WO2001093521A2

WO2001093521A2 - Frequency calibration for burst data reception systems

Info

Publication number: WO2001093521A2
Application number: PCT/US2001/017192
Authority: WO
Inventors: Thomas J. Campana, Jr.; Al Jewer; Gordon Smith; Robert Fries; Michael P. Ponschke, Sr.
Original assignee: Sophia Communications, Inc.
Priority date: 2000-05-31
Filing date: 2001-05-30
Publication date: 2001-12-06
Also published as: AU2001265058A1; WO2001093521A3

Abstract

In a wireless communications systems, a digital signal processor (DSP) periodically pre-monitors a frequency channel containing message traffic destined for other receivers Such monitoring is digitally performed in the DSP through the demodulation and the analysis of a fixed frequency reference tone (pilot) which is included in a data communications portion of the received signal. A plurality of internal radio parameters are then calibrated and stored in order to set the frequency of the receiver to be close to that needed to accurately and quickly receive a following burst data message addressed to said receiver, thus the frequency acquisition and lock the time of the receiver.

Description

A METHOD FOR RADIO RECEIVER FREQUENCY CALIBRATION FOR BURST DATA RECEPTION SYSTEMS

FIELD OF THE INVENTION

This invention relates to the field of receiver tuning in wireless

communications systems, and more particularly to a method of receiving

predecessor information to reduce receiver frequency acquisition time during

burst data receptions.

BACKGROUND OF THE INVENTION

A downlink of a wireless data communications systems typically includes

a base station transmitting a plurality of unique messages to one or more of a

plurality of remote receiver/ transmitters (RTs). For communications systems

having a large number of RTs, such base station transmissions tend to be

continuous, even though an individual RT will only receive a single burst

message followed by long periods of inactivity. Thus, an individual RT requires

precision timing components or techniques to quickly and accurately tune to a

particular frequency and receive a message.

However, due to product cost constraints, conventional implementations

of RT hardware rely on inexpensive timing components, such as crystals, which

produce timing errors due to initial purchase tolerances, operational temperature

and humidity variations, and component drift over life. Such timing errors

prevent precise tuning alignment of the RT with an incoming frequency, which leads to data reception errors. Thus, conventional over-the-air messaging architectures append a prefix time period to each transmitted message, wherein a signal having only a carrier frequency is transmitted for a finite amount of time to allow a receiver sufficient time to acquire and lock to that frequency. Such a non-data prefix time period allows frequency tuning using simple automatic frequency control (AFC) circuitry.

A typical AFC circuit is designed with a .wide frequency acquisition range in order to be able to initially detect the carrier frequency. This wide range is req iired due to the initial receiver frequency settings being derived from the above mentioned imprecise timing components, although the intent is to set the initial receiver frequency to be as close as possible to the incoming carrier frequency to reduce the acquisition time. A drawback of using such inexpensive AFC circuits, however, is that a tuning resolution will typically be on the order of 10 to 20 Hertz due to a design trade-off between tuning resolution and circuit settling times. For a communications system having a carrier frequency of several hundred megahertz, this is generally not adequate for higher data rates and more complex modulation techniques. To obtain a higher frequency resolution requires longer frequency acquisition times and thus a longer frequency-only prefix time period, which can typically occupy from 25 to 50% of a transmission time period, and represent a significant loss of otherwise usable channel capacity. Efforts to reduce the channel capacity losses by using a smaller prefix time results in less accurate receiver data sampling, and thus, greater data bit error rates. In some implementations where such data errors cannot be corrected, a retransmission of all or part of the message, each one having a tuning prefix, further reduces available channel capacity.

