WO1999048219A1

WO1999048219A1 - METHOD AND APPARATUS FOR CLOCK TIMING RECOVERY IN ψDSL, PARTICULARLY VDSL MODEMS

Info

Publication number: WO1999048219A1
Application number: PCT/IL1999/000154
Authority: WO
Inventors: Boaz Porat; Amnon Harpak; Shimon Peleg
Original assignee: Infineon Technologies AG
Current assignee: Infineon Technologies AG
Priority date: 1998-03-19
Filing date: 1999-03-18
Publication date: 1999-09-23
Anticipated expiration: 2000-09-19
Also published as: KR100417238B1; EP1060571A1; AU2954199A; US6922436B1; EP1060571A4; IL123739A0; KR20010052213A; IL123739A

Abstract

Method and modem for fast timing recovery of transmitted data between a master ψDSL modem and a slave ψDSL modem, over a noisy, high loss, high distortion wiring. Transmitted QAM symbols are received and sampled (10) at the slave modem. The sampled data is split into in-phase (I) and quadrature (Q) channels (11, 12), each of which is filtered by matched filter (13, 14). The filtered I and Q outputs are sampled at twice the symbol rate and the lower and upper band edge components are extracted by modulating each of the sampled sequence of I and Q outputs with two discrete time sequences: cos(0.5πn) = ...,1,0,-1,0,... and sin(0.5πn) = ...,0,1,0,-1,... Each of the resulting products is filtered with a first order low-pass filter (26, 27) and re-sampled again at the symbol rate. The Bit Error Rate is computed (28), and the slave modem switches from blind timing recovery mode, to data directed timing recovery mode, after the Bit Error Rate has sufficiently decreased.

Description

METHOD AND APPARATUS FOR CLOCK TIMING RECOVERY IN DSL.

PARTICULARLY VDSL MODEMS

Field of the Invention

The present invention relates to digital data communication between two locations over

the connecting wiring. More particularly, the invention relates to the use of Very High

speed Digital Subscriber Loop (VDSL) modems for ttansferring data at high rates

between two locations connected at least partially by conventional, twisted copper

wires.

Background of the Invention

The art has devoted considerable attention to the problem of transmitting data in a high

rate between users being at different locations. Such users, may be home Personal

Computers (PCs), office desktop workstations, cable television broadcasting services,

Local Area Networks (LANs) and others. In some applications users are connected to

each other by modems (modulator-demodulator) which encode the digital data to be

delivered from one point (user) to another point and transmit the encoded data through a

data link which may be, for instance, an analog communication channel. Such data

comprises voice, digital video movies and software data files.

Digital Subscriber Loops (DSLs) comprise several technologies for high data rates, e.g.,

Asymmetric Digital Subscriber Loops (ADSLs), High speed Digital Subscriber Loops

(HDSLs) and Very High speed Digital Subscriber Loops (NDSLs). Generally, the whole

family of DSLs is commonly known as DSL. In some VDSL applications, like video -2- transmission, data should be transmitted in very fast rates, usually up to 12.96 Mb/Sec

or even exceed 25.92 Mb/Sec.

Analog modems were developed to deal with data rates up to 33.6 Kb/Sec. This rate is

unacceptable for many applications, e.g., picture transmission where pictures are

constructed from large data files. Digital modems which are developed to work on

leased copper lines between two locations can reach higher data rates, up to 64 Kb/s or

even 128 Kb/s. However, this rate is still too low for many applications. Any

transmission medium interferes with the transmitted data by adding noise, by

attenuating its amplitude, and by changing its phase. Digital modems suffer from these

phenomena, reducing their ability to receive data without errors. Errors are critical in

digital modems.

LANs are very intensively used to connect users, usually in the range of a single

building but in many cases the range is expanded to several buildings. Since in many

cases it is desired to connect users being in different buildings to share same data base,

it is generally desired to exploit for this purpose an existing PSTN twisted pair line, or

preferably a leased line. Moreover, in many cases it is desired to make high rate data

communication between two LANs, for example, LANs of two offices located in

different cities a hundred miles or more away from one another.

There are known connections that can provide higher bandwidth than twisted pair

copper lines, for example, 107100-Base-T coaxial cables and fiber-optic lines. The

copper lines between PABXs were already replaced by fiber-optic lines in most cases, -3- and have become standard. However, it is not foreseen that in the near future the twisted

pair copper lines between the telephone end users and the PABXs be replaced, due to

their huge number, and to the complexity of replacing them. Therefore, it is desirable to

provide a much higher rate modem communication on the relatively narrow bandwidth

twisted pair copper lines. Significant efforts are now put in order to develop higher rate

modems, which are commonly called in the art, VDSL modems.

Basically, the conventional unshielded copper wire twisted pair was originally designed

to provide a medium for voice transmission, and when it is used in telephone

communication its bandwidth is confined by filters in its two ends to between 300 Hz to

3.4 KHz. In leased lines, a wider bandwidth is available, however the possible data rate

is still limited by the fact that long lines introduce very large attenuation, especially in

the higher range of the bandwidth, which exceeds 8 MHz in VDSL modem

transmission. This relatively wide bandwidth is required to enable full duplex

communication channel, utilizing the known Frequency Division Duplex (FDD).

Moreover, telephone lines pass through switching exchanges conducted by the local

telephone companies, and this may be a very noisy environment which disrupts the

transmitted data.

