Transcript Towards 4G IP-based Wireless Systems

A 4G System Proposal
Based on Adaptive OFDM
Mikael Sternad
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.
• 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
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; 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 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
Work in Progress
•
Evaluation of system level performance
–
–
–
•
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
–
•
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.)
Study of TCP/IP interaction
Design of uplink system
–
Single carrier modulation or OFDM?
Work in Progress (cont.)
• 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
Network
Server
Thank you!
Questions?