AirBeam: Experimental Demonstration of Presenter: Kaushik R. Chowdhury Distributed Authors: Subhramoy Mohanti Kumar Vijay, Beamforming by a Carlos Bocanegra , Swarm of UAVs Jason Meyer Gokhan Secinti† , Mithun Diddi, Hanumant Singh and Kaushik Chowdhury Next GEneration NEtworks and SYStems Lab
Aerial distributed beamforming < 2 >
Beamforming concept < 3 > The transmitters cooperatively organize themselves into a virtual antenna array and focus their transmission in the direction of the receiver, such that they add up coherently at the receiver Tx Rx Tx The transmitters make sure their transmission has a constructive interference effect at the sensor, which leads to a factor of N^2 gain in power efficiency, where N is the number of collaborating transmitters
Traditional aerial network performance < 4 > BER at receiver (B) with UAV (C) static (on ground) 2 hop topology with UAV as mid-hop router and hovering
Static on-ground multi-user transmit beamforming testbed < 5 >
UAS test-bed setup to evaluate multi-user transmit beamforming < 6 >
Gold code sequence < 7 > Gold sequence, is a type of binary sequence, having bounded small cross-correlations within a set Gold codes have bounded small cross-correlations within a set, which is useful when multiple devices are transmitting in the same frequency range A set of Gold code sequences consists of 2n + 1 sequences each one with a period of 2n − 1 Tx Gold sequence 1 Rx Gold sequence 2 Tx
Benefit of beamforming for disconnected aerial networks < 8 > Probability of BER below X
CSI estimation, BF weight calculation < 9 > The beamforming system can be modeled as: y[m] = h^x[m] + n[m] where h =[h1,...,hN] are the fixed channel gains from the transmit antenna to the receive antenna, and n[m] represents the additive white Gaussian noise (AWGN) at the receiver with a Normal distribution Aiming to achieve constant data-rate over time and targeting low latency applications, the system employs closed-loop Channel State Information (CSI) feedback where the receiver updates the transmitter constantly This way, the transmitted symbols s[m] are multiplied by the beamforming weights w as: is the average energy of the transmitted signal x[m] with normalized constellation symbols at any instant 1, with E being the mean function The weights are selected as to maximize the average mutual information function: offering a total capacity:
Beamforming with distributed aerial transmitters < 10 >
Challenges and Solutions < 11 > Sync problem weight restrictions for carrying heavyweight processors and bulky X310 radios Solutions: NVIDIA Jetson TX2 Ettus B210 SDR Fast processing in Gnuradio
Relation between UAV hovering and channel gain < 13 > Channel gain with 1, 2 and 4 Tx UAVs Gaussian distribution of UAV lateral displacement In the presence of classical GPS, the lateral displacement of Low channel gain fluctuations,can be observed during the UAV is typically +/- 0.5m, even when the UAV is moments of high stability of the UAVs assigned to a fixed location in space
GnuRadio based aerial beamforming algorithm < 14 > Start Wait 50ms Payload Data BPSK QPSK 8-QAM 16-QAM 32-QAM 64-QAM Symbol generation Multiplex FFT CSI feedback Beamformed payload Receiver feedback adapter Training signal Multiplex Zero padding USRP transmit Stop
UAV solution stack < 15 >
Faster receiver and wireless feedback < 16 > CSI estimation time in MATLAB, GnuRadio and fast processing enabled GnuRadio
Pulse Per Second (PPS) < 17 > Some radio clocks and related timekeeping gear have a pulse-per- second (PPS) signal that can be used to discipline the local clock oscillator to a high degree of precision, typically to the order less than 50 us in time. PPS signal does not specify the time, but merely the start of a second. The pulse width is generally 100 ms, but many receivers allow the user to specify the pulse width, as long as it is less than one second.
10 MHz reference signal < 18 > The 10 MHz frequency reference is used to phase lock the local oscillator, making the local oscillator frequency very stable . It ensures, the carrier burst arrives at the receiver at exactly the same frequency as the burst from another transmitter.
