U.S. patent application number 14/204922 was filed with the patent office on 2014-07-10 for method and apparatus for eliminating the effects of frequency offsets in a digital communication system.
This patent application is currently assigned to QUALCOMM Incorporated. The applicant listed for this patent is QUALCOMM Incorporated. Invention is credited to Teresa H. Meng, David Kuochieh Su, Masoud Zargari.
Application Number | 20140192850 14/204922 |
Document ID | / |
Family ID | 23648523 |
Filed Date | 2014-07-10 |
United States Patent
Application |
20140192850 |
Kind Code |
A1 |
Meng; Teresa H. ; et
al. |
July 10, 2014 |
METHOD AND APPARATUS FOR ELIMINATING THE EFFECTS OF FREQUENCY
OFFSETS IN A DIGITAL COMMUNICATION SYSTEM
Abstract
The present invention aims at eliminating the effects of
frequency offsets between two transceivers by adjusting frequencies
used during transmission. In this invention, methods for correcting
the carrier frequency and the sampling frequency during
transmission are provided, including both digital and analog
implementations of such methods. The receiver determines the
relative frequency offset between the transmitter and the receiver,
and uses this information to correct this offset when the receiver
transmits its data to the original transmitter in the return path,
so that the signal received by the original transmitter is in
sampling and carrier frequency lock with the original transmitter's
local frequency reference.
Inventors: |
Meng; Teresa H.; (Portola
Valley, CA) ; Su; David Kuochieh; (Cupertino, CA)
; Zargari; Masoud; (Cupertino, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
QUALCOMM Incorporated |
San Diego |
CA |
US |
|
|
Assignee: |
QUALCOMM Incorporated
San Diego
CA
|
Family ID: |
23648523 |
Appl. No.: |
14/204922 |
Filed: |
March 11, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13736825 |
Jan 8, 2013 |
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14204922 |
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09416098 |
Oct 12, 1999 |
8363757 |
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13736825 |
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Current U.S.
Class: |
375/219 ;
375/327 |
Current CPC
Class: |
H04L 2027/0022 20130101;
H04B 1/38 20130101; H03J 7/04 20130101; H04L 27/148 20130101; H04L
7/033 20130101; H04L 27/0014 20130101 |
Class at
Publication: |
375/219 ;
375/327 |
International
Class: |
H04L 27/148 20060101
H04L027/148; H04B 1/38 20060101 H04B001/38 |
Claims
1. A device in a communication system including a first transceiver
unit and a second transceiver unit remote to the first transceiver
unit, the second transceiver unit operable to communicate in a
bi-directional manner for direct exchange of information with the
first transceiver unit, the device comprising: means for detecting
a sampling frequency offset between a first sampling frequency used
by the first transceiver unit to transmit a first signal, and a
second sampling frequency used by the second transceiver unit to
receive the first signal, wherein detecting the sampling frequency
offset comprises comparing a first digital signal produced by
digitally sampling the first signal at the second sampling
frequency to the received first signal detected by the second
transceiver unit; and means for adjusting sampling points of data
to be transmitted by the second transceiver unit in response to the
detected sampling frequency offset, whereby an error associated
with the sampling frequency offset is reduced.
2. A device according to claim 1, wherein the means for detecting
the sampling frequency offset includes means for locking onto the
sampling frequency offset by introducing a variable delay to the
first digital signal.
3. A device according to claim 1, wherein the means for adjusting
the sampling points includes means for digitally interpolating the
data to be transmitted.
4. A device according to claim 1, wherein the means for detecting
the sampling frequency offset includes: means for generating an
output frequency corresponding to the second sampling frequency
having a desired phase and frequency in accordance with a control
signal; means for comparing the first digital signal and the output
frequency to determine a phase difference between the first digital
signal and the output frequency; and means for adjusting the
control signal in response to the determined phase difference.
5. A device according to claim 4, wherein the means for generating
the output frequency includes means for selecting one of a
plurality of phases of the second sampling frequency in response to
the control signal.