Usually, digital tDSL modems utilize Quadrature Amplitude Modulation (QAM)

techniques to encode data. In this technique, the transmitted information-carrying signal

appears in pre-defined amplitude and phase states, each state representing a pre¬

determined number of bits, and is termed "a symbol". Conventional QAM techniques

utilize 16 states (symbols) or 64 states. In case of 64-QAM, each symbol represents 6 -4- bits. Therefore, for a desirable VDSL modem transmitting at a rate of 12.96 Mb/Sec,

2J6*10⁶ symbols have to be transmitted in each second. A one kilometer twisted pair line has a propagation delay (impulse response time) in the range of about 12 μSec,

whereas each symbol duration is 0.463 μSec in the above case. Thus, the effective duration of the line impulse response is about 25 symbols. This long duration of the impulse response of the line leads to a severe Inter symbol Interference (ISI) which may result in a large errors at the receiving modem if cannot canceled, and practically limits the data rate.

The communication between two tDSL modems is carried out while one modem is the transmitter (master) and the other is the receiver (slave). Data directed to the slave modem are termed "downstream" while the data directed to the master modem are termed "upstream". Communication between the two modems requires synchronization between their timing clocks. Proper operation of tDSL systems requires almost perfect synchronization between master and slave clocks, which means that they must work at the same frequency. Any constant frequency offset leads to a constant growing phase error which may lead to mismatch between the number of transmitted and received symbols per time unit, which is unacceptable. Different clocks always have somewhat different frequencies due to manufacturing tolerances, aging (changes in then- component characteristics versus time), temperature variations, power supply tolerances, random noise deviations, etc. Therefore, synchronization means are required in the slave modem to recover the master clock frequency (timing) from the transmitted symbols, together with a correction apparatus to lock the slave clock frequency to the master clock frequency. -5-

One known method for synchronization between receiving and transmitting modem

clocks is performed by the transmission of a pilot tone from the master modem to the

slave modem. However, in case of pilot tone transmission the energy is concentrated in

a single frequency, violating the Power Spectral Density (PSD) constraints and

interfering with other systems operating in the same frequency range. It is generally

desirable that the power of the synchronizing signal will be distributed on a wide

frequency band, but usually these signals are not periodic. Therefore, using distributed

power signals for synchronization of 0tDSL systems is problematic.

Considering the aforementioned problems, an . DSL system is required to synchronize

in "blind" mode, which means operating in a very noisy environment when initially

there is no information about the transmitted symbols at the receiving modem. This

mechanism is known as Blind Timing Recovery (BTR). It is characterized by the fact

that all symbols have equal probabilities and some or most of them are attenuated,

resulting in a very bad Signal to Noise Ratio (SNR) and/or being received with a

random phase-shift and with high additive noise. BTR algorithms face significant

difficulties when trying to reconstruct the master clock. Thus, an effective error

correction mechanism is required, without reducing the data rates.

Several suggested solutions for BTR have been proposed. "Passband Timing Recovery

in an All-Digital modem receiver" by D. Godard, IEEE Transactions on

Communications, Vol. COM-26. No. 5, 1978. p.p. 517-523. the disclosure of which is

incorporated herein by reference describes a method of performing BTR. However, this -6- reference does not provide a mathematical proof, or show any means for carrying it out.

An effort to carry out Godard's method is discussed in "Joint Blind Equalization,

Carrier Recovery, and Timing Recovery for High Order QAM Signal Constellation",

IEEE Transactions on Signal Processing. Vol. 40. No. 6. 1992. p.p. 1383-1398 the

disclosure of which is also incorporated herein by reference. This reference describes

means for performing BTR by applying a complicated algorithm, based on Godard's

theory. Particularly, these means require complicated hardware having extremely high

processing power.

It is an object of the present invention to provide a synchronization method useful for

fast bi-directional data transmission , between .ΛDSL modems over conventional

unshielded copper or the like wiring, for example connecting LANs.

It is another object of the present invention to provide a simple fast method for

accurately recovering the clock frequency of the transmitting JtDSL modem at the

receiving modem, without the need of a predetermined training sequence.

It is a further object of the invention to provide a method for fast synchronization of the

receiving tDSL modem clock to the transmitting tDSL modem clock, while operating

in blind mode

It is still another object of the invention to provide adaptive, fast converging error

correction apparatus for carrying out the method of the invention. -7-

Other objects and advantages of the invention will become apparent as the description

proceeds.

SUMMARY OF THE INVENTION

The invention is directed to a method for fast timing recovery of transmitted data

between two ^t SL modems, said data is transferred trough a noisy, high loss, high

distortion wiring, comprising the steps of:

a) Providing a master tDSL modem, synchronized by its own timing clock, for data

transmission to a second slave DSL modem;

b) Providing a second slave -#DSL modem, synchronized by its own timing clock,

for data reception from said master ΛDSL modem;

c) Providing a communication wiring connecting said master modem to said slave

modem;

d) Encoding and transmitting the desired data as a sequence of symbols to the slave

modem using pre-determined QAM states;

e) Receiving the transmitted symbols at the slave receiver (demodulator);

f) Sampling the received signal;

g) Splitting the sampled data to in-phase (I) and quadrature (Q) channels;

h) Filtering each channels of step g) above with digital low-pass filters, said filters

being matched to the transmitting filters at the master modem;

i) Tiαrning the master clock timing recovery into blind mode, by the steps of:

(1) Sampling the filtered I and Q outputs at twice the symbol rate;

(2) Extracting the lower and upper band edge components by modulating each of -8- the sampled sequence of I and Q outputs of step (1) above with two discrete time

cos(0.5 -w) =...,1,0,-1,0,... sequences: . ;

SIΠ(0.5ΛW) =...,0,1,0,-1,...

