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 2User 4User 5 User 3 time space SDMA: random assignment User 1 User 3User 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)

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

Generic OFDMA broadband wireless network IFFT Serial To Parallel To Serial OFDM User signal Coding COFDM MC-CDMA OFDMA Spread spectrum 1 1 FH-OFDM t IFFT Serial To Parallel User signal Coding Spread spectrum 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 omitted VoIP

Intelligent MAC with sub-optimum channel scheduling Two-step algorithm with linear complexity 1.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)

Outline Multiuser/network diversity Applications –OFDMA network –Rapidly deployable, self-configurable WAN Concluding Remarks

Micro-MobilityManagement Macro-MobilityManagement 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-based Configurable ground network

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}

Semi-distributed resource scheduling 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

Performance Gains Random channel selection [bit/Hz] Channel selection with interference avoidance [bit/Hz] Improvement Perfect cell configuration % Cell configuration w. misalignments % 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