A 4G System Proposal Based on Adaptive OFDM

1
A 4G System Proposal Based on Adaptive OFDM

• Mikael Sternad

2
The Wireless IP Project

• Part of SSF PCC, 2000-2002
• A SSF funded project2002-2005
• Vinnova funding
• www.signal.uu.se/Research/PCCwirelessIP.html

3
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

4
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)

5
Short-term Channel Properties

• Typical time-frequency channel behavior (6.4 MHz,
  50 km/h)
• Data from Stockholm, Sweden _at_1900MHz (by
  Ericsson)? Accurate channel prediction is needed

Coherence bandwidth 0.6 MHz
Coherence bandwidth 4.9 MHz
6
Channel Prediction
7
Adaptive Modulation and Prediction Errors

• Modify thresholds to keep BER constant
  (single-user)

8
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 Improv
ement of SNR (robust)
9
Scheduling Among Users in a Sector

• 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

1
4
3
5
2
user
freq
time
10
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
1
1
2
1
2
2
2
1
1
2
2
time
11
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

12
Physical Layer

• OFDM system with cyclic prefix yielding low
  inter-channel interference
• Symbol period is 111 ms (10011 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 ?/4 - ?/2 ? 2 - 4 slots _at_ 1900 MHz
  and 100 km/h

13
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

14
Analysis of Throughput (cont.)

• Spectral efficiency (L antennas, K
  users)Cyclic prefix
• Pilots

15
Thresholds
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
16
Spectral Efficiency and Throughput(one sector,
16 dB)
25
20
Throughput Mbit/s
15
10
17
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

18
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 K1
  user/sector
• (Reuse 1 combined with reuse 3,
  area-fair scheduling,
• interference limited, full load,
  Rayleighpath loss, L1 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?

19
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

Server
Network
20
Thank you! Questions?