6. A device according to claim 1, wherein the means for adjusting
the sampling points includes: means for generating an output
frequency corresponding to the second sampling frequency having a
desired phase and frequency; and a digital to analog (D/A)
converter that operates in response to the output frequency.
7. A device according to claim 4, wherein the means for adjusting
the sampling points includes means for generating a second output
frequency corresponding to the second sampling frequency having a
second desired phase and frequency.
8. A method used in a communication system including a first
transceiver unit and a second transceiver unit disposed remotely
from the first transceiver unit, the second transceiver unit
operable to communicate in a bi-direction manner for direct
exchange of information with the first transceiver unit, the method
comprising: detecting a sampling frequency offset between a first
sampling frequency used by the first transceiver unit to transmit a
first signal and a second sampling frequency used by the second
transceiver unit to receive the first signal, by comparing a first
digital signal produced by digitally sampling the first signal at
the second sampling frequency to the received first signal detected
by the second transceiver unit; and adjusting sampling points of
data to be transmitted by the second transceiver unit in response
to detecting the sampling frequency offset, wherein an error
associated with the sampling frequency offset is reduced.
9. The method according to claim 8, wherein detecting the sampling
frequency offset includes locking onto the sampling frequency
offset by introducing a variable delay to the first digital
signal.
10. The method according to claim 8, wherein adjusting the sampling
points comprises digitally interpolating the data to be
transmitted.
11. The method according to claim 8, wherein detecting the sampling
frequency offset includes: generating an output frequency
corresponding to the second sampling frequency having a desired
phase and frequency in accordance with a control signal; comparing
the first digital signal and the output frequency to determine a
phase difference between the first digital signal and the output
frequency; and adjusting the control signal in response to the
determined phase difference.
12. The method according to claim 11, wherein generating the output
frequency includes selecting one of M phases of the second sampling
frequency in response to the control signal.
13. The method according to claim 8, wherein adjusting the sampling
points includes: generating an output frequency corresponding to
the second sampling frequency having a desired phase and frequency;
and operating a digital to analog (D/A) converter in response to
the output frequency.
14. The method according to claim 11, wherein adjusting the
sampling points includes generating a second output frequency
corresponding to the second sampling frequency having a second
desired phase and frequency.
15. A device adapted to be used in a first unit that communicates
with a second unit using a common sampling frequency, the device
comprising: a delay lock loop that is coupled to receive digitally
sampled data of a first signal transmitted by the second unit, and
digitally detected data of the first signal, the delay lock loop
being adapted to detect a sampling frequency offset in the first
signal in response to the digitally sampled data of the first
signal and the digitally detected data of the first signal, and to
produce offset information corresponding thereto; and a first
digital low-pass filter that is coupled to receive the offset
information and data to be transmitted by the first unit in a
second signal to be received by the second unit, the first digital
low-pass filter being adapted to digitally interpolate the data to
be transmitted in accordance with the common sampling frequency and
the sampling frequency offset wherein an error associated with the
sampling frequency offset is reduced.
16. The device of claim 15, further comprising: a timing
acquisition unit configured to produce the digitally sampled data
of the first signal based at least in part on an output of the
delay lock loop.
17. The device of claim 16, further comprising: a data detection
block configured to produce the digitally detected data of the
first signal; wherein an output of the data detection block is
communicatively coupled with the delay lock loop.
18. The device of claim 17, further comprising: a second digital
low-pass filter communicatively coupled with an output of the
timing acquisition unit and an input of the data detection
block.
19. The device of claim 15, further comprising: a digital-to-analog
converter configured to receive the interpolated data to be
transmitted from the first digital low-pass filter.
20. The device of claim 19; further comprising: an analog
transmitter front-end; wherein an output of the digital-to-analog
converter is communicatively coupled with an input of the analog
transmitter front-end.
21. The device of claim 15, further comprising: a data modulation
block configured to modulate the data to be transmitted; wherein an
output of the data modulation block is communicatively coupled with
an input of the first digital low-pass filter.
