Title: Fine-stage automatic frequency compensation in post-detection short-range wireless applications
Abstract: A data-pattern feedback mechanism is introduced into the peak detection process of an automatic frequency compensation system in a Gaussian Frequency Shift Keying (GFSK) modulated system, providing fast and accurate fine-stage automatic frequency compensation (AFC). Maximum positive and negative peak registers are updated with new values as necessary based on detection during a sequence of identical binary bit values (e.g., during a "00" for detection of maximum negative peak frequency, or during a "11" for detection of maximum positive peak frequency), in a particular data frame. As soon as an initial value is determined for both the maximum positive and negative peak frequencies (e.g., after the first occurrence of a "11" and a "00", in any order), fine-stage automatic frequency compensation can be initiated. Subsequent adjustments to the VCO of the local oscillator will further refine the frequency offset towards the ideal of zero. Quick determination of the maximum positive and negative peak frequencies is made based on data pattern feedback in accordance with the principles of the present invention, allowing for a fast and accurate fine-stage automatic frequency compensation adjustment of a local oscillation clock signal.
Patent Number: 6,934,524 Issued on 08/23/2005 to Hansen, et al.
| Inventors:
|
Hansen; Eric John (Lynchburg, VA);
Luo; Wenzhe (Allentown, PA);
Ma; Zhigang (Allentown, PA);
McDowell; Richard L. (Chalfont, PA)
|
| Assignee:
|
Agere Systems Inc. (Allentown, PA)
|
| Appl. No.:
|
131214 |
| Filed:
|
April 25, 2002 |
| Current U.S. Class: |
455/318; 455/317; 455/258; 455/136; 375/344 |
| Intern'l Class: |
H04B 001/26 |
| Field of Search: |
455/318,258,255,317,136,164.1,164.2,192.2,256
375/344,343,136,147
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Urban; Edward F.
Assistant Examiner: Le; Lana
Claims
1. An offset frequency determination system, comprising:
a peak detection module to detect a maximum positive frequency peak and a maximum
negative frequency peak;
a data pattern detection module to detect sequential occurrences of a same binary
value of data, and upon said detection to direct sampling of an associated maximum
positive frequency peak and an associated maximum negative frequency peak.
2. The offset frequency determination system according to claim 1, wherein:
said maximum positive frequency peak is associated with a sequential occurrence
of binary "1" data.
3. The offset frequency determination system according to claim 2, wherein:
said sequential occurrence of binary "1" data is a pattern "11".
4. The offset frequency determination system according to claim 1, wherein:
said maximum negative frequency peak is associated with a sequential occurrence
of binary "0" data.
5. The offset frequency determination system according to claim 4, wherein:
said sequential occurrence of binary "0" data is a pattern "00".
6. The offset frequency determination system according to claim 1, further comprising:
a local oscillator including a voltage controlled oscillator adjusted based on
an offset frequency determined by said offset frequency determination system.
7. The offset frequency determination system according to claim 6, further comprising:
a demodulator to demodulate a received RF signal in accordance with a receive
data clock based on an output of said local oscillator.
8. The offset frequency determination system according to claim 7, wherein:
said demodulator is a Gaussian Frequency Shift Keying demodulator.
9. The offset frequency determination system according to claim 1, wherein:
said offset frequency determination system is installed in a piconet network
device.
10. The offset frequency determination system according to claim 9, wherein:
said piconet network device is a BLUETOOTH conforming device.
11. A method of automatically compensating an offset frequency between a received
RF signal and a local oscillation source, comprising:
demodulating said received RF signal into a data signal;
detecting in said data signal a sequential occurrence of a first binary state
of bits and a sequential occurrence of a second binary state of bits;
sampling a maximum positive frequency peak in response to a detection of said
sequential occurrence of said second binary state of bits;
sampling a maximum negative frequency peak in response to a detection of said
sequential occurrence of said first binary state of bits;
calculating a frequency offset based on said sampled maximum positive frequency
peak and said sampled maximum negative frequency peak; and
adjusting an output frequency from said local oscillation source based on said
calculated frequency offset.
12. The method of automatically compensating an offset frequency between a received
RF signal and a local oscillation source according to claim 11, wherein:
said first binary state is "1"; and
said second binary state is "0".
13. The method of automatically compensating an offset frequency between a received
RF signal and a local oscillation source according to claim 11, wherein:
said received RF signal is Gaussian Frequency Shift Keying (GFSK) modulated.
14. The method of automatically compensating an offset frequency between a received
RF signal and a local oscillation source according to claim 11, wherein:
said local oscillation source includes a voltage controlled oscillator.