(3) Filtering the four resulting products with four first order low-pass filters and

re-sampling the results at the symbol rate;

(4) Computing the real and imaginary parts of the spectral line vector using the

products of step (3) above;

(5) Filtering both the real and the imaginary parts of step (4) above, using

another first order low-pass filter;

(6) Normalizing the magnitude of the spectral line vector to unity using a

suitable automatic gain control circuitry;

(7) Extracting the phase of the spectral line vector from the normalized

imaginary part of step (6) above;

(8) Feeding the sampled imaginary part of step (7) above as a phase-error signal

to a controller of a phase-locked loop (PLL), said PLL utilizing a frequency

controlled clock oscillator, the frequency of which is tuned to track the

frequency of the incoming symbols (the master modem clock frequency);

(9) Converting the digital control word to analog control voltage supplied to the

tracking oscillator of step (8) above, using a Digital to Analog Converter (DAC);

and

(10) Using a secondary accumulator to correct the control word supplied to the

DAC of steρ (9) above; j) Feeding the I and Q filtered outputs to a complex linear equalizer for coarse phase

and amplitude error correction; -9- k) Computing the symbol state data decisions using a sheer circuitry;

1) Fine equalization of the channel distortions by feeding the I and Q outputs of the

sheer to a decision feedback equalizer, the outputs of which are extracted from the

sheer I and Q inputs, respectively;

m) Computing the extracted symbols error rate at the sheer outputs; and

n) After the error probability decreases to a given Bit Error Rate (BER), switching

from blind mode timing recovery to data directed timing recovery mode.

According to a preferred embodiment of the invention the transmission medium is a pair

of copper wires, which may be a telephone line. High data rates may be transmitted on

relatively long conventional telephone lines, occupying corresponding frequency bands.

The timing oscillator of the receiving modem may be a Voltage-Controlled Crystal

Oscillator (VCXO), utilized by a phase-locked loop.

According to a preferred embodiment of the invention, blind timing recovery is

achieved using a reduced constellation that includes only equal amplitude symbols. This

reduced constellation simplifies and accelerates the equalizing process. Error correction

process is performed to control the frequency of the PLL tracking oscillator. The error

signal produces a digital correction signal which is converted to an analog control signal

by a simple Digital to Analog Converter (DAC). Additional secondary accumulator

circuitry is utilized to correct the input word to the DAC to attenuate frequency jitter,

comprising the steps of:

a) Rounding the double precision control signal; -10- b) Generating an error signal between the double precision value and the rounded

value;

c) Accumulating the error signal in a secondary accumulator;

d) Adding the error signal to the output signal of the secondary accumulator;

e) Comparing the result of step d) above with half the value of the DAC's Least

Significant Bit (LSB);

f) Compensating the rounded value according to the result of step e) above by the

steps of:

(1) Adding the value of the DAC's LSB to the accumulator output, if the

output value is larger than half the value of the DAC's LSB; or

(2) Subtracting the value of the DAC's LSB from the accumulator output, if

the output value is smaller than half the value of the DAC's LSB;

Using this a simple DAC together with the digital compensation circuitry simplifies and

reduces the cost of the control circuitry, and still maintains a stable, accurate control

voltage to the VCXO.

Brief Description of the Figures

The above and other characteristics and advantages of the invention will be better

understood through the following detailed description of preferred embodiments thereof,

with reference to the appended figures, wherein:

Fig. 1 schematically illustrates a full duplex data communication channel

between master and slave JXDSL modems;

Fig. 2A is a graph of typical frequency bands occupied by VDSL transmission; -11- Fig. 2B is a graph of the attenuation of a typical copper wire communication

line;

Fig. 3 is a graph of the Impulse-Response (IR) of the communication line of

Fig. 2B;

Fig. 4 illustrates a 16 QAM generation and the resulting 16 state constellation;

Fig. 5 A is a block diagram of the demodulator of the slave modem;

Fig. 5B is a block diagram of a first order low-pass filter of Fig. 5 A;

Fig. 6 schematically illustrates the output decisions of the slicer of Fig. 5 A;

Fig. 7 is a block diagram of the controller of Fig. 5 A; and

Fig. 8 schematically illustrates the phase shift of a 16 QAM constellation

resulting from frequency mismatch between master and slave modems clock.

Detailed Description of Preferred Embodiments

Fig. 1 illustrates a full duplex data communication channel between master and slave

0tDSL modems, using standard telephone line made from copper wires pair. A tinting

clock 4, which may be supplied by the local telephone system or by a Synchronous

Digital Hierarchy (SDH) system, drives the master modem to transmit data (symbols)

downstream to the slave modem 2. The slave modem is driven by another clock 5,

which is a part of the slave's demodulator 3. Clock 5 should be synchronized to clock 4

since the clock defines the difference between symbols. After synchronization, clock 5

times the downstream symbol reception and upstream symbol transmission to the

master modem. -12-

The frequency spectrum of an -# SL channel utilizes two separated frequency bands

using Frequency Division Duplex (FDD) as shown in Fig. 2A. The first band 6 occupies

the range from 0.9 MHz to 3.5 MHz and is used for down-stream transmission, whereas

the second band 7, occupies the range from 4 to 7.9 MHz and is used for up-stream

transmission. A 500 KHz Guard-Band (GB) 8 remains unused (by øtDSL systems) due

to amateur radio interference constraints.