Description
CROSS REFERENCES
[0001] The present Application for Patent is a continuation of U.S.
patent application Ser. No. 13/736,825 by Meng et al., entitled
"Method and Apparatus for Eliminating the Effects of Frequency
Offsets in a Digital Communication System," filed Jan. 8, 2013;
which is a divisional of U.S. patent application Ser. No.
09/416,098 by Meng et al., entitled "Method and Apparatus for
Eliminating the Effects of Frequency Offsets in a Digital
Communication System," filed Oct. 12, 1999; each of which is
assigned to the assignee hereof, and expressly incorporated by
reference herein.
BACKGROUND
[0002] 2. Field of the Invention
[0003] The present invention relates to digital communications, and
more particularly, to methods for correcting carrier frequency and
sampling frequency at the transmitter to eliminate the effects of
offsets in such frequencies.
[0004] 2. Description of the Related Art
[0005] In a digital communication system composed of at least two
transceivers, one serving as a transmitter and the other as a
receiver, problems occur if the reference frequencies of the two
transceivers are not exactly the same.
[0006] There are in general two sources of frequency offsets:
carrier frequency offsets and sampling frequency offsets. Carrier
frequency offsets result in the received signal being demodulated
by a wrong carrier frequency, while sampling frequency offsets
result in the data being sampled at the wrong time instants.
Typically, the percentages of the carrier frequency offset and the
sampling frequency offset would be the same if a single oscillator
reference is used to generate the two frequencies at both the
transmitter and the receiver. The percentages of the two frequency
offsets will be different if different oscillator references are
used.
[0007] Conventionally, such frequency offsets are only detected and
corrected during processing at the receiver end. For example, the
receiver can employ a carrier frequency lock loop to determine the
carrier frequency offset and a delay lock loop to determine the
sampling frequency offset. Such mechanisms are only used to detect
the frequency offset between the transmitter and the receiver, and
to compensate for offset effects at the receiver end. See, for
example: H. Meyr, M. Moeneclaey, and S. Fechtel, Digital
Communication Receivers, Wiley-Interscience Publication, 1998; S.
Kay, "A fast and accurate single frequency estimator," IEEE Trans.
on Acoustics, Speech, and Signal Processing, December 1989; Viterbi
and A. Viterbi, "Nonlinear estimation of PSK-modulated carrier
phase with application to burst digital transmission," IEEE Trans.
on Information Theory, July 1983; M.
[0008] Fitz, "Further results in the fast estimation of a single
frequency," IEEE Trans. on Communications, February 1994; and D.
Messerschmitt, "Frequency detectors for PLL acquisition in timing
and carrier recovery," IEEE Trans. on Information Theory, September
1979.
[0009] Conventional techniques for reducing the effects of
frequency offsets at the receiving end have many shortcomings For
example, if narrow-band frequency division multiple access (FDMA)
is used to provide multiple access from different users, the
carrier frequency offsets in the reverse link (from end-users to
the base-station) might cause data to overlap in frequency at the
base-station receiver. On the other hand, if time division multiple
access (TDMA) is used to provide multiple access from different
users in the reverse link, sampling frequency offsets might cause
data to overlap in time at the base-station receiver.
[0010] In more advanced communication systems that employ either
multi-user detection in a code division multiple access (CDMA)
system, or multi-carrier modulation in an orthogonal frequency
division multiplexing (OFDM) system, frequency offsets are
particularly damaging in signal detection in the reverse link For
example, in CDMA systems where multiple user access is provided via
multiple remote units, multi-user detection can be employed at the
base station for interference rejection. See, for example, S.
Verdu, Multiuser Detection, Cambridge University Press, 1998.
However, the carrier frequency offsets introduced in the reverse
link by the various remote units, if not corrected during
transmission by the remote units, will destroy the stationary
properties of the combined signal as received by the base station,
thus greatly degrading the multi-user detection performance.
