WO1999026339A1 - Method and apparatus for locating digital carrier signals - Google Patents

Abstract

In a receiver for receiving a carrier modulated with data, there is provided a method and apparatus for searching L-Band spectra for digital carriers within a mixed analog/digital environment. Carrier lock is used as a pre-filter prior to checking for final forward error correction lock. Additionally, when a valid signal has been acquired, the system frequency plan is downloaded to further refine the search.

Description

METHOD AND APPARATUS FOR LOCATING DIGITAL CARRIER SIGNALS

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to digital signal transmission and reception, and

more particularly, is directed to a method and apparatus for searching L-Band spectra for

digital carriers within a mixed analog/digital environment. The method and apparatus

searches for the correct frequency and Forward Error Correction (FEC) rate given the

correct symbol rate. The system is compatible with hardware and firmware which

supports variable symbol rate.

B. Description of Related Art

The background of the present invention is described herein in the context of pay

television systems, such as cable television and direct broadcast satellite (DBS) systems,

that distribute a variety of program services to subscribers, but the invention is by no

means limited thereto except as expressly set forth in the accompanying claims.

In the pay television industry, "programmers" produce programs for distribution

to various remote locations. A "program" may consist of video, audio and other related

services, such as closed-captioning and teletext services. A single programmer may wish

to supply many programs and services. Typically, a programmer will supply these

services via satellite to individual subscribers (i.e., DBS subscribers and/or cable

television operators). In the case of cable television operators, the services transmitted via

satellite are received at the operator's "cable head-end" installations. A cable operator

typically receives programs and other services from many programmers and then selects

the programs/services it wishes to distribute to its subscribers. In addition, a cable operator may insert locally produced services at the cable-head end. The selected services

and locally produced services are then transmitted to the' individual subscribers via a

coaxial cable distribution network. In the case of DBS subscribers, each subscriber is

capable of receiving a satellite down-link from the programmers directly.

In the past, pay television systems, including cable and DBS systems, have

operated in the analog domain. Recently, however, the pay television industry has begun

to move toward all digital systems where prior to transmission, all analog signals are

converted to digital signals. Digital signal transmission offers the advantage that digital

data can be processed at both the transmission and reception ends to improve picture

quality. In addition, digital data compression techniques have been developed that

achieve high signal compression ratios.

In commonly owned U.S. Patent No. 5,400,401 entitled "System and Method for

Transmitting a Plurality of Digital Services," there is described an encoder for generating

a multiplexed data stream carrying services to remote locations via, for example, a

satellite or a cable distribution network. The generated data stream comprises a

continuous sequence of frames, each frame comprising two fields, and each field

comprising a plurality of lines. A first group of lines of a field defines a transport layer

and a second group of lines defines a service data region. A feature of the disclosed

scheme is the ability to dynamically vary the multiplexed data stream from field to field.

A further feature of the disclosed scheme is that the data transmission rate of the

multiplexed data stream is related to the frequency of known analog video formats, i.e.

frame, field and horizontal line rates.

In commonly owned U.S. Patent No. 5,319,707 entitled "System and Method for

Multiplexing a Plurality of Digital Program Services for Transmission to Remote Locations," there is described another system, this for multiplexing a plurality of digital

program services comprising a collection of, for example,' video, audio, teletext, closed-

captioning and "other data" services. According to the disclosed scheme, a plurality of

subframe data streams are generated, each having a transport layer region and a program

data region. These subframe data streams are then multiplexed together into superframes

having a transport layer region and a subframe data region.

While these disclosed transmission systems permit a variety of services to be

transmitted over various media to remote locations, there remains a need to provide yet

other alternative arrangements more particularly adapted to the wide variety of services

that may be offered over various media and permit the end user at the remote location

greater flexibility over the data content the user is ultimately enabled to receive.

Moreover, such a system should be able to be easily adapted to transmit an increasing

number of different services in an increasingly efficient manner, for example, utilizing the

same or less bandwidth.

Digital data is usually modulated onto a carrier prior to transmission. Digital

video or audio may be compressed prior to modulation by well known means, such as

those in conformity to MPEG standards (ISO 11172 or 13818).

