Oscillators – Automatic frequency stabilization using a phase or frequency... – Afc with logic elements
Reexamination Certificate
2002-04-25
2003-11-04
Choe, Henry (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Afc with logic elements
C329S300000, C375S324000, C455S316000
Reexamination Certificate
active
06642797
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piconet wireless networks. More particularly, it relates to frequency offset compensation between piconet devices such as BLUETOOTH™ conforming wireless piconet 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 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.
The BLUETOOTH specification allows for up to +/−75 kHz of initial frequency offset for a transmitter at the start of a burst. If one assumes that both transmitter and receiver have the same tolerance, then this would imply that up to +/−150 kHz offset may exist between a transmitter of a first wireless piconet device and a receiver of another wireless piconet device at the beginning of any given packet. Furthermore, since from a master's perspective consecutive slot packets coming from different slaves have no relationship to each other in terms of frequency offset, they too could be as much as 150 kHz different. This is a fairly significant offset considering that the minimum FSK tone deviation is only 115 kHz.
BLUETOOTH devices typically require a receiver in any given BLUETOOTH device to perform a frequency offset correction during the preamble of each and every packet. However, this requirement poses a challenge because there are only five (5) bits of preamble 10101 used to train. the receiver before the sync word needs to be demodulated and recognized.
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).
To receive a radio frequency (RF) signal from another piconet device, the receiving device must lock onto the transmitted frequency. Moreover, all receiving devices have a local oscillation usually provided by a local oscillator (LO), from which all local frequencies in the received device are derived.
In an ideal world, all piconet devices would have exactly the same local oscillation, and thus all derived frequencies in all devices would be exactly identical. Unfortunately, the real world is far from this ideal. Rather, local oscillations vary, or have an offset, due to, e.g., temperature differences, device differences, local oscillator differences. Moreover, the received signal may be interfered with in transmission and may, in fact, be varied from the exact ideal RF carrier frequency.
Automatic frequency compensation (AFC) is employed in piconet devices (e.g., in BLUETOOTH device) to compensate for variances in local oscillations and align the local oscillator to the frequency of the received RF signal. Automatic frequency compensation (AFC) is particularly important in the design of piconet RF transceivers.
BLUETOOTH RF signals are modulated, using Gaussian Frequency Shift Keyed (GFSK) modulation, with the binary 1's and 0's being distinguished by the direction of deviations in the frequency from a center frequency. In BLUETOOTH devices, the maximum deviations are +150 KHz and −150 KHz, under ideal conditions.
FIG. 3
shows the conventional reception of an ideal RF signal having 1's and 0's represented by frequency deviations F
−(ideal)
, F
+(ideal)
as expected, e.g., +/−150 KHz about a center frequency F
c
.
When there is an offset between a received RF signal and a local oscillator, the offset becomes added to the deviations in the received RF signal, causing difficulties in correct demodulation thereof.
FIG. 4
shows the conventional reception of an ideal RF signal having 1's and 0's represented by frequency deviations F
−(actual)
, F
+(actual)
which are moved in frequency (i.e., offset) from the expected locations +/−150 KHz about the expected center frequency F
c
, respectively.
In particular, as can be seen in
FIG. 4
, this causes a greatly reduced signal in the expected locations +/−150 KHz about the center frequency F
c
, significantly raising the number of errors in the detection of the received signal.
To reduce the number of errors, the local oscillator of BLUETOOTH devices employ automatic frequency control to adjust the local oscillations commensurate with an expected offset between the received RF signal and the local oscillations. Thus, AFC reduces the offset between the received RF signal and the local oscillator.
As can be imagined, detection of the frequency offset is the difficult part of automatic frequency control.
FIG. 5
shows a conventional technique for providing automatic frequency control in the analog domain to adjust for a frequency offset between a received RF signal and a local oscillator.
In particular, as shown in
FIG. 5
, the conventional AFC technique utilizes a series connection of a demodulator
202
, an analog peak detector
204
, a midpoint determiner
206
, a register
208
and control logic
210
to provide a control signal to a local oscillator of the receiving device.
The demodulator
202
(e.g., a GFSK demodulator) demodulates an RF frequency signal S
f
(t) to produce an analog amplitude received signal S
a
(t).
The analog peak detector
204
detects peaks in the amplitude signal S
a
(t) using a peak detection method. In particular, the analog peak detector
204
determines the positive and negative peaks in the amplitude signal S
a
(t) as the maximum positive deviation (V
+
) and the maximum negative deviation (V
−
), respectively.
The mid point detector
206
determines the mid point V
m
between the maximum positive deviation V
+
and the maximum negative deviation V
−
using the simple algorithm V
m
=(V
+
+V
−
)/2. The mid point Vm is presumed to be, ideally, the center frequency F
c
of the received signal. Any difference between the mid point V
m
and the expected center frequency F
c
of the received RF signal is presumed to be equal to the frequency offset.
To this end, a register
208
provides data to control logic
210
, which compares the mid point V
m
to predetermined threshold values to estimate the frequency offset F
o
, which is used to make a corresponding adjustment to a local oscillator to align the frequency of the local oscillator with that of the received RF signal.
The assumption in this conventional technique, however, is that:
S
a
(
t
)=
K
0
*F[
S
f
(
t
)]
wherein F[S
f
(t)] denotes the instant frequency of the received RF signal S
f
(t), and K
o
should be a constant. This proves to be good for detection, e.g., 1000 mV/150 KHz.
However, in reality, for mdst demodulator implementations, K
o
is not purely a constant, but rather depends on |S
f
(t)&
Luo Wenzhe
Ma Zhigang
Agere Systems Inc.
Bollman William H.
Choe Henry
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