[0011] Similarly, in an OFDM system, multiple frequency carriers
are used to transmit data to and from multiple users. See, for
example, B. Le Floch, M. Alard, and C. Berrow, "Coded Orthogonal
Frequency Division Multiplex," Proceedings of IEEE, pp. 982-996,
Vol. 83, No. 6, June 1995. If multiple remote users use different
carrier frequencies to transmit data at the same time in an OFDM
system, as in the reverse link of a CDMA system, the frequency
offsets in both carrier frequency and sampling frequency will cause
the data from different users to overlap in both frequency and
time, again greatly degrading the multi-carrier detection
performance.
[0012] Accordingly, there remains a need in the art for techniques
for reducing frequency offsets that improve the signal detection
capability of the combined signals received from multiple remote
units in a base station, and in general between any two
transceivers. The present invention fulfills this need, among
others.
SUMMARY
[0013] Accordingly, an object of the present invention is to
overcome the problems of the prior art, including the problems
identified above.
[0014] Another object of the present invention is to improve signal
detection capability of combined signals received from multiple
remote units in a base station, and in general between any two
transceivers.
[0015] Another object of the invention is to correct frequency
offsets between transceivers, and particularly between a base
station and a remote unit.
[0016] Another object of the invention is to correct carrier
frequency offsets between transceivers, and particularly between a
base station and a remote unit.
[0017] Another object of the invention is to correct sampling
frequency offsets between transceivers, and particularly a base
station and a remote unit.
[0018] Another object of the invention is to correct for frequency
offsets between transceivers, and particularly a base station and a
remote unit, by adjusting a carrier frequency before
transmission.
[0019] Another object of the invention is to correct for frequency
offsets between transceivers, and particularly a base station and a
remote unit, by adjusting a sampling frequency before
transmission.
[0020] To achieve these objects and others, the invention aims at
eliminating the effects of frequency offsets between transceivers
by adjusting frequencies used during transmission. In this
invention, methods for correcting the carrier frequency and the
sampling frequency during transmission are provided, including both
digital and analog, and closed and open loop implementations of
such methods. The receiver determines the relative frequency offset
between the transmitter and the receiver, and uses this information
to correct this offset when the receiver transmits its data to the
original transmitter in the return path, so that the signal
received by the original transmitter is in frequency lock with the
original transmitter's local frequency reference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] These and other objects and advantages of the present
invention, along with the best mode for practicing it, will become
apparent to those skilled in the art after considering the
following detailed specification, together with the accompanying
drawings wherein:
[0022] FIG. 1 illustrates a communication system in accordance with
the invention;
[0023] FIG. 2 illustrates a receiver in a remote unit that detects
carrier frequency offsets in accordance with a first embodiment of
the invention;
[0024] FIG. 3 illustrates a transmitter in a remote unit that
corrects for carrier frequency offsets in accordance with a first
embodiment of the invention;
[0025] FIG. 4 illustrates a receiver in a remote unit that detects
sampling frequency offsets in accordance with a second embodiment
of the invention;
[0026] FIG. 5 illustrates a transmitter in a remote unit that
corrects for sampling frequency offsets in accordance with a second
embodiment of the invention;
[0027] FIG. 6 illustrates a transceiver in a remote unit that
corrects for carrier frequency offsets in accordance with a third
embodiment of the invention;
[0028] FIG. 7 illustrates a receiver in a remote unit that detects
sampling frequency offsets in accordance with a fourth embodiment
of the invention;
[0029] FIG. 8 illustrates an alternative receiver in a remote unit
that detects sampling frequency offsets in accordance with a fourth
embodiment of the invention;
[0030] FIG. 9 illustrates a receiver in a remote unit that detects
sampling frequency offsets in accordance with a fourth embodiment
of the invention; and
[0031] FIG. 10 illustrates a transmitter in a remote unit that
corrects for sampling frequency offsets in accordance with a fourth
embodiment of the invention.
DETAILED DESCRIPTION
[0032] FIG. 1 illustrates a preferred embodiment in which a base
station (transceiver 120) communicates with multiple remote (e.g.
hand-held and/or mobile) units (transceiver 100-1, transceiver
100-2, etc.).