Fig. 2B shows the attenuation of typical copper lines for common diameters (0.35, 0.4,

0.5, and 0.6 mm). From the figure, it can be seen that a typical, 100 m long, telephone

line with 0.4 mm copper wire diameter has large attenuation characteristics with sharp

attenuation from low to high frequencies. Thus, transmitted symbols propagating along

a 1 Km long line reach the slave modems with power attenuation of up to 50 dB.

Moreover the line causes a substantial shift of the symbols phase. In addition, since the

line passes through switching junctions and other telephone service paths, a lot of noise

and cross-talk are added to the attenuated symbols. All these factors distort the

amplitude and phase characteristics of the transmitted symbols, making the task of their

timing recovery very complicated.

The attenuation of the line is smaller at the downstream band. Therefore, the BTR

process at the slave modem can work with a better SNR than by working on the

upstream data. By the SNR consideration, downstream data is encoded with 64-state

QAM whereas upstream data is encoded with only 16-state QAM. Basically, 16 QAM

has better noise immunity than 64 QAM, but wider bandwidth. -13-

Fig. 3 is a graph of the Impulse-Response (IR) of a 1 Km long line. The IR of a system

represents the output of the system when applying a unit sample δ(n) at the input. It is

seen that the IR lasts a time period equal to the duration of 25 symbols. Thus, large

errors may occur at the slave modem. In addition, fast data rates require extremely fast

processing speeds, which limits the complexity of the BTR that may be used.

Fig. 4 illustrates a 16 QAM generation and the resulting state constellation. Two carriers

V*cos(ω₀t) and V*sin(ω₀t), shifted by 90°, forming an In-phase channel (I-channel) and

Quadrature channel (Q-channel) respectively, are modulated by two information signals

Sj; and S_Q respectively. Each signal Sj or S_Q assume four values: ±1 and ±3. I and Q

channels are summed forming a 16 state QAM signal. These 16 states are

distinguishable by their amplitude/phase combinations as illustrated in the constellation

diagram in the complex plain. Each state is a vector (symbol) which is the sum of two

vectors, I and Q. Since each of S_j and S_Q have four different amplitudes, each may

represent four different logic combinations (00, 01, 10, 11). Thus, summations of their

respective modulated carriers provide 16 different vectors (symbols), representing 16

logic combinations (0000, 0001, 0010, J i l l).

Fig. 5A is a block diagram of the demodulator of the slave modem. Symbols are

sampled and converted to a digital form by an Analog to Digital (A/D) converter 10.

The samples are fed into two multipliers 11 and 12, which are phase-shifted by 90°,

forming the I and Q channels. These I and Q channels are filtered by Low Pass Filters

(LPF) 13 and 14, respectively. These filters (commonly known as "Nyquist Filters") are -14- identical, and similar filters exist in the master modem to give it a raised cosine shape.

An excess bandwidth of approximately 20% results in these filters.

The filtered I and Q channels are fed into a complex Linear Equalizer (LEQ) 5, which

functions as an adaptive filter for a coarse error correction mechanism. LEQ 5 is able to

correct both amplitude and phase errors caused by the line. LEQ 5 feeds both I and Q

corrections to a slicer 18, which provides a decision for any received symbol in order to

classify each symbol to one of the ideal QAM states. The outputs from the slicer 18 are

reconstructed symbols, which are fed back into the complex Decision Feedback

Equalizer (DFE) 17, via the adder 16, into the slicer 18. The DFE provides an additional

fine error correction mechanism which is adaptive according to the resulting errors from

the slicer.

Fig. 6 illustrates the output of the slicer 18. The slicer 18 slices the I-Q complex plain to

16 identical squares. For each equalized (by LEQ 15) symbol that falls into one of these

squares, a decision is taken to associate it to one of the states. In practice, each received

symbol appears with an error in its amplitude as well as in its phase. As a result, all

symbols that are associated with a state form a "cluster" around their state. Each DFE

reads the errors (distance from the theoretical state) of each symbol, processing the

information to predict a better correction step and feeds an input back to the slicer to

reduce the error at the next symbol.

Looking back at Fig. 5 A, a Timing Recovery Loop (TRL) 40 samples the information of

both I and Q channels filtered by LPF 13 and 14 respectively, and provides an error -15- signal e(n) which is fed to controller 29. The controller accepts the error signal and

provides a correcting control voltage (in a direction that reduces the error signal) to a

Voltage Controlled Crystal Oscillator (VCXO) 30 that determines the sampling rate of

the incoming symbols. This sampling rate should follow the incoming symbol rate

(synchronization) and therefore, the TRL 40 together with controller 29 and VCXO 30

are essentially a Phase-Locked Loop (PLL).

TRL 40 may function in two possible modes. The first mode is a blind mode which

operates first, until the symbol error rate at the output of the slicer 18 is better than

10^" or any other desired error-rate. After the desired error rate is obtained, the TRL

switches to Decision Directed Timing Recovery (DDTR) mode, which is relatively

simple and widely used in modems.