PPS misalignment of distributed B210s and solution < 19 > PPS misalignment Host system time synchronized with common clock reference (local server) Tx1 Tx1 PPS Tx2 Tx2 PPS aligned at the synced system clock reference point Tx1 Tx2 Transmission starts after a global #seconds, which now coincides with the PPS start time
Inaccurate cross-correlation due to PPS misalignment < 20 > Transmitters: 2 B210 radios each interfacing with their individual Jetson TX2 host Receiver: 1 B210 radio connected to another Jetson TX2 host Issue: W ithout any common time reference, and even with external 10 MHz reference and PPS from Octoclock, the distributed B210 radios were not able to beamform in a time coherent manner Solution (Phase 1): One host elected to provide common time reference • to all distributed radios • PPS of each radio hardware then locked on to this common time reference Correlation inaccuracy brought down to 20us from • 2000us But still not good enough for beamforming •
Degraded BER due to inaccurate cross-correlation < 21 >
Resolving cross-correlation issue < 22 >
BER target achievement < 23 > Probability of BER below certain value (x-axis) with various modulation schemes, 4 UAV-Tx and 1 ground-Rx.
Conclusion < 24 > • With coherence gap of 1us between the four transmitted signals, faster GnuRadio based signal processing and stable hovering of UAVs, cross-correlation was able to get the required starting index of the received payloads from the two signals • Result was low BER in all the modulation schemes used, as given in the figure • Beamforming was achieved with distributed low capability COTS radios • Next Steps: • Replacing the Octoclock with the GPSDO as the last step in truly wireless beamforming with distributed radios
Why do wireless radios have Clock Frequency Offset? < 26 > 1PPS Reference Ideal 10 MHz/0pp b -100ns -200ns Faster +1ppb +100ns +200ns Slower -1ppb
Distributed Clock Synchronization < 27 > TX1 Reference Amplifier Clock TX2 RF Clock TX Clock Generation Envelope Removing Converting Detection DC Offset Logic Level Control Unit TOF Measurement PPS Jitter Generation Reduction Delay Generation 10 MHz RF Clock RX Radio Radio Radio Radio 1 PPS Distributed RF Clock Recipents RF Front Reference and End System Clock Generation FPGA RF Clock Transceiver RF Clock Receiver USRP Radio Two single-toned Transmission USRP Radio
REAL TEST RESULTS: RF CLOCK < 27 > Clock Transmitter: Transmitting two-tone over air f 1 – f 2 Time Domain Behavior Stage 1: Envelope Detection Clock TX: • B210s were used to send two-tone signals at 900 MHz and 940 MHz with 40 MHz separation over the air. This separation freq can be chosen anything. (e.g 10 MHz) Stage 1 • Detect envelope of two-single tone signal by its separation frequency as 40 MHz. • Envelope detector sensitivity is -25 dBm, allows to large range signal detection. • Envelope signal peak-to-peak voltage level is about 1 V and need to remove DC offset from the signal.
RFCLOCK TEST RESULTS < 28 > Stage 2: Removing Offset and Filtering Stage 2 • Removing DC Offset from the envelope. • Harmonics were filtered from the signal and distortion on the signal were eliminated. Stage 3 • Translating sinewave signal to logic level with minimal additive noise. Stage 4 • Frequency divider designed with flip flops and counters is used to divide clock signal to generate 1 PPS signal. Stage 4: PPs Generation Stage 3: Converting Logic Levels
RFClock Recipient < 29 > Clock Generation Delay Line Time of Flight Measurement 10 MHz Output Envelope Removing Converting Start GPS PPS Programmable Detection DC Offset Logic Level Delay Time-To-Digital 10 MHz 40 MHz Converter Stop Jitter Reduction Jitter Reduction 8-bit latch PD LPF 10 MHz Frequency Divider %M SPI PPS Generation Control Unit Control Logic SPI 1 PPS Output MCU Significant jitter reduction obtained at output of clock signal by using DSPLL with narrow loop filter bandwidth. • • Even if independent clocks have same clock rate (freq synchronization), each might have different start time, resulting misalignment at the edges. • Misalignment issue solved by delaying clock edge to common PPS signal.
Measurement of Clock Frequency Offset < 30 > TX RX Two USRP B210 nodes equipped with • RFClock recipients, one acting as a transmitter and the other as a receiver. The transmitter transmits packets • consisting of a single repeated OFDM 10 Mhz symbol. 1 PPS 1 PPS The receiver computes its CFO with • respect to the transmitter using the traditional correlation-based OFDM CFO 10 Mhz estimation Algorithm. RFClock RFClock
Measurement of Clock Frequency Offset < 31 >
Distributed RFClock Setup < 32 > UAV equipped with RFClock/B210 Reference Amplifie TX2 r TX1 RFClock-1 RFClock-2 RFClock Transciever
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