[0033] In the discussion below, CDMA is preferred as the carrier
modulation technique to provide multiple user access and multi-user
detection is preferably employed at the base station for
interference rejection. The carrier and sampling frequency offsets
introduced by various remote units, if not appropriately corrected
during transmission, will destroy the stationary properties of the
combined signal as received by the base station. According to an
aspect of the invention, therefore, each remote unit 100 corrects
the frequency offsets during transmission as will be described in
more detail below, thus creating a low IF modulation and
interpolation effect, before sending the signal to the analog
front-end circuitry. Although the invention is particularly useful
for CDMA modulation techniques, the invention is not limited to
this example, but should be particularly useful for many other
modulation techniques where frequency offsets can introduce
difficulties such as multi-user detection systems employing antenna
diversity or smart antennas. In fact, any modulation or diversity
schemes that rely on accurate frequency and/or time resolution can
benefit from this invention, which include, but not limited to,
previously mentioned OFDM systems, discrete multiple tone (DMT)
systems, multiple antenna systems, narrow-band FDMA systems, or
TDMA systems. Moreover, although the invention is particularly
useful and generally described herein with reference to multi-user
systems with a base station and multiple remote units, it should be
apparent that the principles of the invention can be extended to
two or any number of transceivers in mutual communication.
[0034] A first preferred embodiment of the invention provides for
digital correction of carrier frequency offsets. In this
embodiment, a receiver in a remote unit 100 employs a
frequency-lock loop 202, as illustrated in FIG. 2, to detect the
carrier frequency offset between the base station and the remote
unit. More particularly, the received signal from the base station,
after down-conversion mixing to the baseband and A/D conversion, is
digitally shifted in frequency to DC by multiplying the received
signal with a complex sinusoidal (or simple sinusoidal for real
signal only) using frequency shift block 204. The frequency of the
complex sinusoidal is the carrier frequency offset between the
base-station and the remote unit. The actual frequency used in the
multiplication is determined by carrier frequency control block
206. This is done by either feed-forward or feed-back frequency
lock loop (FLL 202) using the correlation between the received and
the detected data.
[0035] FLL 202 can be implemented by any one of many conventional
frequency lock loop methods including the publications referred to
above, and a detailed description thereof is not considered
necessary for an understanding of the present invention. Frequency
shift block 204 can be implemented by, for example, a
dedicated-hardware complex or simple multiplier or software
executing on a digital signal processor. Frequency control block
206 can be implemented by, for example, dedicated-hardware control
circuits or software executing on a digital signal processor. The
designs of both blocks are commonly known by those skilled in the
art.
[0036] When there is a difference between the carrier frequency
used by remote unit 100 in performing the down-conversion mixing
and the carrier frequency used by the base station, the baseband
signal will have an offset. This offset is detected and effectively
cancelled by FLL 202. The resulting baseband signal is then usable
for data detection by block 208, which data detection can be
performed using conventional baseband demodulation techniques such
as QPSK, for example. The offset detected by FLL 202 is supplied to
carrier frequency control 206. Further offset information can be
determined during data detection by continuous comparison of the
received signal and detected signal in block 208, which information
can be used to further refine the remote unit carrier
frequency.
[0037] It will be understood by those skilled in the art that other
remote unit receiver or transceiver components are possible in
addition to those described above and below. However, a detailed
description thereof is not necessary for an understanding of the
present invention.
[0038] When the remote unit transmits data to the base station, the
remote unit performs a frequency shift in the digital domain before
sending the data to the DAC. More particularly, in the remote unit
transmitter as illustrated in FIG. 3, data to be transmitted is
assembled in packets (in accordance with the communication protocol
used by the system) by assembler 302. From there, the assembled
data is digitally shifted in frequency by block 304 using the
carrier offset information detected by the receiver in FIG. 2. The
shifted digital data is then converted to analog by DAC 306 and
transmitted by transmitter 308.
[0039] Frequency shift block 304 performs a multiplication of the
assembled data and a complex sinusoidal (or a simple sinusoidal if
the data is real only) and can be implemented by, for example, a
dedicated-hardware multiplier or software executing on a digital
signal processor. Transmitter 308 performs filtering, up-conversion
mixing and amplification before sending the data to the antenna for
transmission.