Generally, PLLs operate as Frequency Modulation (FM) demodulators. In this case, the

frequency of the VCXO should follow the frequency of the master modem clock

(incoming symbol rate). A PLL is used to lock the frequency of the timing clock of the

slave to that of the master. Any change in the master clock frequency (FM), causes TRL

40 to generate an error signal and the controller reacts by forcing the control voltage of

the VCXO to change its frequency to the new frequency. Thus, the VCXO control

voltage detects the frequency changes of the master modem clock.

For a simpler and easier understanding of loop operation, a mathematical representation

of the VCXO operation is provided below. The VCXO, which is the plant of the control

loop, can be mathematically represented as an integrator, because its phase is -16- proportional to the integral of the frequency and the control voltage of the VCXO

determines the instantaneous frequency. Mathematically:

Vvcxo(t) = sin[2πKvcxo \c(t)dt ] [Eq. 1]

where c(t) is the VCXO control voltage, v_vcxo(t) is the VCXO output voltage, and K_vcxo

is a proportionality constant.

The critical parameter of the VCXO is its instantaneous phase, which is given by:

φ(t) = IΉKV XO \c(t)dt [Eq. 2]

If φ_in(t) denotes the phase of the mcoming signal, then the output of the TRL 40 (which

functions as an error signal generator) is given by:

e(t) = sin[φ_in(t) - φ(t)] [Eq. 3]

Here, the phase error is small and the approximation sinx « x may be used. Hence, the

phase error is given by e(t) » φ_in(t) - φ(t) and both the VCXO 30 and the error signal

generator are considered as Linear Time Invariant (LTI) systems. Therefore, the

controller 29 may also be LTI.

The mathematical description of the above and of the following functions of the loop

utilizes both continuous and discrete time analysis, for the sake of convenience. Since

digital processing techniques are implemented, a sampling time interval T₀ of the

incoming signals is defined, enables normalizing frequencies to the sampling frequency

1/T₀ and expressing phase in terms of periods. Using the well known Laplace Transform

(LT) for transforming time presentation of signals to s domain presentation (s = σ+jω, -17- where j = -1), the transfer function of the VCXO is given by:

G(s) = ^≡^. [Eq. 4] s

Fig. 7 is a block diagram of the controller, comprising two functional blocks: an LPF

50, which attenuates the additive noise of the error signal e(t) and a proportional/integral

controller 51, which provides a correction voltage proportional to the frequency offset

between the VCXO and the master modem clock, reducing the steady state phase error

to zero. In addition, an integrative controller smoothes the transition of the TRL from

blind mode to DDTR mode. The expression of the controller transfer function in the s

domain is given by:

_w.^j_.fuaa ' . _[Eq.₅₁

2πK_vcxo s l + (s / ω₀r)

where ω₀ is the loop cutoff frequency (the maximum frequency error that the loop is

able to track) and r is the loop damping factor (an indication of the loop reaction and

stability). The first expression of H(s) stands for constant gain, the second for

proportional/integral part of the controller, and the last one stands for LPF. At the cutoff

frequency ω₀, the magnitude of the open loop transfer function is given by (for s =jω₀):

| Gϋ^'ω₀) HUωo) l = 1 [Eq. 6]

The denominator (known as the characteristic polynomial) of the loop transfer function

G(s) H(s) is given by:

P(s) = s³ + ω₀rs² + ω\rs + ω . [Eq. 7]

this cubic polynomial has one real root and two complex conjugate roots, which

determine the damping factor r of the loop. For r < 1 the loop is unstable (oscillatory) -18- and for r > 3 the loop is overdamped.

According to another preferred embodiment of the present invention, r is chosen to be r

= 2.8. Since the loop is a PLL, there is a maximum frequency offset Δf_max between the

master modem clock and the VCXO for which the loop can achieve locking. According

to a preferred embodiment of the present invention, ω₀ is chosen to satisfy the condition

ω₀ > 2πΔf_max to enable locking.

Rearranging the expression for H(s) a product of two factors gives:

H(s) = H,(s) H₂(s), or

K

H(s) := - K l -t- ω -r o [Eq. 8]

where

ω

K

¹ 2-π -K ^vcxo [Eq. 9]

ω ^r [Eq. 10]

According to a preferred embodiment of the present invention, K_vco is chosen to be K_vco

= Δf_max. This means that a unity control signal supplied to the VCXO is able to shift its

frequency by Δf_max. -19-

According another preferred embodiment of the present invention, ω₀ is chosen to be:

ω₀ = 2πβAf_msκ [Eq. 11]

where l≤β<2. Hence, under the above selected conditions Ki and K₂ are given by:

K\ = β [Eq. 12]

_{Kι =} 2πβ^^ j r

After the analysis of the controller has been done in s domain, a digital implementation

of the LPF H_J S) and the proportional/integrative controller H₂(s) is done using the well

known 2 transform. Applying the 2 transform on the LPF transfer function H^s), the

expression in z domain is given by:

a

H\ 0) = - — — T ' ^{where a = ω} o^r- [Eq. 14] (l^~a)z-

The above expression is a good approximation to perfect discretization of H^z), since

the loop bandwidth is very small compared with the symbol rate. Thus , ω₀r - 1 and

the difference equation related to H_t(z) is given by:

u[k] = u[k-l] +a - (e[k]-u[k-l]) where e[k] and u[k] are the input and output signals of

the LPF, respectively. Since a is very small, the output u[k] is accumulated in double

precision.