A return signal (uplink) employs the same message construction having the frequency-only prefix time period to communicate with the base station. However, since a base station can be implemented using more expensive components and have more sophisticated receiver circuitry, the amount of time required for frequency acquisition and lock can be reduced over that of the simpler, and less expensive RTs. A further significant drawback of conventional implementations is that the same inaccurate timing components are typically used to generate the transmission frequencies in the RT, thus causing wide variations in these frequencies. These frequency variations make it more difficult for the base station receiver to acquire the transmitted signal and in some cases can cause violations of authorized channel boundaries

The shortcomings of the prior art are remedied by the current invention, wherein a Digital Signal Processor (DSP) pre-monitors a • frequency channel containing message traffic destined for .other receivers and measures the frequency of a signal that is contained in a modulation constellation of the received signal. A plurality of internal radio parameters are then calibrated and stored in order to conditionally align the receiver operating frequency to be close to that needed to quickly acquire and lock to the frequency of a succeeding received signal. .

By periodically scheduling such pre-monitoring actions to balance battery power consumption with the calibration requirements of the components of a particular receiver, the frequency acquisition times are significantly reduced over that of conventional methods. A higher spectral efficiency that results from such ^'reduced acquisition time increases channel utilization.

SUMMARY OF THE INVENTION It is an object of the current invention to provide a method for significantly reducing the amount of time that a receiver requires to acquire and lock to a carrier freq iency of an incoming signal in a wireless communications system. It is a specific object of the current invention to provide such a system easily and inexpensively, with a minimum modification to the existing^* systems. According to one aspect of the current invention, the method is used for calibrating frequency in a remote transmitter/ receivers in a communication system including at least one base stations and several remote transmitter/ receivers. In such systems a number of transmissions are transmitted and received, and communications contain both a data signal and a reference signal. Each such signal is addressed to one or more such receiver/ transmitter at any particular time. The method includes embedding frequency calibration parameters in each base station reference signal, after which the frequency calibration parameters are extracted at each transmitter/receiver. After completion of an instant reception, the next step is the calibrating of the frequency of the transmitter/ receiver, and this is done both when the communication is addressed to the transmitter/ receiver, and when it is not.

According to a second aspect of the invention, the communications are transmitted as modulated signals. As a result, the reference signals must be demodulated in each transmitter/ receiver.

According to a third aspect of the invention, the reference signal is a sinusoidal signal having a fixed frequency and a fixed amplitude.

According to a fourth aspect of the invention, the data signal has a different frequency from that of the reference signal.

According to a fifth aspect of the invention, the modulated data'signal has a pair of pass-band frequencies located in a communications channel,, whereas the reference signal has a center frequency that is located outside the pair of pass- band frequencies.

According to a sixth aspect of the invention, the reference signal has a center frequency that is located inside a pair of pass-band frequencies of the ^' communications channel. According to a seventh aspect of the invention, the processing may be done by either a digital signal processor and/ or a microprocessor.

According to an eighth aspect of the invention, calibration parameters extracted from the reference signal further contains a timing signal, and;/ or a controlling digital data input to a phase lock loop, and/. or a controlling voltage signal to a voltage controlled oscillator; and/ or a controlling digital signal to a digital filter.

According to a ninth aspect of the invention, the modulated signal is encoded using a phase shift modulation teclinique which may include any or all of the following: binary phase shift keying, quadrature phase shift keying, and quadrature phase shift keying, and quadrature amplitude modulation

According to a final aspect of the invention, a method for calibrating frequency in a receiver in a wireless data communications system includes transmitting a modulated signal containing a data signal and a reference signal. The signals are received in a remote receiver, and are demodulated. The next step is the extracting several characterization signals from the demodulated reference signal. A number of frequency calibration signals are similarly generated and stored in memory. Finally, the receiver is calibrated using these frequency calibration signals. _.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a conventional wireless communications system having a base station in communications with a plurality of RTs.

Figure 2 shows a timing diagram of an exemplary message.

Figure 3 shows a timing diagram of a typical sequence of messages, each message having a prefix and a data portion.

Figure 4 shows an exemplary message attained using methods according to the present invention.

Figure 5 shows a frequency plot of the reference tone relative to the modulated data signal.

Figure 6 shows a block circuit diagram of an exemplary receiver according to the present invention.