[0040] A second preferred embodiment of the invention provides for
digital correction of sampling frequency offsets. FIG. 4
illustrates a remote unit receiver that includes a delay-lock loop
to accurately determine the correct sampling point of the received
signal. If the received signal is generated at a rate faster than
the local sampling frequency, i.e. the base station DAC clock is
faster than the remote unit ADC clock, then occasionally two
samples will be received in one ADC clock cycle. If the received
signal is generated at a rate slower than the ADC sampling
frequency, i.e. the base station DAC clock is slower than the
remote unit ADC clock, then occasionally no sample will be received
in one ADC clock cycle. In the conventional design, the correct
sampling point relative to the ADC clock, .delta..sub..tau.,
detected by the delay-lock loop, is used to interpolate the correct
sample from the ADC output.
[0041] As shown in FIG. 4, a remote unit receiver in this
embodiment of the invention includes a timing acquisition unit 402,
interpolation filter 404, data detection block 406, and delay-lock
loop 408. Timing acquisition unit 402 samples the received data
based on the timing information from delay-lock loop 408;
delay-lock loop 408 compares the sampled data and detected data and
generates the timing offset .delta..sub..tau.; interpolation filter
404 filters the incoming samples and generates the interpolated
sample at .delta..sub..tau. offset in time from the original
sample; data detection block 406 performs the final signal
detection. These components can be implemented, for example, by
either dedicated-hardware or software executing on a digital signal
processor.
[0042] An example of a remote unit transmitter that can be used to
transmit a sampling frequency-corrected signal in this embodiment
of the invention is illustrated in FIG. 5. As illustrated in FIG.
5, this embodiment of the invention uses the detected
.delta..sub..tau. (from delay-lock loop 408, for example) to
perform sampling rate conversion on the data to be transmitted by
way of a digital lowpass filter 506.
[0043] More particularly, the remote unit 100 modulates the data
(after being assembled into packets by assembler 502 in accordance
with the communication protocol) in data modulation block 504
using, for example, QPSK. 506, the rate-conversion and
interpolation block, then interpolates data at the appropriate
sampling points based on the sampling frequency of the base station
which is determined from the detected .delta..sub..tau.. The
interpolation filter 506 can be any low-pass filter with a
reasonable frequency response, but preferably a pulse-shaping
filter so that spectrum-shaping can be performed simultaneously.
The interpolated data is then converted to an analog signal using a
DAC 508, clocked at the remote unit's sampling frequency. The
analog signal will thus display the timing properly synchronized
with the sampling frequency at the base station. The analog signal
is then filtered, up-converted and amplified in the analog
front-end circuitry 510 before being sent to the antenna for
transmission.
[0044] A third preferred embodiment of the present invention
provides for analog correction of carrier frequency offsets, and is
illustrated in FIG. 6.
[0045] In this embodiment, a closed-loop system corrects the
frequency offset between the remote unit and the base station
carrier frequencies. The system of this embodiment functions as
follows. In the remote unit, a frequency-locked loop 602, which can
be the same as FLL 202 in FIG. 2, detects the frequency offset
between the two carrier frequencies and generates a signal Vc that
is proportional to the difference between the two carrier
frequencies. This signal is then used to adjust the capacitance of
capacitor Cc, which in turn changes the resonance frequency of
crystal oscillator 604 in a direction that corrects the frequency
offset. It should be noted that capacitor Cc can be placed either
in parallel or in series with the crystal depending on the
oscillator's architecture, and that other variably adjustable
passive or active devices can be used. The reference frequency
provided by oscillator 604 is supplied to frequency synthesizer 606
which produces the adjusted carrier frequency that is modulated
before transmission to the base station. Since the carrier
frequency is adjusted in transmission, the base station will
properly demodulate the data from the reverse link signal. A
description for the operation of the frequency synthesizer block
can be found in "RF Microelectronics" by Behzad Razavi, Prentice
Hall, 1998.