Applying the 2 transform on the proportional/integral controller transfer function H₂(s),

the expression in z domain is given by:

H₂{z) = K ₊ -^. [Eq. 15]

1 — z

The difference equation related to H₂(z) is given by: -20- y[k] = y[k-l] + (K₂/ K u[k] [Eq. 16]

c[k] = (K,) (u[k]+y[k]) [Eq. 17]

where u[k] is the output signal from the LPF, y[k] is the state variable of the

proportional/integral controller, and c[k] is the output of the controller (control signal to

the VCXO). In this case Ki = 1 and since K₂ is very small, the output y[k] is

accumulated in double precision.

According to a preferred embodiment of the present invention, an 8 bit Digital to

Analog Converter (DAC) 31, is used to generate the control signal for the VCXO, for an

accurate, simple, cost-effective implementation. This requires rounding of c[k] to be a

relatively short number, which results in an unacceptable operation of the loop. The

problem is overcome by a method based on the addition of a dither to the control signal

c[k], the duty-cycle of which is determined by the rounding error, comprising the

following steps:

1) Defining an error c [k] = c[k] - c[k] between the double precision value c[k] and its

rounded value c[k] .

2) Adding the error c[k] to a secondary accumulator (integrator) with output x[k].

Hence, its output is given by:

x[k] = x[k- l] + c[k]. [Eq. 18]

3) Correcting the rounded value of c[k] according to the value of x[k]. If the Least

Significant Bit (LSB) of the DAC is b, the correction is given by:

x[k] > 0.5b = c[k] = c[k] + b, x[k] = x[k) - b, x[k < -0.5b => c[k] = c[k] - b, x[k] = x[k] + b. ^q" -21-

x[k] accumulates the error c [k] . If x[k] becomes larger than 0.5b, b is added to c[k]

(compensation) and subtracted from x[k] (for new error accumulation). If x[k] becomes

smaller than -0.5b, b is subtracted from c[k] and added to x[k].

By using this mechanism, a very accurate control of the VCXO (which is critical to

proper operation of the loop) is obtained with no need for a complex, expensive DAC.

Correction is calculated continuously, and the control voltage to the VCXO is updated

at the right timing, so as to obtain an accurate phase. Moreover, intensive digital

implementation improves the temperature stability and power consumption of the

VCXO control circuitry.

According to a preferred embodiment of the present invention, the method for BTR and

error signal generation in blind mode employs a modification of Band-Edge Timing

Recovery (BETR) method. Looking back at Fig. 5A, an algorithm for extracting the

TRL phase error is described:

1) The incoming signal is sampled, demodulated, passes filters 13 and 14 and the

resulting complex (I and Q) signal is fed to TRL 40. The resulting complex signal is

given by:

z,[^ή\+jz,[ \ [Eq. 20]

2) This signal has bandwidth from -0.5(1 + ά)f_b to 0.5(1 + ά)f_b , where f_b is the symbol

rate and a is the bandwidth excess ratio. Therefore, the two band-edge components

(base-band components at upper and lower frequencies) spectral lines can be recovered -22- by multiplying the demodulated complex signal z[n] by exp(Jπf_bt) and eχτp(-jπf_bt).

According to a preferred embodiment of the invention, the sampling rate in blind mode

is done at twice the symbol rate f_b . This method is known as Fractional Spaced

Equalization (FSE), which is utilized for reducing the line amplitude and delay

distortions appearing when the signal is sampled at the symbol rate.

Since the sampling frequency is 2f_b, the signal is multiplied by the discrete-time

sequences exp( O.5πn) and exp(-/0.5;z7.) . These sequences are very simple since the

only possible values are 1, j, -1, -j. Therefore, the band-edge components can be

formed without any multiplication, which is one of the main advantages of the present

invention. The lower band edge component is given by:

λ,[ +jλ,[ri [Eq. 21]

The upper band edge component is given by:

μ_rM+Jμ,[ri\ [Eq. 22]

where

_r[«] = z_r[n]cos(0.5Λw)-z_I[«]sin(0.5Λw), λ,[ri = z_I[n]cos(0.5πn) + z_r[ ]s (0.5πn), ∞s(05πn) =...,1,0,-1,0,... / _r[n] = z_r[n]cos(0.5m) + zχn]sm(0.5πn), s (0.5πn) =...,0,1,0,-1,... ^q" μ,[ri - z,[«]cos(0.5 zw)-z_r[n]sin(0.5 z7.),

Each of the components λ_r[n], λ,[ _t μ_r[n], μ,[n] is filtered by LPF 21, 22, 23 and 24

respectively, forming a set of filtered values: λ_r[ ], λ,[ \, Ju_r[ \, ^~μ,[^ή\.

These values are multiplied and summed by the spectral line computer 25, and then -23- filtered again by LPF 26 and 27 respectively, in a way forming the I and Q component

of the desired spectral line vector v[n]. Hence, the components of the spectral line vector

v[n] = V,.[H] + j^'v₍[w] are given by:

v ] = I p_r[n] + 1, [«]/ ,[«],

_ _ _ _ [Eq. 24] v, [n] = λ,[riμ_r[ri\- λ_r[ήμ,[ή\.