Figure 7 shows a block diagram of the steps used to periodically receive and pre-store timing information in order to provide continuous calibration of a receiver.

Figure 8 shows a timing diagram comparing the amount of transmission time that is required for a receiver using the methods of the present invention vs. using c nventional implementation methods.

Figure 9 shows a diagram of the steps used to create a signal comprised of two distinct signals each having a separate modulation frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS W

In a wireless communications system a method is disclosed for significantly reducing the amount of time that a receiver requires to acquire and lock to a carrier frequency of an incoming signal over that of conventional methods. According to the present invention, a method for periodically sampling alternate transmissions in a same frequency channel allows a remote receiver/ transmitter (RT) to periodically recalibrate its internal receiver settings to attain a standby quiescent operating point that has a tuning characteristic that is substantially identical to the signal being transmitted. Thus, when a message is transmitted that is addressed to this particular RT, only a small amount of time is required to fine-tune to the frequency of the incoming signal.

Figure 1 shows a conventional wireless communications system having a base station 10 which has a transmitter/ receiver 12 in communications with a plurality of RTs 14. Each message transmitted by base station 10 includes an identifier address embedded in the message that directs the message to one or more of the plurality of RTs 14. Each one of the plurality of RTs 14 uses its unique address to discriminate between incoming messages, causing a particular RT to only receive and process the data content of messages which include its unique address. Communications with a separate plurality of RTs can be accomplished by additional group address and wildcards as is known in the art. Return (uplink) messages originating at each one of the plurality of RTs 14 typically can have the same message construction. In the timing diagrams discussed in the following paragraphs, the horizontal axis represents time.

Figure 2 shows a timing diagram of an exemplary message of a prior art communications system comprising two portions. A first portion, or prefix 16, represents the time required for the receiver to acquire and lock to an incoming carrier frequency, during which no data can be processed, since frequency errors would typically be greater than any compensation capabilities of data extracting circuitry. A second portion 18 is comprised of a modulated data message. As previously discussed, the length of prefix 16 can typically be from 25 to 50% oi the total time of the message.

Figure 3 shows a timing diagram of a typical sequence of prior art messages transmitted by base station 1-0, each message having a prefix and a data portion. Transmission overhead in the form of a guard-band 20 for message separation and a cross-hatched area 22 for each message represents time that is not useable for the data portion of a message. This overhead has the effect of reducing the effective bandwidth of a given channel through the limiting of the amount of transmission time that can be dedicated to usable data. As will be seen, significant improvements can be made using alternate methods, providing that such methods can retain the accuracy required for frequency calibration as discussed above. Figure 4 shows an exemplary message attained using methods according to the present invention. Due to improved receiver tuning techniques, the amount of time required for prefix 16 has been significantly reduced, allowing more of the available transmission time to be used for data messaging traffic. Such tuning techniques involve the incorporation of a "reference tone" which is imbedded in the transmission signal and is extracted at a receiver using special- processing modules in a digital signal processor (DSP). In a preferred embodiment, such a reference tone would be a fixed frequency and fixed amplitude sinusoidal waveform, since more complex waveforms would contain other frequency components that would need more complex filtering and processing.

Figure 5 shows a frequency plot of the reference tone relative to the modulated data signal. Within an FCC spectral mask 24 for a given channel 26, a frequency^' roll-off of filters typically requires a data signal ^' 28 to be centered within mask 24 and to begin attenuation almost immediately, so as to insure that harmonic frequencies of a transmitted signal will not spill over and interfere with adjacent channels. This leaves an area at the shoulders 30 that can be used for additional frequency tones, such as tone 32, providing that the added signal is contained within mask 24. DSPs provide the processing power to enable an extremely high performance filter to be implemented that can both contain tone 32 and, alternatively, to extract such a received signal using a digital high- performance band-pass filter.

