2.5 Gbit/s duobinary signalling with narrow bandwidth ... - IEEE Xplore

2.5 Gbit/s duobinary signalling with narrow
bandwidth 0.625 terahertz source
L. Moeller, J. Federici and K. Su
Following the field’s general trend to enhance system transport capacity
by increasing the formats’ carrier frequencies and payloads, a novel
approach is reported for 2.5 Gbit/s signalling at a carrier frequency
of 625 GHz. Duobinary baseband modulation on the transmitter side
generates a signal with a sufficiently narrow spectral bandwidth to
pass an upconverting frequency multiplier chain. Power, bit error
rate, and signal-to-noise ratio measurements on the receiver side
describe the signal performance.
Introduction: The advantages of terahertz (THz) communications
have been recognised over recent years by the research community
and a trend in THz signalling to increase carrier frequency, pay
load, and transmission distance is noticeable [1, 2]. The rapidly
growing interest can be attributed mainly to three intrinsic advantages
which THz communications possess compared to its rivals at shorter
infrared (IR) wavelengths and at longer wavelengths (millimetre
(mm) wave). For example, T-rays (around 250 GHz) show several
orders of magnitude smaller attenuation in fog at a visibility of a
few hundred metres compared to IR light. Also, scintillation effects
caused by local refraction index changes limit the reach of the IR
beam [3, 4] based systems but exhibit less impairment to T-ray
propagation. Furthermore, the useable bandwidth of T-rays at a
same modulation index is higher than of mm-waves. These advantages
become accessible when T-rays reside in the 200–300 GHz
band and in the 600–700 GHz band where atmospheric absorption
is relatively small.
Several approaches for THz communications have been published
[1, 2] showing progress in device design towards higher modulation
rates. However, to simultaneously obtain high output power and large
transmitter bandwidth at high carrier frequency remains in general challenging.
Often THz sources are operating in resonance to efficiently generate
output power which reduces their effective bandwidth to a few
hundreds of megahertz.
Our approach takes advantage of a format similar to duobinary
modulation – a technique known from wire line and optical communications
[5, 6] that leads to a narrow signal spectrum – which
our frequency multiplier chain based THz source can support. On
the receiver side, a Schottky diode operated in direct detection
mode, converts the THz signal into baseband. We envision one
application of our setup in studying and comparing propagation features
of THz links with those of IR links under different weather
conditions.
Modulation technique: Our commercially available 1 mW (CW operation)
source consists of four cascaded frequency doublers followed
by a frequency tripler, all based on biased Schottky diodes, which
feed a horn antenna with a 2.4 mm aperture (Fig. 1). A launched tone
within the frequency acceptance band between 12.2 and 13.6 GHz saturates
the 2 W input amplifier at about 5 dBm and gets upconverted
into a frequency band between 585 and 653 GHz.
Two identical THz lenses with short focal lengths (≏32 mm) allow
source beam transmissions over distances of a few metres (beam
diameter ≏ 20 mm) followed by refocusing of the beam on the receiver
side into a horn antenna that is identical to the one used in the transmitter.
The receiver horn is connected to a zero biased Schottky diode,
which functions in the low power regime (input power ,10 mW) as a
square law detector with large baseband bandwidth and a responsivity
of about 2500 V/W. We characterise the ‘power transfer function’ of
our system by sweeping the input frequency of the THz source across
its acceptance band and record the output voltage of the Schottky
diode in the receiver. Fig. 2a indicates a passband that is impaired by
a ≏4 dB dip and 1 dB ripples. These ripples cause group delay variations
across the filter passband and amplitude fluctuations, which
together with the limited bandwidth make this source less suitable for
signal modulation with a comparable wide spectrum (e.g. direct
2.5 Gbit/s non-return-to-zero (NRZ)). The spectrum of the signal indicates
in Fig. 2a the optimal frequency position of the carrier at
12.933 GHz (corresponding to T-rays frequency at ≏625 GHz) where
the best system performance is achieved.
