Doppler Compensation for LTE-Based Aeronautical Mobile Telemetry

Transcription

1 Distribution Statement: Approved for Public Release Distribution is Unlimited 412TW-PA nd ITEA Workshop, May 2018 Doppler Compensation for LTE-Based Aeronautical Mobile Telemetry William H Johnson and Kiran M Rege, PhD Acknowledgement formerly Applied Communication Sciences Effort sponsored by the US Government under Other Transaction number W15QKN between the NSC, and the Government The US Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon - A Vencore Company

2 The Need for Doppler Estimation and Compensation One of the key performance-limiting factors in the implementation of LTE-based airborne telemetry systems is the high Doppler shift experienced by the TA and base station transceivers Standard LTE systems are not designed for very high Doppler shifts (They are designed to handle Doppler shifts for speeds up to 350 kmph) At high Doppler shifts, TAs cannot access base stations even at high SNRs Our two-pronged approach to the Doppler Problem: Proactively directed handovers to preferably connect to base stations that have low Doppler shifts as well as good SNR; and Doppler estimation and compensation at the TA (Needed because proactively directed handovers cannot completely overcome the Doppler problem Two possible implementations: Downlink-based requires indication of desired signal Uplink-based does not require indication of desired signal; exploits the fact that the TA receiver derives its frequency reference from the DL signal Slide 2

3 Standard LTE Transceiver Limitations TA can compensate large frequency offsets (can see many enbs) TA frequency locks onto the enb it selects enb cannot compensate large frequency offsets Designed to handle many UEs TA has locked to enb and uplink can be out of enb s locking capability LTE (TDD) enb Tx at: f c TA Rx Locks at: f c +Δf TA Receiver Locking Range f c LTE Bandwidth: 20MHz f c +Δf Doppler Shift Δf f c +Δf TA Speed (Knots) Doppler (Hz) L-Band C-Band TA transmits at: f c +Δf ,000 2, ,000 5, ,000 11,600 enb Rx Locks at: f c +2Δf enb Receiver Locking Range f c +2Δf Doppler Shift Δf Slide 3

4 A Doppler Compensation Appliqué Uses the uplink signal transmitted by the TA to estimate the Doppler shift Does not need indication of the desired base station Operates in a completely asynchronous manner Can be added to any standard LTE User Equipment (UE) device With the current prototype implementation, Doppler shifts of up to 5 khz (Mach 3 in L-Band and 1 in C-Band) can be handled Based on COTS Software Defined Radio (SDR) hardware FPGA implementation on SDR for real-time operation Capable of rapid tracking of Doppler for high-speed operation In lab tests, showed residual error of at most a few hundred Hz, well within the capability of base station receivers Slide 4

5 The Appliqué-Based Solution to the Doppler Problem Operation with Standard UE DDDD LO: ff rrrrrr DL Channel: +f D UUUU LO: ff rrrrrr enb DDDD Tx DL: ff rrrrrr Doppler Channel Rx DL: ff DDDD rrrrrr + f D TA Rx UL: ff UUUU rrrrrr + 2f D Tx UL: ff UUUU rrrrrr + f D DDDD Tx DL: ff rrrrrr UUUU Rx UL: ff rrrrrr DL Channel: +f D Doppler Channel Rx DL: ff DDDD rrrrrr + f D UUUU LO: ff rrrrrr Applique (Doppler Est & Comp) Collocated Entities Rx DL: DDDD +(1α) f D ff rrrrrr Tx UL: UUUU +(1α) f D ff rrrrrr UUUU LO: ff rrrrrr TA Tx UL: ff UUUU rrrrrr +(1α) f D (2-α) f D = ff UUUU rrrrrr - f D UE Augmented with Applique Slide 5

6 A High-Level View of the Doppler Estimator-Compensator From the UE Transmitter Burst Detector Collect samples over 1 ms Doppler Compensator (Frequency Shifter) To the Uplink Antenna Normal data transmission Estimate Doppler using method for data frames Identify frame-type RACH transmission) Estimate Doppler using method for RACH frames Store Estimate Slide 6

7 Timeline of Applique Operation Transform-based freq estimates for RACH frames Subframe (1 ms) CP-based estimates for data frames Subframe (1 ms) CP (103 ms) Preamble (800 ms) Sym bol Data Slot (05 ms) C P Slot (05 ms) Uplink UE Activity Applique Activity 1 ms worth of samples collected Burst Detected Freq Offset Estimated Frame-type Identified 1 ms worth of samples collected Burst Detected Freq Offset Estimated Frame-type Identified Slide 7

8 An Example TA starts at Point A with zero speed The trajectory ends at Point B TA follows the shown trajectory, increasing its speed along the way There are two base stations whose signals are received by the TA enb 2 B A HO to enb 2 Tracking begins HO to enb 1 enb 1 Slide 8

9 Conclusion The viability of Airborne Mobile Telemetry based on 3GPP s LTE standard is severely limited by the very high Doppler shifts encountered at the TA and base station ends Vencore Labs dual approach comprising proactively directed handovers and Doppler estimation/compensation is designed to overcome the Doppler problem that is particularly felt at the base station receiver Experiments with laboratory prototype have demonstrated the ability of the Doppler estimator/compensator to handle Doppler shifts up to 5 khz The residual error is typically on the order of a few tens of Hz Slide 9