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Satellite communication systems operate in thepresence of path loss and atmospherically induced fading,which results in waveform distortion, altering phase and bittime period. After penetrating through several atmosphericlayers, the signal experiences Doppler shift which results infrequency change and phase reversal, thus to sync thereceiver to the incoming signal we make use of the PhaseLocked Loop (PLL) with a filter at its core functioning.To recover and reconstruct the original signal, the use offilter banks in the loop filter block of the PLL will be addedand discussed. The idea of using M-channel uniform filterbanks is to minimize the error by optimizing theperformance in decomposition and reconstruction of signals.Using different types of fundamental coherent digitalmodulation schemes like, M-ary Amplitude/Frequency/Phase shift keying (MASK, MFSK, MPSK), the bestoptimized solution can be determined for all M-channels forhigh data rates and bandwidth constraints.The presentation studies the design of a digital PLL system byusing a variable filter block for several digital modulationschemes used in ground to space communication links. Twobasic designs will be developed, first model with a fixed setof filters, those seen in traditional legacy systems and thesecond model, which consist of a filtering block made up ofQuadrature Mirror Filter banks (QMF). The method is basedon the strength of the incoming signal and modulationscheme, which in turn decides the number of filter banks, tobe used to recover the signal.In this presentation, the focus on pseudo-adaptive nature of thefilter banks is to reconstruct the original signal, consideringthe effects of Doppler shift for fast moving airborneplatforms. Subsequently, the comparison between theperformance of the fixed filter based architecture and theadditional design with the variable pseudo-adaptive filterbanks design includes the QMF and Discrete CosineTransform Filter banks (DCT). This will follow with thesoftware oriented simulation of the performance of theproposed design method, in different scenarios experiencedin satellite links.
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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 ??