PPG
2.5 Gbit/s
ELECTRONICS LETTERS 21st July 2011 Vol. 47 No. 15
V pp , a.u.
700
600
500
400
300
200
100
frequency
synthesiser
13 GHz
T
0
–150 –100 –50 0 50 100 150
DC voltage at IF port, mV
bal.
mixer
LPF_1
x 2 x 2 x 2
x 2
x 2 x 2
frequency multiplier chain
THz source
LPF_2
lens
625 GHz
clk scope
BERT
iris
RF power
ESA
Fig. 1 Schematic of THz transmission link and balanced mixer
characteristics
5 dB/div.
input
THz source
power
transfer fcn
10 11 12 13 14 15 16
frequency, GHz
a
b c
Fig. 2 Power transfer function (fcn) of system and spectrum of data signal at
THz source input (Fig. 2a). Eye diagrams at 100 ps /div. after lowpass filter
LPF1 (Fig. 2b) and (Fig. 2c) at THz source input (carrier at 12.5 GHz)
Pre-coding the data reduces signal bandwidth but keeps its payload
the same. Thus, the aforementioned filter impairments affect signal performance
less strongly. Our pulse pattern generator (PPG) produces a
2.5 Gbit/s NRZ signal that gets split into two branches using a wideband
6 dB electrical power splitter. One version is delayed by the
duration of a bit (400 ps) before both replicas are combined using
another wideband 6 dB power splitter. The resulting signal possesses
three amplitude levels (21, 0, 1) and an advantageous phase coding,
which significantly shrinks its bandwidth compared to its corresponding
NRZ format. A following quasi Gaussian lowpass filter (LPF_1) with
about 1700 MHz 3 dB bandwidth reduces further the signal spectrum,
mainly by cutting off its tail (Fig. 2b), which drives as baseband
signal a balanced mixer to imprint its data on a carrier at a frequency
accordingly to the acceptance band of the THz source. It is a specific
feature of the balanced mixer that negative amplitudes of the driving
signal result in a 1808 phase shift of the carrier. A high-speed scope
can conveniently be triggered to visualise the output of the balanced
mixer as an eye diagram when the product of the carrier frequency
and bit duration equals an integer. Therefore, we choose for the eye
recording a 12.5 GHz carrier frequency, which is close to our operation
frequency of 12.933 GHz, where the system best performs (Fig. 2c). For
DC-voltages in the range of the voltage swing of the launched data
signal and applied to the intermediate frequency (IF) port of the mixer
we record the amplitude swing of the output signal when a
12.933 GHz tone is connected to the local oscillator port of the mixer.
A small offset for applied DC-voltage between 25 and +5 mV is
visible (Fig. 1) and could explain the unproportional widening of the
‘0’ level at the mixer output (Fig. 2c).
The spectrum of the data signal at the THz source input (Fig. 2a) for
the carrier frequency that gives the best system performance resides at a
position within the passband of our system which exhibits relatively
small filterband tilt and ripples. It has to be further investigated if our
source and detector system can support even wider signal spectra by
using different modulation formats and perhaps by including signal
equalisation techniques
Detection technique: On the receiver side the output of the Schottky
diode is amplified by about 42 dB using two amplifiers (80 kHz–

7 GHz passband, 6 dB noise figure, maximum output power ≏19 dBm)
and filtered by a quasi Gaussian lowpass filter (LPF_2) with 3 GHz
3 dB bandwidth. A 6 dB electrical power splitter launches one output
(Vpp ≏ 500 mV) to a high-speed scope or a bit error rate tester
(BERT) and the other to a 2.5 Gbit/s NRZ clock recovery circuit that synchronises
our measurement equipment.
We emulate transmission losses by reducing the detected T-ray power
with an iris inserted concentrically into the THz beam to limit its effective
diameter (Fig. 1). The total receiver output power is measured with
an RF power meter while we record the corresponding bit error rate
(BER) with and without threshold optimisation (Fig. 3). After optimising
the data decision threshold at a BER of about 1 × 10 23 (BER level
that can be reduced to ≏1 × 10 215 by forward error correction coding
with 7% overhead used in lightwave transmission systems [7]) increasing
of the detected THz power leads to smaller BER up to 2.5 × 10 27
where further power enhancement does not reduce the error count
(error floor) for long (2 31 2 1) pseudorandom bit sequences (PRBS).
