Download - ECE 5221 - Lecture23
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Florida Institute of technologies
ECE 5221 Personal Communication SystemsPrepared by:
Dr. Ivica Kostanic
Lecture 23 Basics of 3G - UMTS
Spring 2011
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W-CDMA (UMTS-FDD)
3G and 2G completely different air interfaces
Advanced radio resource management required
by diverse 3G applications
Multi-rate spreading
W-CDMA is interference limited
Provides soft capacity and
Coverage, capacity, qualitytradeoffs
2
10 ms 10 ms 10 ms
Power spectrum
density
frequency
time
f1
f2
frame
W-CDMA
Channel
DL TX Diversity, DL and UL RAKE receiverreception, UL space diversity
Diversity support
Coherent on both UL and DLDetection
Open loop and closed loop with 1500commands/sec
Power control
10msFrame Length
15,30,60,120,240,480,960,1920 kb/sec
Up to 3 code aggregations
Single code user rates
DL(after coding)
15,30,60,120,240,480, 960 kb/sec
Up to 6 code aggregation
Single code user rates
UL (after coding)
Variable: UL 1-512 (power of 2), DL(1-256)Spreading
3.84 Mc/secondChip rate
5MHzBandwidth
DS-CDMA with FDD and asynchronous operationAccess scheme
ValueProperty
Summary of W-CDMA properties
UL uplink; DL - downlink
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DS SS Systems - basic principles
Three basic stages
Spreading
RF Modem
De-spreading
Page 3
xShaping
Filter xInput data at
user rate
Spreading
sequence at
the higher rate
than user data
Baseband Spreading RF Modulation
( )to
wcos
CDMA Transmitter
Digital processingRF (Analog)
processing
PAFront
End
Filterx
( )to
wcos
Base
Band
Filterx
RF Demodulation
Spreading
sequence at
the higher rate
than user data
Wireless
Channel
CDMA Receiver
"RF MODEM"
Integrate
and dump
Output data
at user rate
Baseband De-Spreading
Processing of the signal for a single CDMA user
RF modem part is independent of CDMA
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Page 4
DS CDMA - multiple users
After spreading signals from multiple users are summed
Signals from multiple users co-exist in time and frequency
The spreading codes have to be orthogonal
x
X
X
S
X
X
X
S1
S2
Sn
S1
S2
Sn
C1
C1
Integrate
and Dump
Integrate
and Dump
Integrate
and Dump
C2
C2
Cn Cn
X= S1C
1+S
2C
2+S
3C
3
Ci
- Spreading code
Si
- User data
X - Spread spectrum signal
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Example of DS CDMA - two users same PG
Processing gain (PG) is the ratio of chip and bit rates
Page 5
x
X
S
X
X
S1
S2
C1C
1
Integrate
Integrate
C2
= 1
= -1
1 11 1
1 11 1
1 1-1 -1
0 0 2 2
X= S1C
1+S
2C
2
S1
C1
x
S2
C2
x
1 11 1
1 1 -1 -1
1 1 -1 -1
C2
0 0 2 2
0+0+2+2 = 4 > 0
0 0 - 2 -20+0-2-2 = -4 < 0
X C1
x
X C2
x
1 was
sent
-1 was
sent
Two signals coexist in
time and frequency
b
c
R
RPG
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UMTS Voice Example
Vocoder rate 12.2kbps
Chip rate 3.84Mbps
6
Processing gain
dB25102.121084.3log10dBPG
3
6
Required S/N ratio for voice after de-
spreading is around 5dB
Signal can have S/(N+I) of -20dB and still be
received successfully
DS CDMA allows demodulation of signals
that are below interference and/or noise floor
At RF (before de-spreading)
At the base-band (after de-spreading)
Note: processing gain is derived through
reshaping of the power spectrum density
in the frequency domain
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Orthogonal Variable Spreading Factor codes (OVSF)
UMTS-FDD uses OVSF codes
OVSF codes preserve
orthogonality even for different
code lengths
Codes are designated with 2numbers
first number is the length
second number is the
position in the code tree
OVSF codes are orthogonal ifthey are not on the same path
from the root of the code tree
Page 7
(1)
(1,1)
(1,-1)
(1,1,1,1)
(1,1,1,1,1,1,1,1)
(1,1,-1,-1)
(1,1,1,1,-1,-1,-1,-1)
(1,1,-1,-1,1,1,-1-,1)
(1,1,-1,-1,-1,-1,1,1)
(1,-1,1,-1,1,-1,1,-1)
(1,-1,1,-1,-1,1,-1,1)
(1,-1,-1,1,1,-1,-1,1)
(1,-1,-1,1,-1,1,1,-1)
(1,-1,1,-1)
(1,-1,-1,1)
C1,0
C2,0
C2,1
C4,0
C4,1
C4,2
C4,3
C8,0
C8,1
C8,2
C8,3
C8,4
C8,5
C8,6
C8,7
OVSF code generation
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OVSF code - orthogonality
OVSF codes are orthogonal if they are not on the same path Consequence: assignment of a given code eliminates all codes
down the path
Note: in CDMA unique code is the channel. In 3G one may havemany low rate or few high rate channels
Page 8
Example: Illustration of the orthogonality
0,0
X
Code C8,1