At the receiver, the data modulated onto the carrier must be demodulated. First,

however, the receiver must be tuned to the carrier frequency where the data is located, the

frequency of the quadrature phase shift keying (QPSK) carrier. The receiver must then

track the QPSK carrier. This is because the QPSK carrier will inevitably suffer some drift

because of limitations in the RF circuitry. Referring to Figure 1, an apparatus is shown

for tracking the QPSK carrier. Tuner 101 receives the RF television signal. Tuner 101

has an automatic frequency control (AFC) input for adjusting the tuning of tuner 101. The AFC is used to track the QPSK carrier. The output of tuner 101 is input to QPSK

demodulator 102 and then digitized by A/D converter 103: Digital PAL Logic 104 then

determines whether the receiver is tuned to the QPSK carrier. Digital PAL Logic 104

generates a phase error signal. The phase error signal is passed through the loop filter and

sweep circuit 105 and then to the AFC input of the tuner 101, which adjusts the tuner 101

to track the QPSK carrier.

To acquire the carrier initially, the AFC of tuner 101 is swept by the Loop Filter

and Sweep Circuit 105 to search for the carrier frequency. When a earner is found, the

system goes into the tracking mode described above.

In many digital data systems a wide sweep range, such as on the order of two

megahertz and up, is required to find a desired carrier. This is because communications

equipment, such as satellite dishes, often contain wide tolerance oscillators and other

components with wide tolerances, so that the equipment is economical. Thus, the

reception equipment must be able to compensate for these wide tolerances, and a wide

sweep is required. Because of this wide sweep, false locks can occur. False locks occur

when the demodulator thinks it is tracking a carrier, but in reality it is off frequency and

the data recovered is invalid. This condition occurs at QPSK offset frequencies of

multiples of -4 the A/D converter sampling frequency or symbol clock. The problem is

common to all QPSK demodulators that use wide sweep ranges.

Referring to Figures 2(a) - 2(c), examples of true and false locks are shown.

Consider a sampled QPSK demodulated data rate of 24.57 Mbits per second. A 24.57

Mbit per second demodulated data rate translates into a 49.14 Mbits per second total data

rate since most satellite digital systems use two bit streams (I and Q) to transmit

information. Potential false locks will occur in intervals of 3.07 MHz from the following calculations: 21.5 Mbits data x(8/7) = 24.57 Mbits per second; 24.57/2 x (907360°) =

3.07 MHz. Note that in actuality, data in the proposed system described by the

calculation is not sampled at 24.57 MHz but at 25.47/2 MHz because, in this system, the

data carrier is modulated by two different data streams that are time-division multiplexed,

either one of which may be used. The two data streams may represent, for example, two

different channels in this proposed system. This is an example system only. If data was

sampled at a full 24.57 MHz, the false locks would occur at approximately 6 MHz

intervals. The (8/7) fraction takes into account the error correction coding added to the

actual data to be transmitted.

Figure 2(a) shows the sweep of the tuner AFC for an input IF of 479.5 MHz. The

true lock occurs in the center of the sweep with potential false locks occurring at each end

of the sweep. Figure 2(b) shows the sweep of the tuner AFC for an input IF of 482.5

MHz. The true lock occurs near the beginning of the sweep, with potential false locks

occurring in the center and near the end of the sweep. Finally, Figure 2(c) shows the

sweep of the tuner AFC for an input IF of 476.5 MHz. The true lock occurs near the end

of the sweep, with potential false locks occurring near the beginning and at the center of

the sweep. Of course, the data rate of 24.57 Mbits per second is for example only. Other

data rates will result in other lockup points.

Several solutions to this problem of false locking have been suggested. It has

been suggested that the sweep range be limited, in effect, requiring that all RF frequencies

be kept to a high stability. However, this stability could be provided only by expensive

RF hardware.

It has also been suggested that the output signals from the demodulator be

compared for phase shift difference. A false lock would produce a different level of phase shift than a true lock. A level circuit would then restart the sweep of the tuner AFC if a

false lock was detected. This would require more expensive equipment and be less

flexible than a microprocessor controlled approach, given that a microprocessor and error

correctors are already present in most digital transmission and reception systems.

It also has been suggested that double sampling be performed at the A/D converter

while using faster digital PAL logic. This technique would allow sweeps without false

locks. However, it would require more expensive logic and a faster A/D converter and

symbol clock, all of which adds to the expense of the system.

Finally, in co-pending and commonly owned U.S. application Serial No.

