A 4G System Proposal Based on Adaptive OFDM

Presentation Transcript

Mikael Sternad

The Wireless IP Project

Part of SSF PCC, 2000-2002

A SSF funded project2002-2005

+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 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

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

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

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

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):Cyclic prefix:

Pilots:

Thresholds

Select the modulation level i as

Spectral Efficiency and Throughput(one sector, 16 dB)

25

20

Throughput [Mbit/s]

15

10

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