Phase Locked Loop with Filter Banks for High Data Rate Satellite Link

Chirag WartyRF and Wireless EngineerIEEE Associate Member

Richard Wai YuSystem Engineer

NAVSEA – Port Hueneme

INDEX

• Introduction

• Conventional PLL Design

• Digital Modulation schemes• Amplitude Shift Keying (ASK)• Frequency Shift Keying (FSK)• Phase Shift Keying (PSK)

• Quadrature Mirror Filter Banks• Two Channel Model• Multiple Channel Filter Bank

• Discrete Cosine Transform Filter Banks • Practical model with delays

• Digital PLL Design Implementation

• Conclusion

Introduction• Satellite Communication System

– LEO (Low Earth Orbit)– MEO (Medium Earth Orbit)– GEO (Geosynchronous orbit)

• Atmospheric effects and ambient noise effect the link

• Global Connectivity

• Immediate signal lock – Key to minimal data loss

• M-channel uniform filter banks to minimize error by optimizing the performance in decomposition and reconstruction of signals

• Two basic designs– Quadrature Mirror Filter banks (QMF)– Discrete Cosine Transform Filter banks

(DCT)

• Effects of variable architecture of filter banks on probability of recovering the original signal

Transmitter PLL Receiver

Noise Channel

Conventional PLL Design

• PLL compares phase of the incoming signal with the output of the voltage controlled oscillator (VCO) and adjusts the frequency of its oscillator to keep the system in phase with the received signal

• Loop filter – Band pass operation

• To extract the original signal from the incoming signal the receiver has to be synchronized with the received carrier

• When in sync the transmitter and the receiver would have the same bit sequence going through zero simultaneously

Fin

Fout

Fout/N

Phase Detector Loop Filter VCO

Feedback Counter

Phase Lock Loop• Phase Detector

• Carrier waveform :

• Output of the phase detector

• Phase error – Posetive : – Negative:

• Loop Filter

• Core Decision Making Block

• Two response models :– Transient response – Steady state response

• Applying Fourier Transform to input signal

Phase Detector Loop Filter VCO+

Feedback Counter

Noise

F in

F out

Fout/N

Error Signal e(t)

Phase Lock Loop• Voltage Controlled Oscillator (VCO)

• Rate of change in the output frequency due to an incremental change in the input signal

• Loop transfer function

Phase Detector Loop Filter VCO+

Feedback Counter

Noise

F in

F out

Fout/N

Error Signal e(t)

Channel Conditions

The communication link for a terrestrial ground station and the airborne platform has conventionally been in C-band (4GHz – 8GHz) to Ku – Band (12GHz – 18GHz)

GEO platforms docked in the Exospheric layer of the Earth’s atmosphere and are in a state of constant motion, travelling at almost 10 miles/sec

Atmospheric effectsfrequency selective fadingDoppler shift

The noise is induced in the form of additive white Gaussian noise (AWGN) at the loop filter.

The filter plays a decisive role in recovering the original signal from the noisy version of the received signal

Ozone layer

10 miles

50 miles

300 miles

Ground noise

UV/Visible Light

TROPOSPHERE

STRATOSPHERE

IONOSPHERE

EXOSPHERE

M–ary Amplitude Shift Keying (MASK)

• Earliest Forms of Radio Telegraphy

• Simple implementation but susceptible to noise and distortion

0 5 10 15 20 25

10-6

10-4

10-2

100

Eb/No [dB]

Pro

bab

ility

of

Bit

Err

or

Rat

e

SNR vs. P(BER) for M-ary Amplitude Shift Keying

BASK4-ASK8-ASK16-ASK

M–ary Frequency Shift Keying (MFSK)

• Performance decreases as SNR increases

• Immune to amplitude changes but susceptible to ambient frequencies

0 5 10 15

10-6

10-4

10-2

100

Eb/No [dB]

Pro

bab

ility

of

Bit

Err

or

Rat

e

SNR Vs. P(BER) for M-ary Frequency Shift Keying

BFSK4-FSK8-FSK16-FSK

M–ary Phase Shift Keying (MPSK)

• Receiver and Transmitter need to be synchronized

• Better performances than MASK and MFSK• BPSK and QPSK show similar performance

0 5 10 15 20

10-6

10-4

10-2

100

Eb/No [dB]

Pro

bab

ility

of

Bit

Err

or

Rat

e

SNR vs. P(BER) for M-ary Phase Shift Keying

BPSKQPSK8-PSK16-PSK

Phase component of the signal varies in time

Quadrature Mirror Filter Banks

• Basic Building Block – Two Channel model

• Analysis Section - Two decimator blocks

• Synthesis Section – Two Interpolator Block

• Where is the frequency response of the LPF and is a mirror image HPF frequency response

• To achieve perfect reconstruction the output of the QMF bank should be identical to the input

…. …. …

…. …. ….

Two Channel

QMF Bank

Multiple QM Filter Banks

Even Set of band Pass Filters

Consist of Analysis Section and Synthesis Section

The analysis filter bank consist of N filters with where L = 0, 1, 2 …….. N-1, as a system function, which can be obtained by uniformly shifting the frequency response of a low pass filter (LPF) by multiples of

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Time (microseconds)

Mag

nitu

de (V

olts

)

PLL Tracking response in Time Domain

InputPLL Tracking

Discrete Cosine Transform Filter Banks

• Real valued transform that map integer valued signals to floating point coefficients

• Where x(n) is real and even, by using symmetry property of DFT reduces to

Feedback Counter

VCOR(s) E(s)

Y(s)

Loop Filter

H(s)

-

+

n(s)

++

Fout/N

Phase Detector

Noisy signal

Digital PLL Design Simulation

• Implementing a Low Pass Filter at PLL with stop band at -50 dB• Fast response compared to traditional PLL loop filter• Phase response : Similar to FIR filter response • With satellite bands advancing in to Ka and Ku bands the filter banks need to employ fast

algorithms that can analyze traffic nearly instantaneously

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2000

-1500

-1000

-500

0

Frequency - Normalized (radians/sample)

Phas

e (d

egre

es)

Phase Response of the Filter Bank

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-150

-100

-50

0

50

Frequency - Normalized (radians/sample)

Mag

nitu

de (d

B)

Frequency Response of the Filter Banks

Conclusion• The loop filter in the PLL is the key element to lock on to the phase of the

received signal to synchronize the receiver with the transmitting entity.

• Filter banks provide pseudo adaptive characteristics to PLL, to suite the desired modulation scheme and the varying order of M

• PLL can operate and adapt to several different environments, atmospheric conditions and available bandwidth.

• DCT filter banks are acutely sensitive to image processing, thus giving the PLL circuit an edge on satellite transmissions that include video streaming

• Possibility to introduce other types of filter banks, which reduce clutter.

QUESTIONS ??