Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Frequency or repetition rate conversion or control
Reexamination Certificate
2003-03-06
2004-09-07
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Frequency or repetition rate conversion or control
C375S130000, C375S295000, C455S323000
Reexamination Certificate
active
06788117
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to wireless personal area networks and wireless local area networks. More particularly, the present invention relates to how to generate frequency stable wavelets for transmission in an ultrawide bandwidth device.
UWB Wavelets
UWB systems often use signals that are based on trains of short duration wavelets (also called chips or pulses) formed using a single basic wavelet shape. The interval between individual pulses can be uniform or variable, and there are a number of different methods that can be used for modulating the wavelet train with data for communications.
An important point common to UWB systems is that the individual wavelets are very short in duration, typically much shorter than the interval corresponding to a single bit of information being passed, which can offer advantages in resolving multipath components. In one embodiment, a UWB signal includes a series of wavelets formed according to the following equation:
s
(
t
)
=
∑
k
=
-
∞
∞
a
k
p
(
t
-
t
k
)
(
1
)
Here s(t) is the UWB signal, p(t) is the basic pulse shape, and a
k
and t
k
are the amplitude and time offset for each individual pulse. Because of the short duration of the pulses, the spectrum of the UWB signal can be several gigahertz or more in bandwidth.
FIG. 1A
shows an exemplary UWB wavelet.
In this example the pulse is a third derivative Gaussian wavelet with a peak-to-peak time (T
p-p
) of a fraction of a fraction of a nanosecond, and a bandwidth of several gigahertz.
FIG. 1B
shows the frequency response of the wavelet shown in FIG.
1
A.
FIG. 2
is a block diagram showing an exemplary wavelet circuit for generating the third derivative Gaussian wavelet of FIG.
1
A. As shown in
FIG. 2
, the wavelet generating circuit
200
includes a Gaussian low pass filter
210
and first through third derivative circuits
220
,
230
, and
240
. The Gaussian low pass filter
210
receives an impulse signal and produces a Gaussian signal p(t) as an output. This Gaussian signal p(t) is provided to the first derivative circuit
220
, which outputs a first derivative Gaussian signal p′(t). This first derivative Gaussian signal p′(t) is then provided to the second derivative circuit
230
, which outputs a second derivative Gaussian signal p″(t). Finally, the second derivative Gaussian signal p″(t) is provided to the third derivative circuit
240
, which outputs a third derivative Gaussian signal p′″(t).
The first, second, and third derivative circuits
220
,
230
, and
240
are often implemented to provide approximate derivative signals.
FIG. 3
is a block diagram showing an exemplary derivative circuit for generating an approximate derivative of an input signal. As shown in
FIG. 3
, the derivative circuit
300
includes a delay
310
, an inverter
320
, a summer
330
, and a scaling circuit
340
.
The delay
310
receives an input signal x(t) and delays it by a delay period &pgr;. The inverter
320
then inverts the delayed signal and provides it to the summer
330
. The summer
330
receives the inverted delayed signal and the input signal, and adds them together, and the scaling circuit divides the sum by &pgr;.
The output of the derivative circuit
300
can thus be described by the following equation:
x
′
(
t
)
=
x
(
t
)
-
x
(
t
-
τ
)
τ
(
2
)
which is an acceptable approximation of the derivative of the input signal x(t).
The wavelet output from the wavelet generating circuit
200
is used to carry data for the UWB system. Information is encoded into a series of wavelets that are wirelessly transmitted from a first device to a second device as a wireless signal.
In order to properly decode the incoming signal, the second device uses a correlation circuit. This correlation circuit allows the second device to determine the timing of an incoming signal, and the data encoded in it.
FIG. 4
is a block diagram showing a portion of a wireless receiver according to a preferred embodiment of the present invention. As shown in
FIG. 4
, the receiver
400
includes a pulse forming network (PFN) and timer
410
and a correlation circuit
420
. The correlation circuit further includes a mixer
430
and a decision circuit.
The PFN and timer
410
preferably generates local copies of the wavelets that are the basis of the transmitted signal. (See FIG.
1
A). These locally generated wavelets are provided to the correlation circuit
420
and are preferably nearly identical to the wavelets transmitted by the transmitter.
The correlation circuit
420
receives a wavelet stream that has preferably been wirelessly transmitted to the receiver
400
, received at an antenna (not shown), and processed by a front end (not shown). The wavelet stream is then mixed in the mixer
430
with the locally generated wavelets to provide a correlation value.
The decision circuit receives the correlation value and uses that value to decode the information in the wavelet stream and to generate certain control signals. For example, during signal acquisition the decision circuit
440
uses the correlation value to generate control signals for the PFN and timer
410
to adjust the phase of the locally generated wavelets to match the phase of the wavelet stream. When a data signal is coming in, the decision circuit
440
uses the correlation value to decode data from the wavelet stream.
FIGS. 5A and 5B
are graphs showing the autocorrelation function of the wavelet of FIG.
1
A.
FIG. 5A
shows the autocorrelation in terms of amplitude, while
FIG. 5B
shows the autocorrelation in terms of dBr. Autocorrelation refers to when the wavelet is correlated with a duplicate of itself, as is done by the mixer
430
during signal acquisition.
The time axis of both of
FIGS. 5A and 5B
show the relative difference in wavelet starting points, delayed by the amount of time it takes for the mixer
430
to output the correlation value (a little over 400 ps in this embodiment). In the alternative, the autocorrelation graphs could be normalized to zero, setting the maximum points in the curves in
FIGS. 5A ands
5
B at zero on the x-axis. These maximum points show where the wavelets are perfectly aligned. The remainders of the autocorrelation curves show results for varying degrees of phase shift for the two copies of the wavelet that are being autocorrelated.
However, this open loop method of generating wavelets exhibits poor frequency stability. For example, there is a significant variation in the peak-to-peak time T
p-p
in wavelets depending upon temperature, component tolerances, etc. This is particularly true for the Gaussian low pass filter
210
and the delays
310
in the first, second, and third derivative circuits
220
,
230
, and
240
. For example, the use of a delay having an LC circuit can cause a T
p-p
variance of 20% by itself.
FIG. 6
is a graph showing the spectrum variance for the wavelet of
FIG. 1A
as its peak-to-peak time is varied. In particular,
FIG. 6
shows three wavelet spectrum curves: a first spectrum curve
610
in which the peak-to-peak time T
p-p
is at an ideal value, a second spectrum curve
620
in which the peak-to-peak time T
p-p
is 20% below the ideal value, and a third spectrum curve
630
in which the peak-to-peak time T
p-p
is 20% above the ideal value.
As shown in
FIG. 6
, the spectrum variance between these three curves is significant. For example, the center frequency changes from about 4.5 GHz at an ideal T
p-p
up to about 7.5 GHz at −20% from the ideal T
p-p
and down to about 3 GHz at +20% from the ideal T
p-p
. This is an unacceptable variance in many UWB applications.
It is therefore desirable to provide a way of generating wavelets that have a stable frequency over a variety of conditions.
SUMMARY OF THE INVENTION
Consistent with the title of this section, only a brief description of selected features of the present invention is now presented. A more complete description of the present invention is the subject of this enti
Freescale Semiconductor, Inc
Posz & Bethards, PLC
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