network diversity in broadband wireless system onr workshop 2003 hui liu department of electrical...
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Network diversity in broadband wireless system
ONR workshop 2003
Hui Liu
Department of Electrical Engineering
University of Washington
Outline
• Multiuser/network diversity• Applications
– OFDMA network– Rapidly deployable, self-configurable WAN
• Concluding Remarks
Multiuser/network diversity
• Wireless users’ characteristics are heterogeneous.
– Dynamic channels (space/frequency/time)
– Diverse QoS requirements.
– Varying mutual interference.
• Multi-user/network diversity: diversity exploited by the MAC layer for efficient resource sharing
Example: spatial multiuser diversity
User 5
User 1
User 2
User 3
User 4
User 1
User 2 User 4 User 5
User 3
time
space
SDMA: random assignment
User 1
User 3 User 4User 5
User 2
time
space
SDMA: channel-aware assignment
• spatial correlation: orthogonal coherent• intelligent SDMA: near-orthogonal users in one time slot • significant gain without undue PHY cost
Spatial signatures
Example: frequency multiuser diversity
• frequency-selective broadband channels
• spontaneous packet interference
• each narrowband resource likely to be a very good channel for some users
• capacity maximized when channel to the best user (fairness ignored)
Frequency
SN
R
Frequency
SN
R
Frequency
SN
R
OFDM:User #1
OFDM:User #2
OFDMA
Broadband resource scheduling
video
data
data
voice
user 1
video
data
data
user n
voice
user 2
RF
chan
nel
RF
chan
nel
RF
chan
nel
video 1
data 1
data n
voice 1RFchannel i
data 1
data n
video n
voice 2 RF channel j
Example: traffic multiuser diversity
• diverse QoS (e.g., delay and throughput)
• instantaneous resource unit (time-frequency-space) bears different significance
• cross-layer designs enable maximum gain
• Challenge is to exploit multiuser diversity while sharing the benefits fairly and timely to users
Outline
• Multiuser/network diversity• Applications
– OFDMA network– Rapidly deployable, self-configurable WAN
• Concluding Remarks
The trend
WLAN
BWA
Cellular 2G 4G3G
802.16Proprietory
802.11/Hiperlan
DVB-T
Generic OFDMA broadband wireless network
IFFTSerial
ToParallel
ParallelTo
SerialOFDM
Usersignal
Coding
COFDM
MC-CDMA
OFDMA
Spreadspectrum
1-11-1
FH-OFDM
t
IFFTSerial
ToParallel
Usersignal
Coding Spreadspectrum
1 1-1 -1 f
• No intracell interference
• Finest granularity
• Simple, low cost PHY
• Capacity achieving
Exploiting multiuser diversity in OFDMA
• Maximizing the total throughput (easy) while satisfying– individual users rate requirements (NP-Hard)
– and outage probability constraints (NP-Hard)
Video
Data
Subcarriers omittedVoIP
Intelligent MAC with sub-optimum channel scheduling
• Two-step algorithm with linear complexity1. Bandwidth allocation – to decide the amount of bandwidth for
users based on rate constraints and channel mean/variances
2. Channel allocation - to determine specific channel sets for users based on actual channel characteristics
Network setup
• System Parameters– Bandwidth of the system: 1 MHz; Number of users: 64;– Number of traffic channels: 128; Buffer size: 20 packets/user– Traffic intensity among users: Random
• Algorithms in comparison– BCOFDMA: proposed two-step algorithm– AOFDMA : adaptive modulation/coding with no regard to rate constraints– IAOFDMA : Improved AOFDMA with bandwidth proportional to users’ rate constraints– ROFDMA : randomly assignment of channels with bandwidth proportional to users’ rate constraints
Performance gain
• Throughput vs. traffic intensity (fixed outage probability)
100 150 200 250 300 35060
80
100
120
140
160
180
200
220
240Total throughput vs. total arrival rate
total arrival rate
Tot
al t
hrou
ghpu
t
UpperboundBCOFDMA IAOFDMA ROFDMA AOFDMA
Outline
• Multiuser/network diversity• Applications
– OFDMA network– Rapidly deployable, self-configurable WAN
• Concluding Remarks
Micro-MobilityMicro-MobilityManagementManagement Macro-MobilityMacro-Mobility
ManagementManagement
Rapidly deployable data network
• Flexible and scalable wide area network using satellite backhauls and configurable wireless ground access
• Supports broadband multimedia applications and all-IP based mobility management
• VPN security and other benefits from both existing and emerging IP-related technologies and services
• Minimum to none cell planning
• Self-organized base-stations
• Maximum capacity for broadband multimedia
OFDMA-basedOFDMA-basedConfigurable ground Configurable ground networknetwork
Traditional cellular network
Lasting (voice) interference from neighboring cells
Spatial Frequency Reuse
OFDMA-based, opportunistic frequency reuse
• Self-provisioning cells– Intra-cell interference free– Inter-cell interference avoidance– Overlapping cell redundancy
• Maximum flexibility – Allow “peak capacity” access by one user if
other uses are idle– Configurable to circuit-switched FDMA
• Suitable for bursty packet network
OFDMA radio resources
More network flexibility more multiuser/network diversity
• Inputs
• Users’ broadband channel characteristics
• Users’ rate requirements
• # of users/cell
• Outputs
• Radio channels for each cells
• {Frequency_IDs, Terminal_IDs}
RNC
BTS BTSBTS
UE 1 UE 2 UE 1 UE 2 UE 1 UE 2
Ai r i nterface
BTSAPBTSAPBTSAP
Semi-distributed resource scheduling
1 10. . .2 1 10. . .2
traff i c sl otcontrol
sl ot
1. BS recei ves RNCadvi ses2. BS broadcasts Beacon3. BS recei ves CSI
1. channel assi gnmentto users
controlsl ot
traff i c sl otcontrol
sl ottraff i c sl ot
super f rame100ms
super f rame100ms
. . .
1. BS recei ves RNCadvi ses2. BS broadcasts Beacon3. BS recei ves CSI
1. channel assi gnmentto users
1. BS recei ves RNCadvi ses2. BS broadcasts Beacon3. BS recei ves CSI
• Users determine strongest signals and interference
• Base-stations collect traffic and channel information
• RNC provides low-rate IA decisions every super-frame
• BTS performs high-rate channel assignment based on traffic and channel characteristics
Performance comparison
20 40 60 80 100 120 14010
20
30
40
50
60
70
80
90
100
110
traffic load
thro
ughput
(kbits/)
throughput vs. traffic load
RAND RAND-misallignedIA IA-misallgned
Performance Gains
Random channel selection [bit/Hz]
Channel selection with interference avoidance[bit/Hz]
Improvement
Perfect cell configuration
3.1261 3.7088 20%
Cell configuration w. misalignments
2.0503 3.24 61%
Degradation due to misalignments
35% 12%
Concluding remarks
• Multiuser/network diversity– Diverse fading and selectivity desirable in multiuser network
– Intelligent MAC increases system capacity and network flexibility
– Limiting factors including overhead/update rate, optimization complexity, and architectural support
• A new paradigm for broadband network – From point-to-multipoint to multiuser packet data
– From decoupled PHY and MAC to cross-layer
– From CDMA to OFDMA