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Part of SSF PCC, 2000-2002
A SSF funded project2002-2005
+Vinnova funding
www.signal.uu.se/Research/PCCwirelessIP.html
The Wireless IP Project
Visions and Goals
• A flexible, low-cost general packet data system allowing wide area coverage and high mobility (vehicular velocities)
– Perceived performance of 100 Mbit/s Ethernet– High spectral efficiency (10 fold increase vs.
3G)– Quality of service and fairness
Leads to an extreme system based on adaptive resource allocation
Design concepts
• Use short term properties of the channelinstead of averaging (predictive link adaptation)
• Interference control (smart antennas etc.)
• Scheduling among sectors and users (combined MAC and RRM)
• Cross-layer interaction(soft information)
Short-term Channel Properties• Typical time-frequency channel behavior (6.4 MHz, ~50 km/h)
• Data from Stockholm, Sweden @1900MHz (by Ericsson) Accurate channel prediction is needed
Coherence bandwidth 0.6 MHz
Coherence bandwidth 4.9 MHz
Smart Antennas: Simplest Case
MRC in mobile stations (MS)
Advantages BS: Efficient use of space (robust)
Low interference levels
MS: Improvement of SNR (robust)
Fixed lobes (sectors, cells) at base stations
Scheduling Among Users in a Sector
time
freq
user
4
53
2
1• Feedback info from each
mobile: Appropriate modulation level for each bin in a time slot.
• Perform scheduling based on predicted SNR in bins
• For each bin let the “best” user transmit; use adaptive modulation and ARQ scheme
• Modify to take QoS and fairness into account
Minimizing Interference Among Sectors• Exclusive allocation of time-frequency bins to users
within border zones between sectors of a base station.
• Frequency reuse 1 in inner parts of sectors
• Frequency reuse 3 in outer parts of sectors
• Multi-antenna terminals (IRC)
• (Power control)
• Slow resource reallocation
between sites and sectors,
based on traffic load
11
1 1112
22
222
f
time
Design Example: An Adaptive OFDM Downlink
• Maximize throughput. Ignore fairness and QoS
• Target speed 100 km/h +large cells Frequency-selective fading
• WCDMA frequency band (5 MHz bandwidth, 1900 MHz carrier)
• Adaptive modulation. Fixed within a bin (BPSK, 4-QAM, 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256-QAM)
• Simple ARQ
• No channel coding
Physical Layer
• OFDM system with cyclic prefix yielding low inter-channel interference– Symbol period is 111 s (100+11 cyclic prefix)– 10 kHz carrier spacing (500 subcarriers in 5
MHz)
• Time-frequency grid 0.667 ms x 200 kHz (120 symbols/bin; 5 are pilots)
– Channel ~ constant within each bin– Design target speed is 100 km/h
• Broadband channel predictor– Accurate over λ/4 - λ/2 2 - 4 slots @ 1900 MHz
and 100 km/h
Analysis of ThroughputSimplifying assumptions:
• Flat AWGN channel within each bin; Independent fading between bins
• MRC with L antennas at mobiles (one sector of BS)
• Average SNR = 16 dB / receiver antenna and info symbol (same for all users; slow power control)
• Adaptive modulation. Selection based on perfect channel prediction
• K users. Fairness between users, QoS requirements, and delay constraints are neglected
Analysis of Throughput (cont.)
Spectral efficiency (L antennas, K users):
Cyclic prefix:
Pilots:
11
,0
(1 ( )) ( )i
i
N
c p i FER ii
G G k P p d
1e,
LK
K
Kp L L
L
100 /111cG
108 /120pG
Thresholds
*,arg max (1 ( ))i i FER ii k P
Select the modulation level i as
Modulation
0 BPSK 1
1 4-QAM 2 8.70
2 8-QAM 3 13.53
3 16-QAM 4 16.89
4 32-QAM 5 20.46
5 64-QAM 6 23.59
6 128-QAM 7 26.86
7 256-QAM 8 29.94
ik (dB)ii
Observations• Scheduling gives multiuser selection
diversity (from both time and frequency selectivity of the channels)
• MRC leads to good initial SNR
• Good spectral efficiency improvement already at low to moderate load (#users)
• Not all bins can be used in every sector due to interference
• Uplink control information is required to signal modulation level
Work in Progress
• Evaluation of system level performance
– Intercell interference, QoS, and fairness– First indications give a reuse of 1.7, average SIR
16dB– Results in 1.25 bits/s/Hz/sector at K=1 user/sector (Reuse 1 combined with reuse 3, ”area-fair scheduling”, interference limited, full load, Rayleigh+path loss, L=1
ant.)
• Improved adaptive modulation systems
– TCM (See presentation by Sorour Falahati)– Prediction errors ( - ” -)– Feedback information– MIMO ( 2 x 2 MIMO quite reasonable)
• Development of a network simulator
– Study of TCP/IP interaction
• Design of uplink system
– Single carrier modulation or OFDM?
Work in Progress (cont.)
Network
Server
• Optimize scheduler
– QoS and fairness– Maximum Entropy Scheduler (using information about
buffer influx). Minimize average buffer contents.– Intercell scheduling
• Soft information
– Passing PHY soft information to application– JPEG 2000 application– Modifications to TCP and UDP– Format for soft information