The phase of the spectral line vector is given by:

an^'^VitnjV v-fn]) [Eq. 25]

Since the phase error is small, the approximations x « sinx ∞ tgx and e(t) « sin[φ_in(t) -

Φ(^t)]~ Φi_n(t) - Φ(t) may be used. Therefore, the phase error of the timing loop is proportional to v,[«]. The proportionality factor is a function of the signal amplitude

which may vary. Therefore, v[n] = v-[«]+ v,[«] is fed to an amplitude normalizer 28,

which normalizes the magnitude of v-[«] +_ v,[«] to be 1. This normalization is achieved

by a widely used Automatic Gain Control (AGC) circuitry. After normalization, the

normalized imaginary part of the spectral line, which is the required error signal of the

loop, is sampled again at the symbol rate f_b, and fed to the controller 29 to lock the

loop. From this point, blind equalization is performed until symbol error rate of less

than 10^"3 (or any other desired error rate) is achieved. Using the preferred embodiment

of the present invention described above, blind equalization is accomplished in less than

0.1 Sec.

All the LPFs of TRL 40 are first-order Infinite Impulse Response (IIR) filters. Fig. 5B is

a block-diagram of this filter, and its mathematical representation is given at Eq. 14

above. Going back to the 2 domain, the output T (z) is multiplied by (1-α), time shifted

by T, and added to the input X(z) multiplied by a. According to the invention, a is -24- selected to be a = 2^~k (k is an integer), leading to the filter's difference equation:

v[n] = v[n-l] + 2-k*(x[n]- [n-l]) [Eq. 26]

Since 2"^ is equivalent to a time-shift k, each LPF may be realized with no need for any

multiplication.

According to a preferred embodiment of the present invention, a reduced constellation is

transmitted by the master modem for the blind mode operation. This reduced

constellation comprises only four symbols, each having the same amplitude. This

method simplifies the equalization during blind mode, since the symbols differ from

each other only in their phase. After equalization using reduced constellation, the line

characteristics has been "extracted" and full constellation is started.

After blind equalization, the slave modem switches to the well known DDTR mode, as

mentioned above. The operation of this mode is illustrated in Fig. 8, in which a 16

QAM constellation is presented. If there is any offset between the clock frequencies of

the master and slave modems, phase error is generated, shifting the phase of any symbol

in time. This shift is illustrated by arrows pointing towards the shifting direction of the

phase. These phase shifts are detected by measuring the deviations of pre-detected

symbols to from their post-detected symbols, and as a result the loop is adjusted to shift

the VCXO frequency to the direction that minimizes the clusters that are generated

around each state of the constellation. Since blind equalization has been already

implemented, these clusters are relatively small, and the DDTR mode is utilized to

maintain only fine corrections. -25-

All the above description and examples have been provided for the purpose of

illustration, and are not intended to limit the invention any way. Many modifications

and additional operations can be effected in the method, and many different hardware

elements, wiring and components can be used, all without exceeding the scope of the

invention.

Claims

-26- Claims

1. A method for the fast, timing recovery of transmitted data between two 0tDSL

modems, said data is transferred along a noisy, high loss, high distortion wiring,

characterized by that data received at the slave modem as a sequence of symbols, is

sampled at the symbol rate, converted to digital form, said sampled data been split to In-

phase (I) and Quadrature (Q) channels, filtered with a digital Low-Pass Filter (LPF),

sampled again at twice the symbol rate, and modulated each with the two discrete-time

cos(0.5^w) =... ,1,0,-1,0,... sequences _sin(0.₅^_{) =}... ,0,1,0,-1,../

2. A method according to claim 1, comprising the steps of:

a) Providing a master 0tDSL modem, synchronized by its own timing clock, for data

transmission to a second slave DSL modem;

b) Providing a second slave tDSL modem, synchronized by its own timing clock,

for data reception from said master tDSL modem;

c) Providing a communication wiring connecting said master modem to said slave

modem;

modem using pre-determined QAM states;

e) Receiving the transmitted symbols at the slave receiver (demodulator);

f) Sampling the received signal;

g) Splitting the sampled data to in-phase (I) and quadrature (Q) channels;

h) Filtering each channels of step g) above with digital low-pass filters, said filters -27- being matched to the transmitting filters at the master modem;

i) Turning the master clock timing recovery into blind mode, by the steps of:

(1) Sampling the filtered I and Q outputs at twice the symbol rate;

(2) Extracting the lower and upper band edge components by modulating each of

the sampled sequence of I and Q outputs of step (1) above with two discrete time

cos(05^w) =...,l,0,-l,0,... sequences: . » _{1 n} , ; sιn(0.5 zw) =...,0,1,0,-1,...

re-sampling the results at the symbol rate;

products of step (3) above;

(5) Filtering both the real and the imaginary parts of step (4) above, using

another first order low-pass filter;

(6) Normalizing the magnitude of the spectral line vector to unity using a

suitable automatic gain control circuitry;

(7) Extracting the phase of the spectral line vector from the normalized

imaginary part of step (6) above;

to a controller of a phase-locked loop (PLL), said PLL utilizing a frequency

controlled clock oscillator, the frequency of which is tuned to track the

frequency of the incoming symbols (the master modem clock frequency);

and -28-

(10) Using a secondary accumulator to correct the control word supplied to the

DAC of step (9) above;

j) Feeding the I and Q filtered outputs to a complex linear equalizer for coarse phase

and amplitude error correction;

k) Computing the symbol state data decisions using a slicer circuitry;

slicer to a decision feedback equalizer, the outputs of which are extracted from the

slicer I and Q inputs, respectively;

m) Computing the extracted symbols error rate at the slicer outputs; and

n) After the error probability decreases to a given BER, switching from blind mode

timing recovery to data directed timing recovery mode.