In the case of threshold adjustment error-free operation is achieved at
power levels above 214 dBm at the same pattern length. We investigated
the impact of PRBS length by performing similar BER measurements
with shorter patterns. For this we directly triggered the data
decision with the pulse pattern generator (PPG) clock in order to
avoid artifacts caused by the clock recovery when driven with long
PRBSs. The short PRBS does not show error floors as exemplified
with a 2 7 2 1 pattern in both cases with and without decision threshold
adjustment. Reasons for this could be the saturation effects of the receiver
Schottky diode and bandwidth limitations of the receiver amplifiers.
The clock recovery did not further degrade system performance.
log (BER)
3
4
5
6
7
8
9
10
11
2 31 –1 fixed threshold
2 31 –1 threshold optimised
2 7 –1 fixed threshold
2 7 –1 threshold optimised
35 mV/div.
100 ps/div.
–22 –20 –18 –16 –14 –12 –10 –8 –6
RF power at decision gate, dBm
Fig. 3 BER curves for long and short PRBSs with and without decision
threshold optimisation
Eye diagram at BER ¼ 10 29 at decision gate input for 2 31 2 1 PRBS
Our current detector design based on commercially available Schottky
diodes is suboptimal for high-speed signalling. The video resistance of
the diodes, under low input power, is nominally ≏1.5 k.V and connected
via a series of 50 V transmission lines and cables to an external
high-speed electrical amplifier with matched input impedance [8]. For
low-speed applications and after replacing the amplifier with a high
impedance device about 270 mV at the Schottky diode output is detectable.
But with 50 V termination of the diode significantly smaller
voltage is accessible at the high-speed amplifier input. A rough estimate
(applying voltage divider rule 50/1550) shows thus only about 3% of
the signal voltage generated in the Schottky diode is applied to the
amplifier input. We use this estimate to determine the operation
regime of the diode. At the amplifier output RF powers of about
214 dBm for a BER around 10 29 are typical. Assuming an amplifier
gain of about 42 dB, the aforementioned voltage conversion factor of
the diode, and a diode responsivity of 2500 V/W, yields an effective
launched THz power of about 20 mW. This is significantly more than
the vendor specs for the linear operation range of the device.
To determine the signal-to-noise ratio (SNR) we modulated with a
short and periodically repeated PRBS (2 7 2 1, 2 15 2 1) and measured
with a high resolution electrical spectrum analyser the noise level at
the receiver amplifier output. The noise level was found to be about
18 dB above the intrinsic noise floor of the electrical spectrum analyser
(ESA) and independent of the launched THz power. The total receiver
noise was measured with an RF power meter to ≏ 2 36.4 dB. The
obtained error rates are in fair agreement with the theoretical amounts
expected for bipolar coding if vertical eye closure as visible in Fig. 3
is accounted for in the calculation. The internal preamp of the used
12.5 Gbit/s BERT does not significantly add noise to the data signal.
Conclusion: We have demonstrated 2.5 Gbit/s error-free signalling at
625 GHz carrier frequency over lab distance. BER curves as a function
of measured receiver power are discussed. Our setup has the advantages
of combining high output power on the emitter side with a comparable
less complex receiver architecture.
# The Institution of Engineering and Technology 2011
17 May 2011
doi: 10.1049/el.2011.1451
One or more of the Figures in this Letter are available in colour online.
L. Moeller (Bell Laboratories/Alcatel-Lucent, 791 Holmdel-Keyport
Rd, Holmdel, NJ 07733, USA)
E-mail: lothar.moeller@alcatel-lucent.com
J. Federici and K. Su (New Jersey Institute of Technology, 323 King
Boulevard, Newark, NJ 07102, USA)
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