15. Apparatus for automatically compensating an offset frequency between a received
RF signal and a local oscillation source, comprising:
means for demodulating said received RF signal into a data signal;
means for detecting in said data signal a sequential occurrence of a first binary
state of bits in said data signal and a sequential occurrence of a second binary
state of bits in said data signal;
means for sampling a maximum positive frequency peak in response to a detection
of said sequential occurrence of said second binary state of bits;
means for sampling a maximum negative frequency peak in response to a detection
of said sequential occurrence of said first binary state of bits;
means for calculating a frequency offset based on said sampled maximum positive
frequency peak and said sampled maximum negative frequency peak; and
means for adjusting an output frequency from said local oscillation source based
on said calculated frequency offset.
16. Apparatus for automatically compensating an offset frequency between a received
RF signal and a local oscillation source according to claim 15, wherein:
said first binary state is "1"; and
said second binary state is "0".
17. Apparatus for automatically compensating an offset frequency between a received
RF signal and a local oscillation source according to claim 15, wherein:
said received RF signal is Gaussian Frequency Shift Keying (GFSK) modulated.
18. Apparatus for automatically compensating an offset frequency between a received
RF signal and a local oscillation source according to claim 15, wherein:
said local oscillation source includes a voltage controlled oscillator.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piconet wireless networks. More particularly,
it relates to frequency compensation in radio frequency (RF) devices.
2. Background of Related Art
Piconets, or small wireless networks, are being formed by more and more
devices in many homes and offices. In particular, a popular piconet standard is
commonly referred to as a BLUETOOTH™ piconet. Piconet technology in general,
and BLUETOOTH technology in particular, provides peer-to-peer communications over
short distances. The BLUETOOTH™ specification Version 1.1 is available at
www.bluetooth.com, and is explicitly incorporated herein by reference.
The wireless frequency of piconets may be 2.4 GHz as per BLUETOOTH standards,
and/or typically have a 20 to 100 foot range. The piconet RF transmitter may operate
in common frequencies which do not necessarily require a license from the regulating
government authorities, e.g., the Federal Communications Commission (FCC) in the
United States. Alternatively, the wireless communication can be accomplished with
infrared (IR) transmitters and receivers, but this is less preferable because of
the directional and visual problems often associated with IR systems.
A plurality of piconet networks may be interconnected through a scatternet connection,
in accordance with BLUETOOTH protocols. BLUETOOTH network technology may be utilized
to implement a wireless piconet network connection (including scatternet). The
BLUETOOTH standard for wireless piconet networks is well known, and is available
from many sources, e.g., from the web site www.bluetooth.com.
According to the BLUETOOTH specification, BLUETOOTH systems typically operate
in a range of 2400 to 2483.5 MHz, with multiple RF channels. For instance, in the
US, 79 RF channels are defined as f=2402+k MHz, k=0, . . . , 78. This corresponds
to 1 MHz channel spacing, with a lower guard band (e.g., 2 MHz) and an upper guard
band (e.g., 3.5 MHz).
The BLUETOOTH specification requires transmission using Gaussian Frequency Shift
Keying (GFSK), with a binary one being represented by a positive frequency deviation,
and a binary zero being represented by a negative frequency deviation. FIG. 4 shows
the function of a conventional peak detector in determining a maximum positive
peak offset frequency and a maximum negative peak offset frequency in a GFSK system.
According to the BLUETOOTH piconet standard, the minimum deviation shall never
be smaller than 115 KHz. Also, the transmitted initial center frequency accuracy
must be +/-;75 KHz from the desired center frequency.
All receiving devices have a local clock on which a baseband receive clock signal
in an RF section is based. To receive a radio frequency (RF) signal from another
piconet device, the receiving device must lock onto the transmitted frequency.
It is important to note that in the real world, clock signals jitter and vary
somewhat within desired tolerable limits. Other than the frequency requirements,
the BLUETOOTH standard specifies that the clock jitter (rms value) should not exceed
2 nS and the settling time should be within 250 uS. A significant source of clock
variation is the variance between external crystal oscillators installed in any
particular BLUETOOTH device. Temperature also causes variations in clock signals.
In the RF transceiver of a BLUETOOTH conforming device, the alignment of a local
oscillation (LO) with a received RF signal is important to guarantee proper and
correct receipt of the underlying data signal. Thus, to eliminate any frequency
offset of the received RF signal with respect to the local oscillation signal,
automatic frequency compensation is employed.
