Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1999-05-21
2003-10-14
Tran, Khai (Department: 2631)
Pulse or digital communications
Receivers
Automatic frequency control
C342S105000, C367S090000
Reexamination Certificate
active
06633617
ABSTRACT:
BACKGROUND OF THE INVENTION
The Doppler effect is an apparent change in the frequency of propagating waves occurring when a transmitter and a receiver are in motion relative to each other. It causes an increase in the frequency when the transmitter and receiver approach each other and a decrease when they move apart.
The Doppler effect applies in general to all types of propagating waves. Some systems advantageously utilize its effect, while it can introduce undesirable effects in others. One area where the Doppler effect can produce undesirable effects is communications systems, particularly in mobile communication systems.
The third generation of mobile communication systems, known until recently as the Future Public Land Mobile Telecommunication Systems (FPLMTS), and recently renamed International Mobile Telecommunications 2000 (IMT-2000), uses a variety of cell structures from very small indoor picocells to large outdoor terrestrial cells and finally to global satellite coverage. An adaptive radio interface is envisaged for IMT-2000 to optimize performance in these widely differing propagation conditions. This adaptation will be controlled by software using digital signal processing technology. While a detailed description of IMT-2000 is outside of the scope of this invention disclosure, embodiments of the present invention relate to the following components of IMT-2000:
terminal mobility, i.e. the ability to move continuously over large areas while maintaining the ability to use multimedia telecommunication services, video, audio, speech. The speed of movement of multimedia terminals may vary from zero to very high.
High-bandwidth services (data, image, multimedia, etc.)
Intra and inter system handover, i.e. roaming across dissimilar networks;
Service adaptation. No longer will the various elements of the radio interface (coding, modulation, etc.) have fixed parameters, but they can be selected, negotiated and matched according to the requirements of the service and the instantaneous capability of the radio channel.
When there is any motion between the transmitter and receiver in mobile communication systems, the Doppler frequency shift is experienced between the transmitted and received frequency. This causes the frequency content of the received signal to be different from the frequency content of the transmitted signal. In analog cellular communications, this Doppler effect matters little, but it significantly affects digital phase transmission and equalizer operations Therefore, in addition to overcoming multipath distortion, an equalizer must overcome phase and amplitude distortions arising from the Doppler effect. The Doppler effect manifests itself as a random frequency modulation of the transmitted signal and can cause significant impairment of the original signal.
Much research has been done to determine the speed and direction of a moving object. One suitable approach for mobile cellular communications that can be applied at base stations is described in U.S. Pat. No. 5,548,296, by Matsuno, entitled METHOD OF AND APPARATUS FOR DETERMINING POSITION OF MOBILE OBJECT AND MOBILE RADIO COMMUNICATION SYSTEM USING THE SAME, issued on Aug. 20, 1996, assigned to Nippon Steel Corp., herein incorporated by reference in its entirety. Another approach, the result of a very recent work performed at Lucent Technologies, is applicable when it is desirable to obtain an estimate of the Doppler speed at the mobile unit.
In addition to mobile communications, there are other applications where the Doppler effect causes degradation and needs to be compensated, e.g. acoustics, radar, etc. Other work relevant in this respect is described in U.S. Pat. No. 5,642,732, assigned to Acuson Corporation of Mountain View, Calif. and in U.S. Pat. No. 5,647,366, assigned to Siemens Medical Systems, Inc., of Iselin, N.J.
Furthermore, in other applications, digital signal processing may be used to create a Doppler effect. For example, this technique is employed in three dimensional sound systems such as in U.S. Pat. No. 5,337,363, by Platt.
In prior art systems there are two basic digital signal processing approaches to compensate the Doppler effect: processing in the frequency domain and processing in the time domain.
Processing in the frequency domain, illustrated in
FIG. 1A
, requires that the signal is passed through a fast Fourier transformer, a frequency shifter, and an inverse fast Fourier transformer to produce a signal without degradation due to the Doppler effect. From a multirate digital signal processing point of view the task to compensate a Doppler effect is equivalent to changing the sampling rate by a rational factor. Suppose we have a signal x(n), defined for n=0, . . . , N−1. We are looking for another signal {circumflex over (x)} (m) of duration LN/M. The Doppler parameters determine the sampling rate change factor according to:
M
L
=
1
+
V
t
cos
θ
t
-
V
r
cos
θ
r
V
(
1
)
where, as illustrated in
FIG. 1B
, &thgr;t is the speed of the transmitter, V
r
is the speed of the receiver, &thgr;
t
is the direction of travel of the transmitter and &thgr;
r
is the direction of travel of the receiver, and V is the speed at which the signal travels. For electromagnetic waves V=c, while acoustic signals travel at a much slower speed. Both M and L should preferably be integers. Their ranges are determined in accordance with well-known principles. Since the right side of equation (1) is not guaranteed to be a ratio of two integers, in practice this method can only approximate the exact Doppler shift. The frequency shift step of
FIG. 1A
is carried out as illustrated in FIG.
2
. First, a signal with Doppler shift is received. The first processing step is interpolation by a factor of L, in which L−1 samples are inserted between every two samples and L−1 samples are added after the last sample. This step is accomplished by computing the DFT of x(n), corresponding to the step in
FIG. 2
labelled “compute the DFT of X(n)”.
Then the DFT of x
1
(m) is:
X
1
(
k
)
=
{
LX
(
k
)
,
if
0
≤
k
<
N
2
L
2
X
(
k
)
if
k
=
N
2
0
,
if
N
2
<
k
<
lN
-
N
2
L
2
X
(
k
-
(
L
-
1
)
N
)
,
if
k
=
LN
-
N
2
LX
(
k
)
-
(
L
-
1
)
N
)
,
if
LN
-
N
2
<
k
<
LN
(
2
)
Thus, the DFT of X(n) is computed by applying the above formula in the step of
FIG. 2
labelled “apply formula 2”.
The Nyquist component is weighted by L/2, to preserve the Hermitian symmetry. The next step is the decimation by a factor of M. From the DFT X
1
(k) this can be performed as:
X
2
(
k
)
=
{
X
1
(
k
)
,
if
0
≤
k
<
LN
2
M
-
1
&LeftBracketingBar;
X
1
(
k
)
&RightBracketingBar;
,
if
k
=
LN
2
M
X
1
(
k
+
N
(
M
-
1
)
/
M
)
,
if
LN
2
M
+
1
<
k
<
LN
M
-
1.
(
3
)
Thus, the decimation by a factor of M is performed by applying the above formula in the step of
FIG. 2
labelled “apply formula 3”.
Finally, the inverse DFT of X
2
(k) gives the desired signal X
2
(m), which is free of the Doppler shift. Of course, the two steps represented by equations or formulas (2) and (3) above can be combined into one. In the case when it is necessary to create a Doppler effect, the processing steps are the essentially same. This requires significant computing resources. While the frequency-domain approach is feasible for narrow-band speech, the amount of necessary computations makes it unfeasible for wide-band signals.
Time-domain Approach
A time-domain approach is described in U.S. Pat. No. 5,719,944, by Banerjea, entitled SYSTEM AND METHOD FOR CREATING A DOPPLER EFFECT, issued on Feb. 17, 1998, assigned to Lucent Technologies, Inc., of Murray Hill
3Com Corporation
Michaelson Peter L.
Michaelson & Wallace
Tran Khai
LandOfFree
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