3.3 Regenerator
Regenerator – is the basic element of the DTS line path. Regenerator is installed in all intermediate stations and also in the receiving path of LPTE (line path terminal equipment).
Regenerator functions:
The signals amplification attenuated by the line
Distortions correction of the received signal.
Significant time moment forming
Making a decision what signal was transmitted.
Pulse forming with the determined amplitude and duration according to the signal at the output of decision making device.
The simplified functional scheme of the one-way binary signal regenerator (see 116, 120 L.W.)
Line transformer
Correcting amplifier
Decision making device (threshold matching scheme) TMS
Forming device FD
Control device CD
Clock frequency former (clock driver) ClD
Remote supply RS
Correcting amplifier – performs the 1st and the 2nd functions. There is loop with the adjustable frequency dependent circuits, in which the pulse correction is performed.
The correction elements values are regulated with the control device when there is a cable attenuation changing.
In a comparison with the ATS such regulation is performed according to the received signal value but not by the pilot frequencies (Automatic Gain Control by the pilot frequency) – this correction system is named as the adaptive system.
Clock driver (ClD) – performs the 3rd function (= time intervals extractor = clock synchronization device).
ClD forms the narrow pulse sequence from the corrected by the CAmp pulse signal, the pulse-repetition frequency of which is the same with the clock frequency, and the appearance moments (phases) determine the significant time moments (STM), that is such time moments, in which it is necessary to determine what symbol was transmitted.
The control device is the part of threshold matching scheme. The pulse signal at the output of the TMS starts the FD (which forms the rectangular shape pulses with the Upulse = 1,3 V amplitude and Tp duration). Tp is determined by the line transmission rate and by the code type (RZ, NRZ); clock frequency is numerically equal to the bitrate B = fcl.
TMS performs the 4th function.
The FD – the 5th.
It is not possible and there is no need to obtain the rectangular shape pulses at the CAmp output.
I
t
is necessary to have the CAmp amplification equal to the attenuation
of the line when the frequency tends (goes) to infinity in order to
recover the rectangular shape pulse completely.
LPC AMI
Ai
it
is no possible to obtain such CAmp as the immunity will be bad.
CAmp The digital part
s
ignal
noise
only noise
The intended use of the CAmp is to help the TMS (CD) with taking a correct decision.
The intersymbol interference bothers to take the correct decision due to the pulse tails.
The intersymbol overlapping is the noise that overlaps the signals surface and worsens the reception quality.
It is necessary to explain what is the eye-diagram, its designing, what signal should be at the output of CAmp, what the bad and good quality signal eye-diagrams are look like, how many dB decreasing of regenerator immunity from distortions if the amplitude eye-diagram parameters (blur) are known by using the L.W. materials.
It is known that the long-haul communication providing with the required quality is possible only when intermediate stations using.
These are regenerators in DTS. The ATS line path quality is determined by the noises, in the DTS – by the errors during the reception (bit error). Not only the error quantity is important but the their relative value that is determined by the error coefficient
e
rror
bits number
Tmeasured – time interval determined in advance The bit error relative value BER is random.
e
rror
dispersion
T measured
If the time of measurements tends to infinity than the Ker –> constant, it means – to the error probability.
The line path and other digital paths quality in DTS is determined by the BER (error probability). These errors influent on the different signals transmission quality in a different way.
It is acceptable if the BER value is smaller than 10-6 between two maximum length line subscribers within the Earth.
There are norms that are developed by the scientific-research communication institute centre.
n
ational
network
trunk lines
This norm is subdivided into the main, intraareal and local sections.
b
ut
the sections with the different length:
The error probability norms within the each section is recalculated into the norm (Per acceptable) for 1 km so depending on the length – different values are obtained. – This is the old method.
In order to obtain more accurate DTS quality estimate the new parameters were used due to which the error bit-stream analyze can be performed within the small time intervals.
ESR (errored second ratio)
The ratio of ES to total seconds in available time during a fixed measurement interval.
severely errored second ratio (SESR)
The ratio of SES to total seconds in available time during a fixed measurement interval. (till 30% of bits are with errors)
background block error ratio (BBER)
The ratio of Background Block Errors (BBE) to total blocks in available time during a fixed measurement interval. The count of total blocks excludes all blocks during SESs. (for E1 block = 2000 bits)
It is possible to use next formula in order to recalculate new values of Per
Additional
The new norms determine the acceptable value of ESR that depends on transmission rate (B) and on line length (L).
The long-term and short-term (operational) norms are used for the line path quality determination. Operational norms are used for the already operating line path. The long-term norms are used for the new TSs that are going to be implemented.