3. A method according to claims 1 and 2, wherein the transmission medium is a pair of

copper wires.

4. A method according to any one of claims 1 to 3, wherein the pair of copper wires is a

telephone line.

5. A method according to claims 1 and 2 wherein the sampling rate is more than twice

the symbol rate.

6. A method according to claims 1 and 2, wherein the timing oscillator utilized by the

phase-locked loop is a Voltage-Controlled Crystal Oscillator (VCXO) or the like

suitable clock oscillator. -29-

7. A method according to any one of claims 1 to 6, wherein the blind timing recovery is

achieved using a reduced constellation.

8. A method according to claim 7, wherein the reduced constellation comprises only

equal amplitude symbols.

9. A method according to claim 1 to 6, wherein the blind timing recovery is achieved

using full constellation.

10. A method according to claim 1 and 2, wherein the control signal of the PLL tracking

oscillator is provided accurately and converted using an up to 8 bit Digital to Analog

Converter (DAC) means, comprising the steps of:

a) Rounding the double precision control signal;

b) Generating an error signal between the double precision value and the rounded

value;

c) Accumulating the error signal in a secondary accumulator;

d) Adding the error signal to the output signal of the secondary accumulator;

e) Comparing the result of step d) above with half the value of the DAC's LSB;

steps of:

(1) Adding the value of the DAC's LSB to the accumulator output, if the output

value is larger than half the value of the DAC's LSB;or -30-

(2) Subtracting the value of the DAC's LSB from the accumulator output, if the

output value is smaller than half the value of the DAC's LSB;

11. A method according to any one of claims 1 to 10, substantially as described and

illustrated.

12. An JtDSL modem for fast timing recovery of received data, said data transmitted

between two tDSL modems and transferred trough a noisy, high loss, high distortion

wiring, comprising:

a) Circuitry for receiving the transmitted symbols at the slave receiver

(demodulator);

b) Circuitry for sampling the received signal;

c) Circuitry for splitting the sampled data to in-phase (I) and quadrature (Q)

channels;

d) Circuitry for filtering each channels of step c) above with digital low-pass filters,

said filters being matched to the transmitting filters at the master modem;

e) Circuitry for larrning the master clock timing recovery into blind mode, by the

steps of:

(1) Circuitry for sampling the filtered I and Q outputs at twice the symbol rate;

(2) Circuitry for extracting the lower and upper band edge components by

modulating each of the sampled sequence of I and Q outputs of step (1) above

cos(0.5πw) =...,1,0,-1,0,... with two discrete time sequences: . _*n . . _{Λ 1 Λ 1} ; sιn(0.5 z7.) =...,0,1,0,-1,...

(3) Circuitry for filtering the four resulting products with four first order low- -31- pass filters and re-sampling the results at the symbol rate;

(4) Circuitry for computing the real and imaginary parts of the spectral line

vector using the products of step (3) above;

(5) Circuitry for filtering both the real and the imaginary parts of step (4) above,

using one or more first order low-pass filter;

(6) Circuitry for normalizing the magnitude of the spectral line vector to unity

using a suitable automatic gain control circuitry;

(7) Circuitry for extracting the phase of the spectral line vector from the

normalized imaginary part of step (6) above;

(8) Circuitry for feeding the sampled imaginary part of step (7) above as a

phase-error signal to a controller of a phase-locked loop (PLL), said PLL

utilizing a frequency controlled clock oscillator, the frequency of which is tuned

to track the frequency of the incoming symbols (the master modem clock

frequency);

(9) Circuitry for converting the digital control word to analog control voltage

supplied to the tracking oscillator of step (8) above, using a Digital to Analog

Converter (DAC).

f) Circuitry for feeding the I and Q filtered outputs to a complex linear equalizer for

coarse phase and amplitude error correction; g) Circuitry for computing the symbol state data decisions using a slicer circuitry;

h) Fine equalizing the channel distortions by feeding the I and Q outputs of the slicer

to a decision feedback equalizer, the outputs of which is extracted from the slicer I

and Q inputs, respectively; -32- i) Circuitry for computing the extracted symbols error rate at the slicer outputs; and

j) Circuitry for switching from blind mode timing recovery to data directed timing

recovery mode, once the error is reduced to less than a given BER.

13. A modem according to claim 12, further comprising (10) circuitry for accumulation

o correct the control word supplied to the DAC by providing:

a) Circuitry for rounding the double precision control signal;

b) Circuitry for generation of an error signal between the double precision value and

the rounded value;

c) Circuitry for accumulation of the error signal in a secondary accumulator;

d) Circuitry for adding the error signal to the output signal of the secondary

accumulator;

e) Circuitry for comparing the result of step (d) above with half the value of the

DAC's LSB;

f) Circuitry for compensating the rounded value according to the result of step (e)

above and by:

(1) Circuitry for adding the value of the DAC's LSB to the accumulator output,

if the output value is larger than half the value of the DAC's LSB; and

(2) Circuitry for subtracting the value of the DAC's LSB from the accumulator

output, if the output value is smaller than half the value of the DAC's LSB;

14. A modem according to claim 12, comprising means for sampling at a sampling rate

that is more than twice the symbol rate.