For example, assuming an exemplary channel width of 5 KHz, or ± 2500 Hz, with the upper limits of spectral mask 24 established at +2000 Hz, data signal 28 can be attenuated by -lOdb at ±2000 Hz. The margin 30 between signal 28 at a representative 1500 Hz and spectral mask 24 allows the inclusion of tone 32 at a lower amplitude than the peak value of signal 28 at the channel midpoint. The requirements for such inclusion is that the filtering be accomplished using a filter typically having a much higher number of poles, so that the attenuation vs. frequency curve of tone 32 is much steeper than the attenuation curve of signal 28.

While tone 32 can have a voltage amplitude equal to that of signal 28, an implementation of such a filter is impractical, even in digital form. The optimum amplitude of tone 32 is determined by the trade-off between the signal magnitude necessary to be easily detected and interpreted at a remote RT and the signal that can be practically implemented without undue system complexity and cost. It should be noted that a two-way communications system will require tone 32 to be imbedded at both ends of the communication link. So while a single base station can have complex and expensive implementation circuitry, a pkirality of inexpensive RTs cannot practically and economically use the same circuitry and components. Figure 6 shows a block circuit diagram of an exemplary RT according to the present invention. A computing device, such as a powerful microprocessor or DSP 34, controls all aspects of the reception and transmission of signals in the RT. A signal 36 received at antenna 38 is detected by front end circuitry 40, which signals DSP 34 that a carrier signal is present. DSP 34 then enables front- end circuitry 40, a fractional phase lock loop (PLL) 42, and a voltage controlled oscillator (VCO) 44 to provide a frequency control signal to a down-conversion circuit in tuning circuitry 40. DSP 34 filters and processes down-converted signal 46 to extract the frequency information contained in included tone 32. Corrective calculations are then performed in DSP 34 to provide adjustment signals to PLL 42 and VCO 44 to enable frequency acquisition and lock of the receiver circuitry to a succeeding received signal.

Filtering and decoding of tone 32 included in signal 46 provides DSP 34 with calibration data to adjust the frequencies that are supplied by PLL 42 and VCO 44. This calibration data is in the form of a known fixed frequency of tone 32, and enables DSP 34 to digitally determine the precise frequency of the received signal. Since this fixed frequency is predetermined and known to the RT, DSP 34 can compare the received values with its internal values to determine the corrective adjustments needed to generate a set of new parameters for PLL 42 and VCO 44. These new parameters are then stored in memory for use during a- succeeding RT reception. One of the new parameters calculated and stored by DSP 34 is a quantification of any timing errors associated with the internal timing component of the RT relative to the received reference signal. These q iantification parameter can then be used to calculate adjustments needed in the transmission frequencies in the RT. Another parameter that is calculated and stored is a correction factor for the amplitude of the received signal. This correction factor is provided to an automatic gain control (AGC) circuit included in front-end circuitry 40 to insure that the output signal of the AGC and front- end circuitry 40 is of a constant amplitude irrespective of variations in the magnitude of the input signal. Maintaining the constant amplitude of the received signal facilitates the rapid frequency acquisition process.

While the above discussion relates to the use of a fixed and separate tone 32 included in the same channel as shown in figure 5, alternate embodiments can use a variety of frequency acquisition techniques. For example, the fixed tone can be transmitted in an alternate channel that is reserved to serve solely as a reference for all RT's in a communications system, although this represents an inefficient use a valuable wireless channel. An alternative approach is to digitally sample the data transmissions to extract the timing information contained therein. This approach is difficult to implement in systems that have variable data rates, such systems being the norm.

The desired result is to be able to pre-set the frequency of a receiver which uses non-precision timing components to be close to the frequency of an incoming signal. This reduces the time needed for frequency search-and- acquisition. The principal requirements are that the frequency of the reference signal be constant and known to all elements of the communications system, and that the receiver has the ability to derive frequency correction parameters from that reference signal which compensate for local timing offsets. Also, while the derived calibration parameters can be used to re-align the receiver during reception of the current message, this adjustment activity can create unnecessary errors since the receiver would be in lock with, and satisfactorily receiving, the incoming signal, and any such changes can misalign the processing.