3.4 The regenerator section length
10 FOTS line path
It is known that the electrical signal transformation into the light and reverse transformation in the receiver has nonlinear characteristics, so FOC (optical fiber cable) using for ATS designing is not possible due to the harmonics and interchannel influence.
The FOTS is one of the DTS type operating through the FOC.
The 1st FOTS (1st generation) were using the PDH DTS.
The modern FOTS – SDH DTS.
The WDM DTS are used nowadays (many trunk digital bitstream transmission through the FOC in different wavelengths method).
WDM – is the wavelength discrete multiplexing, that is the hybrid of TDM DTS (time division multiplexing) and frequency trunk division.
PDH DTS for the FOTS were realized as the “СОПКА-2” (8.8448 Mb/s) and “СОПКА-3” (34 Mb/s) in Soviet Union.
The differences between the СОПКА and DTS operating through the metal cable (PCM) is in different LPTE using. The regenerator should be supplied with the light-electricity transformers.
In the FOTS LPTE the digital signal of the corresponding interface (AMI, HDB-3) is transformed into the binary signal and transmitted to the optical modulator and reverse transformation – in the receiver.
Such transformation into the binary signal was necessary due to the light modulation by the polarity (phase) wasn’t known in that period of time, and modulation was performed by the light emission intensity changing (amplitude manipulation).
PCM Coder (binary signal) quasiternary signal 1B1T FOTS line code 1T2B
Coding
middle value independence
Decoding
Ternary signal has the redundancy that is used in the regenerator for the correct binary information transmission controlling. Such controlling is performed by the bipolarity violation registration when AMI code using, and by the insert rule violation – in HDB-3 code.
There is also redundancy when line signal in FOTS forming.
1B1B 1B1T (AMI, HDB-3) 1B2B (FOTS)
There are next codes in FOTS: 10B12B (mBnB, n > m).
The higher n the better control of informational accuracy.
Such transmission bitrate raise is strongly not desirable as it is connected with the attenuation of the section and as a result with the regeneration section length so to the more expensive DTS line path.
The regeneration section length almost doesn’t depend on transmission rate in FOTS.
REGENERATION SECTION LEGTH IN FOTS
Is determined by two factors:
Dispersion distortions
The loss in fiber.
The dispersion distorts the shape of the received pulse, they make the duration wider and this leads to the interference between the pulses, that is named as the intersymbol interference.
Trans
L=5 km eye diagram
L=10 km ISI
The dispersion is determined by the next formula:
Tout
– the output pulse duration
Tin – the output pulse duration.
The acceptable condition is
.
The kilometric dispersion σ0 concept is used because of the length dependence of the dispersion.
The regeneration section length is limited by the dispersion distortions:
the regeneration section length that meets the
acceptable ISI caused by the dispersion of the fiber.
The reasons of the dispersion effect are:
The distortions caused by the FOC and optical transmission and receiving transformers
where σ ES –
emission source dispersion;
σrec – dispersion caused by the receiving optical module;
σL – dispersion caused by the FOC.
Optical receiver and transmitter dispersions are often neglected in simplified calculations because these dispersions are smaller than FOC dispersion.
σ01 – specific fiber dispersion;
Δλ – the emission line width. The smaller Δλ, the smaller dispersion influence.
The dispersion influence on the regeneration section length was determinative in FOTS of 1st generation with 8; 34 Mb/s and higher rates.
The narrowband coherent light sources are used in modern FOTS, so the dispersion influence becomes not so important.
2nd reason is the fiber loss (attenuation in the cable).
The attenuation coefficient frequency dependence is different for the metal and FOC.
frequency windows
The transmitted PDH DTS signal takes very small part of these frequency windows and the transmission rate rise even by an order and correspondingly the frequency bandwidth width increasing doesn’t lead to the noticeable loss increasing it almost doesn’t influent on regeneration section length.
The regeneration section length is determined from the immunity equation:
Optical equation of the receiver
energy potential (power budget) – systems coefficient
F
OTS
characteristic
FOC characteristic
EP – is the FOTS characteristic.
The attenuations caused by the plug-type connections (PlC) and by the permanent connections (PC) are not taken into account.
PlC – are used for the equipment connecting to the cable;
PC – are performed by the welding of the FOC factory length.
THE LEVEL DIAGRAM OF THE REGENERATION SECTION
4xPC, PlC, Reg
PC Attenuation
Prec acc
Ai acc
T
he
station cable attenuation, dispersion of the factory lengths,
equipment margin Aeq.
EP-2APLC - Aeq
a+afact+APC/lfact
APLC –the plug-type connections (PlC) attenuation;
APC – the permanent connections (PC) attenuation;
lfact – the factory length taking into account the dispersion (variation).