Transcript Towards 4G IP-based Wireless Systems

A 4G System Proposal
Based on Adaptive OFDM
Mikael Sternad, UU
Joint work with Tony Ottosson, CTH, Anders Ahlen, UU
Arne Svensson, CTH and Anna Brunström, KAU
The Wireless IP Project
Part of SSF PCC 2000-2002
A SSF funded project
2002-2005
+Vinnova funding
www.signal.uu.se/Research/PCCwirelessIP.html
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 channel
instead 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
Channel Prediction
Adaptive Modulation and
Prediction Errors
Modify thresholds to keep BER constant (single-user)
Smart Antennas: Simplest Case
Fixed lobes (sectors, cells) at base stations
MRC in mobile stations (MS)
Advantages
BS:
Efficient use of space (robust)
Low interference levels
MS:
Improvement of SNR (robust)
Scheduling Among Users in a Sector
1
4
5
3
2
• Feedback info from each
mobile: Appropriate
modulation level for each bin
in a time slot.
• Perform scheduling based on
predicted SNR in bins

user
freq
• For each bin let the “best”
user transmit; use adaptive
modulation and ARQ scheme
• Modify to take QoS and
fairness into account
time
Minimizing Interference Among Sectors
• Exclusive allocation of time-frequency bins to users
within border zones between sectors of a base station.
• Novel freqency reuse scheme
• Multi-antenna terminals (IRC)
• (Power control)
• Slow resource reallocation
between sites and sectors,
based on traffic load
f
1
2
1
1
2
1
1
2
2
2
1
2
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 ms (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; 4 pilots and 8 control symbols)
– 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 Throughput
Simplifying 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):
N 1
  GcG p  ki
i 0
 i 1
 (1  P
FER ,i
( )) p ( )d
i
Cyclic prefix: Gc  100 /111
Pilots: Gp  108/120
p   
K e 

  
   L  
K
L
  L     L,    
K 1
Thresholds
Select the modulation level i as
i*  argi max ki (1  PFER,i ( ))
ki
 i (dB)
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
i
Modulation
0
Spectral Efficiency and Throughput
(one sector, 16 dB)
20
15
10
Throughput [Mbit/s]
25
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
Thank you!
Questions?