Automatic frequency compensation (AFC) estimates frequency offset, and
appropriately adjusts the local oscillation signal, typically via a voltage controlled
oscillator (VCO). In BLUETOOTH applications, because of the tight schedule of receive
timing, the AFC is divided into two stages: (1) course-AFC; and (2) fine-AFC.
Course-AFC is performed during the short period when the data frame begins.
During course-AFC, the receiving device roughly adjusts its local oscillation so
that the frequency offset becomes smaller, e.g., allowing for the correct recognition
of an appropriate access code. Thereafter, fine-AFC is performed until the completion
of the data frame. During fine-AFC, the frequency offset is adjusted to a minimum
value such that the frequency offset does not affect the bit error rate (BER) of
the received signal.
Conventional automatic frequency compensation uses peak detection to
determine the frequency offset between the local oscillation and the received RF
signal, and makes a corresponding adjustment to local oscillation based thereon.
However, this conventional method for detecting frequency offset is prone to errors,
largely because the peak detection period is defined without the knowledge of the
particular data pattern being received. Thus, frequency deviations intended to
indicate a binary "1" or "0" may be misinterpreted to be a frequency offset of
the center frequency.
For instance, if a long data string of
0's or a long string of l's is
received, the opposite peak can be correctly detected, causing the frequency offset
to be erroneously calculated, resulting in incorrect adjustment of local oscillation.
Moreover, intersymbol interference may cause the peaks to appear differently depending
on the particular data patterns. Thus, if the opposite peaks are determined based
on a data signal containing various interference effects, the detected offset will
likely be inaccurate.
There is a need for an accurate apparatus and technique for providing fine
automatic frequency compensation, particularly in time-sensitive applications and/or
in the presence of intersymbol interference.
SUMMARY OF THE INVENTION
In accordance with the principles of the present invention, an offset frequency
determination system comprises a peak detection module to detect a maximum positive
frequency peak and a maximum negative frequency peak. A data pattern detection
module detects sequential occurrences of a same binary value of data, and upon
the detection directs sampling of an associated maximum positive frequency peak
and an associated maximum negative frequency peak.
A method of automatically compensating an offset frequency between a received
RF
signal and a local oscillation source in accordance with another aspect of the
present invention comprises demodulating the received RF signal into a data signal,
and detecting in the data signal a sequential occurrence of a first binary state
of bits and a sequential occurrence of a second binary state of bits. A maximum
positive frequency peak is sampled in response to a detection of the sequential
occurrence of the second binary state of bits, and a maximum negative frequency
peak is sampled in response to a detection of the sequential occurrence of the
first binary state of bits. A frequency offset is calculated based on the sampled
maximum positive frequency peak and the sampled maximum negative frequency peak.
An output frequency from the local oscillation source is adjusted based on the
calculated frequency offset.
BRIEF DESCRIPTION OF THE DRAWINGS
Features and advantages of the present invention will become apparent to
those skilled in the art from the following description with reference to the drawings,
in which:
FIG. 1 shows a block diagram of an exemplary fine-stage automatic frequency
compensation (AFC) system including data pattern feedback in peak detection, in
accordance with the principles of the present invention.
FIG. 2 shows a block diagram of an exemplary data pattern feedback module shown
in FIG. 1.
FIG. 3 shows an exemplary state machine timing of fine-stage automatic frequency
compensation, in accordance with the principles of the present invention.
FIG. 4 shows the function of a conventional peak detector in determining a maximum
positive peak offset frequency and a maximum negative peak offset frequency.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The present introduces a data-pattern feedback mechanism in the peak detection
process of automatic frequency compensation.
FIG. 1 shows a block diagram of an exemplary fine-stage automatic frequency
compensation (AFC) system including data pattern feedback in peak detection, in
accordance with the principles of the present invention.
In particular, FIG. 1 shows an automatic frequency compensation system
101
including a peak detection module
106 and a data pattern feedback module
100. A processor may be used to comprise and/or control the various digital
components shown in FIG.
1. The processor may be, e.g., a suitable digital
signal processor (DSP), microprocessor, or microcontroller.
The automatic frequency compensation system
101 comprises an otherwise
conventional demodulator
108, local oscillator
110, and peak detection
module
106. The AFC system
101 also includes suitably sized positive
peak register
102 and negative peak register
104.
The peak detection module
106 receives an RF signal from a transmitting
device, and determines a maximum positive peak value of the signal's frequency,
and a maximum negative peak value of the same signal. The mid-point of the difference
between the maximum positive peak value and the maximum negative peak value represents
the detected center frequency of the incoming RF signal, allowing adjustment of
the local oscillator or reference frequency
110 to allow a match to the
center frequency of the incoming RF signal.