1, 1, 1, 1, -1,-1,-1,-1
Code C4,3
1, -1, -1, 1, 1,-1,-1,1
C8,3
C4,3x
1, -1, -1, 1, -1,1,1,-1
S4
S8 0
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W-CDMA variable spreading - equal powers
User 1 and user 2 have different data rates
User 1 and user 2 use codes of different length
User 2 has a smaller processing gain
Decision making process is easier for user 1
Page 9
x
X
S
X
X
S8
S4
C8,0
C4,3 C
4,3
C8,0
(1,1,1,1,1,1,1,1)
(1,-1,-1,1)(-1,1,1,-1)
2,0,0,2,0,2,2,0
(1,1,1,1,1,1,1,1)
(1,-1,-1,1)
2+0+0+2+0+2+2+0=8
(0-2-2+0)= -4(2+0+0+2)= 4
Decision 1
Decision 1 -1
User 1
User 2
R1
R2
R2= 2R
1
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W-CDMA variable spreading - equal Eb/Nt
User 2 adjusts its power to compensate forsmaller processing gain
With power adjustments, both users havesame symbol energy after de-spreading
Page 10
X
S
X
X
S8
S4
C8,0
C4,3 C
4,3
C8,0
(1,1,1,1,1,1,1,1)
(2,-2,-2,2)(-2,2,2,-2)
3,-1,-1,3,-1,3,3,-1
(1,1,1,1,1,1,1,1)
(1,-1,-1,1)
3-1-1+3-1+3+3-1=8
(-1-3-3-1)= -8(3+1+1+3)= 8
Decision 1
User 1
User 2
R1
R2
R2= 2R
1
X
2
Decision 1 -1
(1,1,1,1,1,1,1,1)
(1,-1,-1,1)
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Multipath and Rake RX (1)
Terrestrial environment multipath
propagation of RF signal
Multipath propagation results: Signal dispersion: at the RX energy is
dispersed among multiple components
Signal fading: each component is subject to
fading
Signal dispersion energy reaches
received through resolvable multipath
components
Components are resolvable if their
relative delay is larger than a chip
interval
In W-CDMA one chip time interval
corresponds to 78m of path difference
Resolvable multipath components are
combined using maximum ratio
combining (MRC)
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Power delay profile
example for 5MHz
channel
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Multipath and Rake RX (2)
Signal fading each delay position usually consists of
several components
The components have random phases causes fading
Fading occurs at the scale comparable to of a
wavelength (~ 7cm)
Fading may be as much as 30dB deep Fading is mitigated through interleaving and coding
12 Example signal variations due to fast fading
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Operation of Rake RX
Rake receiver consist of multiple
receiving fingers and searchers
Rake RX algorithm
Identify the time delay positions
with significant energy and assign
them to fingers
Demodulate resolvable multipathreceptions at each of the fingers
Combine demodulated and phase
adjusted symbols across all active
fingers and present them to the
demodulator
Typically phones rake receiver
has minimum of
Three fingers
One searcher
Delay resolution for a searcher is
typically 14/-1/2 chip interval
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x
x
x
+
Correlator
Correlator
Correlator
t1
t2
t3
Phase and
amplitude
alignment
( )T
dt0. DECISION
Multipath
Response
t1
t2
t3
Finger 1
Finger 2
Finger 3
Simplified diagram of a rake receiver
Note: fingers may be used for multi-paths or
for different cells in soft handover
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Power control
WCDMA implements power control
on both uplink and downlink
On the UL - two loops for power
control Inner (fast loop)
Outer (slow loop)
Inner loop (uplink) Fast adjustments of MS TX power so that
target SIR at the base station is met
Rate: up to 1500 power adjustments/sec
Outer loop (uplink)
Adjustment of the SIR target at the basestation
On the DL Provide increase of power for edge of the
cell mobiles
Compensate for some fast fading effects
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Note: SIR target usually changes as a
function of propagation environment and
mobiles speed
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Soft/softer handovers
Softer mobile is in communication
with sectors of the same cell
Soft mobile is in communication
with sectors of different cells
Essential interference mitigation tool Ensures that the mobile is power
controlled by all cells that cover
certain area
Prevents interference by reducing
near-far problem
Form mobile standpoint soft andsofter are essentially the same
From the system perspective soft
and softer differ in number of
allocated resources
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Softer handover
Soft handoverNote: soft handover requires additional
resources between Node B and RNC