[0046] A fourth preferred embodiment of the present invention
provides for analog correction of sampling frequency offsets, for
example by varying the sampling clock of the analog-to-digital
converter (ADC) and/or digital-to-analog converter (DAC). More
particularly, the sampling frequency and phase offsets can be
corrected in the remote unit in the analog domain by varying the
sampling phases of the ADC of a receiver or the DAC of a
transmitter. In accordance with one aspect of the invention, phase
interpolation can be used to generate the sampling clock of an ADC
for receive, and phase interpolation can be used to generate the
sampling clock of a DAC for transmit.
[0047] Receivers of sampled data systems (such as radio) require
some form of timing recovery mechanism to align the sampling
frequency and phase of the receiver to that of the transmitter.
This operation can be done in the analog domain by adjusting the
sampling clock of the ADC as described in, for example, P. Roo, et
al., International Solid State Circuits Conference 1998, pp.
392-393 and T. Lee, et. al., International Solid State Circuits
Conference 1994, pp. 300-301.
[0048] FIG. 7 shows an analog timing recovery system in accordance
with the analog implementation described above that uses a phase
detector 702, loop filter 704 and a voltage-controlled oscillator
(VCO) 706 to generate the desired sampling clock for the ADC 708 of
the receiver. The phase detector 702, loop filter 704, and VCO 706
form a conventional phase/frequency-locked loop that keeps the
phase and frequency of the sampled input signal equal to that of
the VCO output. The phase detector 702 compares the phase/frequency
of the sampled input signal and the VCO output, loop filter 704 is
a low-pass filter and can be implemented using R-C circuits. The
output of the loop filter is a dc (or low frequency signal) that
sets the phase/frequency of the VCO output so that it is equal to
that of the sampled input signal.
[0049] An alternative implementation of this embodiment is shown in
FIG. 8 whereby the phase detector 802, loop filter 804, and VCO 806
are implemented as digital circuits. A typical way to implement a
digital VCO is to include a divider 808 that derives the sampling
clock from an external reference clock FEXT that is M times faster
than the desired sampling clock (where M can be selected from a
number of alternative integers). The precision of the timing
recovery is quantized to the nearest sampling edge of the external
reference clock FEXT.
[0050] In high speed communication systems where the sampling
frequency can be over 100 MHz, however, the design of a digital VCO
with an external reference clock that is M times faster is not
desirable. According to an aspect of the present invention, a
technique is provided to perform timing recovery using digital
circuits without the need for a reference clock that is M times
faster than the sampling frequency. The basic idea to generate the
M phases of the sampling clock by using phase interpolator, which
can be implemented using techniques such as those described in T.
Knotts, et. al, International Solid State Circuits Conference 1994,
pp. 58-59 and D. Chu and T. Knots, U.S. Pat. No. 5,166,959.
[0051] A closed loop system for digital timing recovery of sampling
frequency offsets in accordance with this aspect of the invention
is illustrated in FIGS. 9 and 10.
[0052] As shown in FIG. 9, the timing recovery circuit consists of
a digital phase detector 902 and a digital loop filter 904, which
can be the same as those described above. Digital VCO 908 in this
example of the invention, however, includes a phase interpolator
910 which generates M phases of the sampling clock using techniques
such as those described in T. Knots, et. al, International Solid
State Circuits Conference 1994, pp. 58-59 and D. Chu and T. Knots,
U.S. Pat. No. 5,166,959. The phase detector 902 compares the
digitized input signal (at Nyquist rate) with the sampling
frequency. This phase comparison can be done using a digital
multiplier or more simply with adders if the characteristics of the
incoming signal are known. The loop filter 904 typically consists
of at least one integrator (accumulator) and compensation network.
State machine 906 monitors the frequency and phase offsets of the
input sampling signal and can be implemented by using standard
digital logic such as those described in (or any other logic text
on state machines) Hill & Peterson, Introduction to Switching
Theory & Logical Design, 2nd Ed., John Wiley & Sons,
1981.