The positive peak register
102 maintains a maximum positive peak value
during a particular sampling period (e.g., during the reception of a data frame),
while the negative peak register
104 maintains a maximum negative peak value
during the same sampling period. Both the maximum positive peak register
102
and the maximum negative peak register
104 are reset corresponding to the
beginning of each received data frame.
The demodulator
108 provides demodulation (e.g., GFSK demodulation) of
an input RF into an output data stream, with an input data clock signal based on
a reference clock provided by an adjustable local oscillator (e.g., a voltage controlled
oscillator (VCO)), in an otherwise conventional manner as is known in the art.
Importantly, in accordance with the principles of the present invention,
a data pattern feedback module
100 monitors the received data stream (e.g.,
output from the demodulator
108) and searches for binary data associated
either with a normal positive deviation from the center frequency (e.g., a '1')
or with a normal negative deviation from the center frequency (e.g., a '0').
The present invention searches for a sequential occurrence of a particular data
pattern (e.g., a "00" or a "11") wherein the received signal will be either negatively
or positively biased from the center frequency, respectively. Using conventional
peak detection methods and apparatus, sequential strings of similar binary data
will cause errors in the calculated frequency offset between the actual center
frequency of the received RF signal and the frequency based on the local oscillator
110. Using data pattern feedback in accordance with the principles of the
present invention, the positive or negative bias of the peak detection due to strings
of similar binary data will be compensated more accurately.
During the fine-AFC stage, the frequency offset is assumed to be not too large,
e.g., less than 75 KHz (for BLUETOOTH apps). In the disclosed embodiment, the processor
initially quickly estimates the offset to a low non-harmful value (e.g., to less
than 5 KHz) sampled only during the reception of corresponding binary data, then
serves as a feedback loop to keep the offset low in case of frequency drift that
might occur during receipt of the data frame.
Frequency offset is measured as the mid-point between the maximum positive
& negative peaks detected by the peak detection module
106. After the frequency
offset has been measured, the automatic frequency compensation (AFC) system
100
adjusts the VCO in the local oscillator
110 accordingly. Preferably, the
VCO is allowed to settle for a given amount of time appropriate to the particular
frequencies used. Then, the frequency offset is measured in the fine-stage AFC,
and frequency adjustments to the VCO in the local oscillator
110 continue
until the data frame or burst ends.
Note that VCO settling does not necessarily mean phase locked loop (PLL) settling,
which takes much longer. Thus, a partial settling is desired after the VCO in the
local oscillator
110 has been adjusted by the AFC scheme. Otherwise, the
automatic frequency compensation must wait for too long before the next iteration begins.
The length of peak detection performed by the peak detection module
106
is not fixed in time, but rather is dependant on the length of the data frame.
The automatic frequency compensation system
101 uses fully extended maximum
positive and negative frequency peaks to estimate the frequency offset between
the received RF signal and the local oscillator
110. In the disclosed embodiment,
as soon as both maximum frequency peaks have been detected by the peak detection
module
106, they are stored in the maximum positive peak register
102
and the maximum negative peak register
104. Based on these maximum peak
register values
102,
104, as soon as they are detected, the frequency
offset between the local oscillator or reference clock
110 and the received
RF signal is estimated, and the VCO of the local oscillator
110 is adjusted
accordingly. Thus, continual updating of the VCO frequency may occur and is likely
during the reception of any particular data frame or burst, particularly in the
case of longer data streams.
The maximum positive peak register
102 and maximum negative peak register
104 are updated with new values as necessary during the reception of a particular
data frame. However, as soon as an initial value is determined for both the maximum
positive and negative peak frequencies (e.g., after the first occurrence of a "11"
and a "00", in any order), fine-stage automatic frequency compensation can be initiated.
Subsequent adjustments to the VCO of the local oscillator
110 will further
refine the frequency offset towards the ideal of zero. However, quick determination
of the maximum positive and negative peak frequencies based on data pattern feedback
in accordance with the principles of the present invention allows for a fast and
accurate fine-stage frequency adjustment.
FIG. 2 shows a block diagram of an exemplary data pattern feedback module shown
in FIG.
1.
In particular, as shown in FIG. 2, the data pattern feedback module
100
includes a "00" pattern detector
202 and a "11" pattern detector
204.
