Robust Adaptive Transmission

Greg Pottie, UCLA Electrical Engineering DepartmentTel: (310) 825-8150; Fax: (310) 206-8495

Research FocusDealing with high channel dynamics: Doppler, fading; modify techniques proposed in other contexts for Orthogonal Frequency Division Multiplexing (OFDM)Objective: robust communication links, at maximum possible rate for wide range of scenarios, using systolic radiosIssues:Jamming: training sequence usually the weak link; apply spread spectrum methods to training signalComplexity: must implement in radios; not merely a study of optimal forms.

Robust and Adaptive TransmissionPrior work on adaptive OFDM has considered both time and frequency domain estimationHas not considered rates of channel change present in air/ground communications, nor jammingPeak to average power ratio (PAR) reduction techniques considered in isolation from other trainingWill consider multiple uses of coded training sequencesPrior work on Multi-Input Multi-Output (MIMO) largely concerned with slowly changing linksWill investigate how to have same adaptive framework for single or many antennas, and fast changing channel

Student Researcher ProgressEqualization (S. Kim)Channel characterization, study of time and frequency domain adaptive equalization techniquesSpread spectrum acquisition and synchronism issues (R. Thrasher)Survey of fast synchronism techniquesControl and sensor traffic modeling (A. Pandya)Gathering data from formation flight experimentsPeak power reduction techniques (H. Chen)Reduced complexity method

Channel Conditions

Air to Air : Fast fading(almost frequency non-selective, very time selective)

Channel Conditions

Ground to Ground : Slow fading(highly frequency selective, not very time selective)

Channel Conditions

Air to Ground : Worst case(frequency and time selective)

Channel EstimationEstimation in Time vs. FrequencyChoice depends on the channel conditionsFast fading with short delay spread (air to air, air to ground)Time domain estimation Slow fading with long delay spread (ground to ground)Frequency domain estimationCan share training sequence among many transceiver functions

Channel EstimationSnap-shot estimation and interpolation

Training symbols for channel estimationEstimated channelInterpolated channel

Conventional RLS equalization

BER performance ( Air to Ground )

Two Algorithmic LevelsChannel identification: fading rate, link loss, interference levels, packet durationdetermines OFDM/spread spectrum mix and basic tracking approachChannel tracking:Synchronization: frequency and time domain methodsEqualization: frequency and time domain methodsPAR reduction: in-band techniques making use of coded training sequenceMIMO: coding and adaptive array techniques

DeliverablesChannel, traffic, and physical link models for simulator and MAC developmentSeveral generations of algorithms for equalization, synchronization, PAR reduction, and use of multiple antennas, taking into account systolic radio architecture and MACAdaptive algorithms running on the systolic radio