When the RT transmits a signal, DSP 34 enables the transmitter control circuitry 48 and loads a data signal into PLL 42 and VCO 44 that is associated with a desired frequency. Since this frequency is also calibrated to the previously received tone 32, an accurate frequency selection is made that allows the transmission to be sent without violating the spectral mask of the transmission channel. To enable base station 10 to also quickly acquire and lock to this frequency, an identical tone 32 is imbedded in the modulated transmission block by DSP 34.

Figure 7 shows a block diagram of the steps used to periodically receive and pre-store frequency timing information in order to provide continuous timing calibration to a receiver. By periodically sampling messages in a given frequency channel that are being, directed to RTs other than the subject RT, a continuous updating of the information contained in tone 32 is made available to the subject RT. After processing by DSP 34 as above, a new set of parameters are stored for use during any succeeding receptions. This allows the RT to be initialized at an operating point that is within 100 Hz of an incoming carrier frequency, and thus minimizes the acquisition and lock time. The storage of the . parameters can be implemented using a variety of storage devices, both volatile and non-volatile. Examples of such devices are: static and dynamic random- access memory (SRAM and DRAM), electronically reprogrammable memory (EEPROM and Flash), and fixed and removable media, such as floppy disks and hard disk.

An exemplary RT would normally be in an inactive mode in step 50 to reduce standby battery consumption. An internal timer would periodically awaken the RT in step 52 to sample a receiver channel in step 54, even though a message would not be addressed to the RT. By processing the message in step 56 to extract the frequency timing information in tone 32 in step 58, DSP 34 can update the internal parameters of the RT in step 60 to correct for any frequency timing errors that result from the above mentioned component tolerances in step 62. After updating the calibration parameters, the RT would return to the inactive state in step 64.

Figure 8 shows two timing diagrams comparing the amount of transmission time that is required for a receiver using the methods of the present. invention, as the upper diagram, vs. using conventional implementation methods, as the lower diagram. A transmitted message in a conventional communications system as shown in figure 2 will have prefix 16 that will occupy up to 50% of the total transmission time window, with data portion 18 occupying the remainder. By implementation of tone 32 and periodically receiving and storing the timing information according to the present invention, an exemplary transmission signal will be characterized by a prefix occupying, for example, from 10 to 20% of the transmission time window. This is accomplished primarily through the high accuracy of the initial frequency setting that significantly reduces the acquisition time of the tuning circuitry.

Figure 9 shows a diagram of the steps used to create a signal comprised of two distinct signals each having a separate modulation frequency. In step 66, a first signal is digitally modulated in DSP 34 at a first frequency. In step 68, a second signal is digitally modulating in DSP 34 at a second frequency. In step 70, the first and second signals are added so as to produce a combined third signal, which is digitally filtered in step 72 to eliminate spectral components that are not contained within a predetermined spectral mask.

Numerous modifications to and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching ^• those skilled in the art the best mode of carrying out the invention. Details of the methods may be varied without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.

Claims

CLAIMS What is claimed is:

1. A method for frequency acquisition and lock in a receiver in a wireless data communications system, comprising the steps of: transmitting in a transmitter a modulated signal comprising a data signal . and a reference signal; receiving said signal in a remote receiver; demodulating said reference signal in said receiver; extracting by a processing means a plurality of characterization signals from said demodulated reference signal; generating a plurality of frequency calibration signals by said processing means; and calibrating said receiver using said plurality of frequency calibration signals.

2. The method according to claim 1, wherein said reference signal is a sinusoidal signal having a fixed frequency and a fixed amplitude.

3. The method according to claim^' 2, wherein said data signal and said reference signal have different frequencies.

4. The method according to claim 3, wherein said reference signal has a center frequency that is located outside .a pair of pass-band frequencies of the modulated data signal.

5. The method according to claim 4, wherein said reference signal has a center frequency that is located inside a pair of pass-band frequencies of a communications channel.

6. The method according to claim 1, wherein the transmitter is a base station and the receiver is one of a plurality of receivers in communications with said base station.