While the present invention relates to the detection of a sequential pattern of
two identical binary bits, larger length patterns may be utilized (e.g., "000"
and "111"; "0000" and "1111", etc.) Moreover, while it is preferred that the same
number of bits be in each binary search pattern, it is possible to utilize peak
frequency values obtained with differing length search patterns (e.g., "00" and
"1111"), in accordance with the principles of the present invention.
The amount of time that the automatic frequency compensation system
100
takes to measure the offset and the time to wait VCO settle should be ratioed appropriate
to the particular application. For instance, in the given BLUETOOTH piconet application,
the peak detection period is fixed at, e.g., 8 uS. During the 8 uS time slot, both
the maximum positive and negative frequency peaks are recorded. Then, at the end
of the peak detection period (e.g, at the end of the 8 uS time period), the maximum
positive and negative frequency peaks are averaged, with the result indicating
the current frequency offset, based on which the frequency output from the VCO
in the local oscillator
110 is adjusted.
FIG. 3 shows an exemplary state machine timing showing the overall process of
interactive, fine-stage automatic frequency compensation adjustment, in accordance
with the principles of the present invention. Curved blocks depict states, and
rectangular blocks depict actions. Thick lines show a transition between states,
whereas thinner lines depict accompanying operations from the state.
In particular, as shown in FIG. 3, a new data frame is input to the automatic
frequency compensation system
101 as shown in state
302, and any
previous peak detection results are cleared, e.g., by resetting appropriate registers,
as shown in action
304.
In state
306, the automatic frequency compensation system
101 waits
an appropriate amount of fixed time for the output of the VCO to settle.
Data pattern detection and feedback is performed as depicted generally in function
370. In action
340, the frequency offset is calculated, and the appropriate
adjustments are made to the VCO in the local oscillator
110, as depicted
in action
360.
Function
370 provides the important data pattern detection and feedback,
in accordance with the principles of the present invention. In particular, after
the VCO has settled in step
306, the next data bit (or symbol) is determined
to fit one of two patterns. For example, in the case of bit detection, either the
"detect '1'" state
310 or the "detect '0'" state will be entered. In the
exemplary case of bit-wise determination, the sign of the data bit can be checked,
e.g., with a comparator. Thus, if a binary "1" is detected, state "detect '
1'"
310 will be entered. Otherwise, the state "detect '
0'"
308
will be entered.
In "det
-1" state
310, if a binary "0" is detected, "det
-0"
state
308 is entered. Otherwise, state "det
-11"
314 is
entered. Thereafter, the output of the demodulator
108 (FIG. 1) is sampled
and held, or otherwise stored as the current maximum positive peak, as depicted
in action
320.
In "det
-0" state
308, if a binary "1" is detected, "det
-1"
state
310 is entered. Otherwise, state "det
-00"
312 is
entered. Thereafter, the output of the demodulator
108 sampled and held,
or otherwise stored as the current maximum negative peak, as depicted in action
330.
In "det
-11" state
314, if a binary "0" is detected and the
"det
-00"
state
312 has not been reached in this portion of the AFC frame, then the
"det
-0" state
308 is entered. Otherwise, the state machine proceeds
to an "end_frame" state
316.
In "det
-00" state
312, if a binary "1" is detected and the
"det
-11"
state
314 has not been reached in this portion of the AFC frame, then the
"det
-1" state
310 is entered. Otherwise, the state machine proceeds
to the "end_frame" state
316.
In the "end_frame" state
316, the frequency offset is calculated with
the
detected maximum positive and negative frequency peaks by averaging therebetween,
and the VCO of the local oscillator
110 is then adjusted accordingly and
fine-stage automatic frequency compensation of the current data frame is complete
in a quick and accurate manner.
To iteratively adjust the frequency output from the VCO of the local oscillator
110, the state machine returns to state
302 for reception of a new
data frame.
By providing data pattern feedback in a peak detection process for reception
of
a modulated data (e.g., a GFSK modulated signal), the time required to achieve
fine-stage automatic frequency compensation is reduced. Moreover, the data feedback
guarantees that both the maximum positive frequency peak and the maximum negative
frequency peak are fully extended, thereby minimizing the effects of intersymbol
interference on the calculation of frequency offset for adjustment of a local oscillation source.
While the present invention has particular applicability to piconet network
devices in general, and BLUETOOTH integrated circuits and devices in particular,
fine-stage automatic frequency compensation in accordance with the principles of
the present invention have applicability in many other types of RF integrated circuits
and devices.
While the invention has been described with reference to the exemplary preferred
embodiments thereof, those skilled in the art will be able to make various modifications
to the described embodiments of the invention without departing from the true spirit
and scope of the invention.
*