08/427,660 entitled "Method and Apparatus For Locating And Tracking a QPSK Carrier"

filed April 21, 1995, (which is incorporated herein by reference) there is described a

method to sweep for acquisition of a QPSK carrier while avoiding false lock conditions

and an apparatus for performing the method. This method and apparatus is described

with respect to the use of a Viterbi error correction chip in combination with a

microprocessor. The Viterbi chip and microprocessor are used to control a sweep circuit

and loop filter to locate and track a QPSK carrier.

While the above described techniques for resolving the problems of carrier false

locking represents an improvement over predecessor systems, there remains a need for

improvements to be made.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method and

apparatus for a carrier lockup system which improves over such systems know in the prior

art. It is a further object of the present invention to provide a method and apparatus for

a carrier lockup system which can be automatically operated without user intervention.

It is a still further object of the present invention to provide a method and

apparatus for a carrier lockup system which is economical to implement and is simple in

operation.

It is a further object of the present invention to provide a method and apparatus for

a carrier lockup system which is self-contained and is not reliant on outside systems for

its operation.

It is a still further object of the present invention to provide a method and

apparatus for a carrier lockup system which is low in cost to implement and can be readily

installed in existing communication equipment designs.

The above and other objects of the present invention are achieved by quickly

acquiring carrier lock (not RF lock). Carrier lock is used as a pre-filter prior to checking

for final forward error correction (FEC) lock. Additionally, when a valid signal has been

acquired, the system frequency plan is downloaded to further refine the search.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of a carrier locating and tracking system in accordance

with the prior art.

Figures 2(a) - 2(c) are graphs showing true and false lock points in sample AFC

sweeps. Figures 3 and 4 are flow charts which illustrate the operation of the carrier

lockup system of the present invention.

Figures 5 and 6 are tables which illustrate one of different sets of various

parameters which may be used with the carrier lockup system of the present invention. BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a means for searching L-Band spectra for digital

carriers within a mixed analog/digital environment. Given the correct symbol rate, the

present invention searches for correct carrier frequency and Forward Error Correction

(FEC) rate. The method and apparatus of the present invention is compatible with

hardware and firmware which supports variable symbol rate. The method and apparatus

of the invention operates by quickly acquiring carrier lock which is used as a pre-filter

prior to searching for final FEC lock. When a valid signal has been acquired, the system

frequency plan is downloaded to refine the search.

As used in this application, symbol rate is directly equivalent to bandwidth. For

example, a 30 MSps rate requires twice the bandwidth of a 15 MSps rate. Both uplink

and receive transmissions must be synchronized to the same symbol rate. A fixed rate

system uses a 28.3465 MSps at both the receive and transmission ends. Variable rate

systems use differing rates to accommodate larger or smaller communication needs

according to budget. These rates are normally set to specific values for hours or days.

When rates change, there is normally a disruption in the RF signal.

The present invention implements "find" and "search" modes which utilize a

common algorithm with differing "extensions" to achieve the specific function. The

"find" mode is intended to assist the equipment installation process by acquiring available

signals in sequence. At each acquisition, the equipment installer can save the parameters

or initiate a new "find." "Search" mode automatically re-acquires the original signal if

there is a change in one of the original parameters (e.g. frequency shift). Several user

selectable search variants may be used to address different applications. In accordance with the present invention, search parameters presently include FEC

rate and frequency. Prior digital lock techniques require that the signal level (measured

with respect to automatic gain control "AGC" values) to be tested as frequency is scanned

from a specified start to a specified end frequency. Changes in AGC are then used to

isolate carriers (e.g. carrier start, overlap, peak). Frequency would then be scanned

narrowly in an attempt to acquire RF lock.

It has been found, however, that AGC levels in some equipment are unreliable for

search purposes. AGC can only be tested for a value greater than the noise floor to see if

a signal is possibly there. No other rejection method is available. Therefore, analog

carriers, oscillator harmonics, and other artifacts cannot be identified and rejected.

Thus, in accordance with the present invention, an attempt to lock the receiver

front end is made whenever a potential digital carrier is identified. There are two main

lockup requirements in a variable symbol rate environment; the first applies to the symbol

rate while the second applies to the FEC rate. Both require the correct frequency to be

tuned. In a fixed rate system, only FEC lockup is required. Note, however, that false

locks are possible for both carrier and FEC stages. Carrier lock indicates that frequency

and symbol rate have been provisionally achieved. FEC lock will confirm that a valid

carrier has been acquired rather than a false lock.