7. The method according to claim 1, wherein the processing means comprises at least one from the group consisting of a digital signal processor and a microprocessor.

8. The method according to claim 1, wherein plurality of characterization signals comprises at least one from the group consisting of a voltage amplitude and a frequency.

9. The method according to claim 1, wherein the plurality of calibration signals comprises of one or more from the group consisting of: a timing signal; a phase lock loop control signal; an automatic gain controller input signal; a voltage controlled oscillator control signal ; and a digital filter control signal.

10. The method according to claim 1, wherein said modulated signal is encoded using a phase shift modulation technique consisting of one or more from the group consisting of: binary phase shift keying, quadrature phase shift keying, and quadrature amplitude modulation.

11. A method for incorporating at least two independent signals having different frequencies in a narrow frequency spectral mask using a digital signal processor, comprising the steps of: digitally modulating in the digital signal processor a first signal at a first frequency; digitally modulating in the digital signal processor a second signal at a second frequency; adding the first and second signals so as to produce a combined third signal; and digitally filtering said third signal to eliminate spectral components that are not contained within a predetermined spectral mask.

12. The method according to claim 11, wherein the first signal comprises a data signal to be transmitted.

13. The method according to claim 11, wherein the second signal comprises a sinusoidal signal that has a fixed frequency and a fixed amplitude.

14. The method according to claim 2, wherein the modulated reference signal is included in a frequency spectral mask that includes a modulated data portion of the transmission signal, but is offset in frequency from the modulated data portion so as to be separately detectable.

15. A method for calibrating the frequency in a remote transmitter/ receivers in a communication system comprising at least one base stations and a multiplicity of remote transmitter/ receivers, in which a multiplicity of communications are transmitted and received, and in which each communication comprises a data signal and a reference signal, and each communication is addressed to one or more such receiver/ transmitter at any particular time, the method comprising: embedding calibration parameters in each reference signal; extracting the calibration parameters at each transmitter/ receiver; calibrating the frequency of said transmitter/receiver after extracting of the calibration parameters; both when the communication is addressed to the transmitter/ receiver, and when it is not.

16. The method of claim 15, wherein the communications are transmitted as modulated signals, and further comprising demodulating said reference signal in said transmitter/ receiver.

17. The method according to claim 16, wherein said demodulated reference signal is a fixed frequency sinusoid, having a fixed amplitude.

18. The method according to claim 17, wherein said data signal comprises a spectrum about a center frequency and wherein said center frequency is different from said reference signal frequency.

19. The method according to claim 18, wherein said modulated data signal further is located inside a pair of pass-band frequencies, and wherein the reference signal has a center frequency that is located outside the pair of pass- band frequencies.

20. The method according to claim 19, wherein said reference signal center frequency is located inside a pair of pass-band frequencies of a communications channel.

21. The method according to claim 15, wherein the processing means further comprises at least one from the group consisting of a digital signal processor and a microprocessor.

22. The method according to claim 15, wherein the reference signal contains a plurality of characterization signals, said characterization signals containing at least one ember selected from the group consisting of a voltage amplitude characterization signal and a frequency characterization signal.

23. The method according to claim 15, wherein the calibration parameters further contains a plurality of signals at least one member of which is selected from the group consisting of: a timing signal; a phase lock loop control signal; an automatic gain controller input signal; a voltage controlled oscillator control signal ; and a digital filter control signal.

24. The method according to claim 3, wherein said modulated signal is encoded using a phase shift modulation technique consisting of one or more from the group consisting of: binary phase shift keying, quadrature phase shift keying, and quadrature amplitude modulation.

25. The method according to claim 1, additionally including the step of storing said calibration parameters in a memory module in said receiver.

26. The method according to claim 25, wherein the memory module comprises one or more from the group consisting of volatile and non-volatile memory.

27. The method according to claim 26, wherein the memory module comprises one or more from the group consisting of: static random access memory, dynamic random access memory, and electronically reprogrammable memory _