Downlink Packet Access (HSDPA)
� Fast Link Adaptation– Data rate adapted to radio conditions– 2 ms time bases– Higher order modulation
� Fast Retransmissions– Roundtrip time of 10-20 ms possible– Soft combination of multiple attempts
� Shared Channel Transmission– Dynamically shared in code and time
� Fast Radio-Dependent Scheduling– Transmit to users with good radio
conditions– 2 ms time basis 2 ms
© Ericsson AB 2006 2006-05-1528 (33)
Fast Link Adaptation
� Adjust transmission parameters to match instantaneousradio channel conditions
– Path loss and shadowing– Interference variations– Fast multi-path fading
� Rate controlled (not power controlled)– Encoding rate, number of channelization codes & modulation
type adapted based on available power
– Adaptation on 2 ms TTI basis ⇒ 500 times/sec!
High data rate
Low data rate
© Ericsson AB 2006 2006-05-1529 (33)
Fast Link Adaptation Power Utilization
Dedicated channels (power controlled)
Common channels
Power usage with dedicated channels channels
t
Unused power
Power
HSDPA with dynamic power allocationt
Dedicated channels (power controlled)
Common channels
HSDPA (rate controlled)
To
tal c
ell p
ow
er
Power
To
tal c
ell p
ow
er
– No need for extra spectrum/carrier– Voice and data on same carrier
Original WCDMA WCDMA HSDPA
© Ericsson AB 2006 2006-05-1530 (33)
Scheduling
� Scheduling = which UE to transmit to at a given time instant
� Basic idea: – Transmit to users based on radio channel quality, targeting
fading peaks– May lead to large variations in data rate between users– Tradeoff: fairness vs. cell throughput
high data rate
low data rate
Time
#2#1 #2 #2#1 #1 #1
User 2
User 1
Scheduled user
© Ericsson AB 2006 2006-05-1531 (33)
Performance Example
� 2-3 times capacity increase– for web browsing– less for streaming
� ~3 times lower download time
– for web browsing– large TCP objects (file
transfer) can show even larger performance gains
� Scheduling strategy has a large impact on the performance.
0 0.5 1 1.5 2 2.5 3 3.50.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Normalized System Throughput
Nor
mal
ized
Del
ay
HS-DSCH, max C/I HS-DSCH, RR64 kbps dedicated
Fast Scheduling
Fast Link Adaptation
Reduced hybrid ARQ round-trip
delay
Normalized throughput 1 equals approx. 600 kbit/s/cell.
Web browsing, PedA channel model
© Ericsson AB 2006 2006-05-1532 (33)
Latency Radio-access-network contribution of Round Trip Time (RTT)
Latency
≈≈≈≈150 ms Initial WCDMA (R99)
HSDPA<100 ms
30-50 msEnhanced uplink
UL 384 kbpsDL 384 kbps
UL 384 kbpsDL 14400 kbps
UL 4700 kbpsDL 14400 kbps
© Ericsson AB 2006 2006-05-1533 (33)
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