[0053] The timing recovery loop works as follows. The phase of the
incoming signal is compared to the sampling clock. The difference
in phase is accumulated by loop filter 904 and then used to produce
a control signal that selects the most appropriate phase of the
sampling clock produced by interpolator 910. Since the operation is
a closed loop system, the phase adjustment need only provide the
direction of the phase change not the absolute phase change.
[0054] The frequency offset adjustment process can be illustrated
with the following example. If the receiver clock Fs is 10% faster
than the transmitter clock and M is chosen to be 10, the timing
recovery logic should reduce the receiver clock period by 1/M, i.e.
one period after every 10 Fs periods. The resultant receiver Fs
should thus have the same average frequency as the transmitter
frequency. The error due to the discrete choice of sampling phases
can usually be tolerated as long as M is sufficiently large. The
use of phase interpolation allows an implementation that does not
require an external high frequency clock.
[0055] The phase interpolation approach described above can be
applied to the transmitter to correct for frequency offsets during
transmission in accordance with the invention. In this approach,
the average change in the phase selection algorithm in FIG. 9
during the receive operation is used for transmit. For example, if
the receiver timing recovery reduces the sampling clock period Fs
by 1/M, or one period for every 10 Fs periods, the transmitter can
use the same phase change for the transmitted data as shown in FIG.
10. The receive circuit in FIG. 9 preferably includes an additional
state machine 906 to monitor the control signal to the digital VCO
908 in order to record the frequency and amplitude of the phase
changes. This information is provided to the transmit circuit in
FIG. 10 and used by state machine 1002 to control the phase
selection of the digital VCO 1004 consisting of phase interpolator
1006 to adjust the sampling frequency for DAC 1008 so that the
transmit signal will have the same average sampling frequency as
the received signal.
[0056] It should be apparent that although the frequency correction
operations of the invention have been generally described
hereinabove as taking place in the transceiver of a remote unit,
that such operations can also take place in the transceiver of a
base station, or in any transceiver or transmitter of a first unit
in communication with another transceiver of transmitter of a
second unit where a shared reference frequency or commonly used
frequency is used by the first and second units.
[0057] Moreover, other embodiments of the invention are possible.
For example, the transmitter-corrected frequency offset scheme can
be applied to any digital communication system for better
performance or lower-cost implementations. It is especially suited
for applications that explore diversity to facilitate multiple
access. Some such examples are multi-user detection systems
employing antenna diversity or smart antennas, multi-carrier OFDM,
DMT, etc. This invention allows the transmitted signals from
multiple sources to be frequency-locked to the receiver, so that
the signals from multiple sources are synchronized in both
frequency and time.
[0058] Other embodiments of this invention can also include any
point-to-point or broadcast channels, such as those used in ADSL or
cable modem systems.
[0059] As a further alternative, the information on the frequency
offsets can be sent from the receiver, after it has been detected,
to the transmitter so that the transmitter can adjust its carrier
frequency and/or sampling frequency accordingly for next
transmission. For example, to reduce the hardware complexity or
power consumption of a first transceiver, such as a remote unit, a
second transceiver in communication therewith, e.g., a base
station, can detect the frequency offset relative to the first
transceiver, and send the offset information to the first
transceiver in the forward link The first transceiver, after
receiving the offset information, probably through a low-rate
channel or a broadcast channel, will correct the frequencies as
described in this invention. This is the scheme for closed loop
frequency control.
[0060] The offset information can also be sent from the receiver of
a first transceiver, to the transmitter of a second transceiver,
for the second transceiver to correct its frequencies during
receiving. For example, to reduce the hardware complexity of a
first transceiver such as a remote unit, the sampling frequency
offset information can be sent to a second transceiver such as a
base station. The second transceiver, after receiving the offset
information, will interpolate the received data accordingly, which
enhances the detection capabilities without requiring the first
transceiver to perform sampling rate conversion.
[0061] Although the present invention has been described in detail
with reference to the preferred embodiments thereof, those skilled
in the art will appreciate that various substitutions and
modifications can be made to the examples described herein while
remaining within the spirit and scope of the invention as defined
in the appended claims.
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