If the carrier detector is optimized to provide very fast carrier lockups, the detector

can serve as a filter in the search process. The search algorithm of the present invention

can then skip through the spectrum looking for carrier locks. If none are found on the

first pass, the time permitted to find a carrier lock on each frequency is extended. If none

still are found, the third pass further increases time tolerances. After that, the algorithm resets to the first time tolerance on the assumption that no

signal is present. If a carrier lock is found, then an FEC lock is attempted. If signal lock

is achieved, the transmitted frequency table is checked for exact frequency. This is

necessary since RF lock can be acquired up to 2 MHz or more off the main carrier.

Figures 3 and 4 illustrate the above described operation of the search algorithm in

accordance with the present invention.

The above described method of the present invention allows for a full L-Band

frequency search (950 to 2150 MHz) to be completed in approximately 2 minutes.

Accordingly, the signal lock method of the present invention represents an improvement

over prior art systems.

Appliances have found that there are three criteria that determines the overall

speed of search on frequency. These are:

1. Amount of time allowed for carrier lock;

2. Size of the frequency step; and

3. Number of carriers in the search band.

In addition, Applicants have found that the amount of time required for carrier

lock is determined by two parameters; symbol rate and predefined search speeds. Since

the amount of time required for carrier lock varies depending on the noise condition of the

communication channel, three search speeds are used to optimize the average search time.

These search speeds are fast, medium and slow.

In accordance with the present invention, the search mechanism starts the

frequency search in the "Fast" mode. If no carrier is found, the next search speed is used.

The search engine continuously cycles through the three search speeds until a valid

carrier is found. Figure 5 illustrates a matrix showing the relationships between the time allowed for carrier lock and the two parameters as follows:

Zone 1 : Symbol Rate >= 20 MSps ^•

Zone 2: 15 MSps <= Symbol Rate < 20 MSps

Zone 3 : 7.5 MSps <= Symbol Rate < 15 MSps

Zone 4: 3.75 MSps <= Symbol Rate < 7.5 MSps

Zone 5: 1.875 MSps <= Symbol Rate < 3.75 MSps

Zone 6: Symbol Rate < 1.875 MSps

The above parameters are for illustration purposes only, and other such sets of

parameters may be used as well.

Applicants have also found that varying frequency step size does not improve the

search accuracy in high noise conditions. Therefore the frequency step size is determined

solely by symbol rate as illustrated by the matrix in Figure 6.

There are usually a mixture of analog and digital carriers on a signal satellite

transponder. The search engine of the present invention ignores regions with signal levels

(AGC) close to the noise floor. Therefore, search speed is determined by the number of

carriers and the bandwidth of those carriers.

By way of example only, it has been determined that the search speed is

approximately 55 seconds in an environment with:

2 digital, 7 analog carriers;

the nominal noise condition is 2.5 db above DVB noise threshold;

digital carrier characteristics is 28 MSps, 7/8 FEC;

the search band is 950 MHz to 1450 MHz; and

the number of search cycles required was 1. It should be obvious from the above-discussed apparatus embodiment that

numerous other variations and modifications of the apparatus of this invention are

possible, and such will readily occur to those skilled in the art. Accordingly, the scope of

this invention is not to be limited to the embodiment disclosed, but is to include any such

embodiments as may be encompassed within the scope of the claims appended hereto.

Claims

1. A receiver for receiving a data carrier having data modulated on it, said

receiver comprising:

tuner means for tuning a range of frequencies, said range containing the

frequency of the data carrier;

tuner control means coupled to said tuner means for controlling the

frequency tuning of said tuner means in incremental frequency steps within said range of

frequencies;

noise level measuring means coupled to said tuner means and to said tuner

control means for measuring the level of noise on the frequency to which said tuner

means is tuned, said noise level measuring means controlling said tuner control means to

cause said tuner means to tune to the next said incremental frequency when said level of

noise is below a predetermined level;

carrier lock means coupled to said noise level measuring means and being

controlled by said noise level measuring means to lock said tuner means on said tuning

frequency when said noise level is above said predetermined level; and

Forward Error Correction lock means coupled to said carrier lock means

for locking said tuner means on a further frequency within said range of frequencies.