Note: Descriptions are shown in the official language in which they were submitted.
W O 93/00777 1 2 ~ ~ 9 ~ ~ n PCT/SE92/00430
Method and apparatus for estimating the Doppler frequ~ncy
of a mobile station.
Field of the Invention
The present invention relates generally to cellular mobile radio
systems having channels for transmitting information between base
stations and mobile stations. More precisely, the invention
relates to a method and apparatus for estimating the doppler
frequency of a mobile station.
Backaroun~ of the Invention
In cellular mobile radio systems, it is fundamental that a mobile
station with an established connection on a radio channel shall
be able to maintain the established connection when moving from
one cell to another cell wherein the cells are serviced by
different or the same base station. It is also highly desirable
that the mobile station with an established connection on a radio
channel shall be able to maintain the established connection on
the radio channel shall be able to maintain the established
connection when moving within the same cell and when the radio
channel which is used is subject to increased interference. A
process by which a mobile station can maintain an established
connection when moving in a cellular radio system is generally
called a handoff.
As the capacity demands on a cellular mobile radio system
increase, it becomes increasingly important to be able to
determine the speed of a mobile station when processing data
received from the mobile station or when handling request for
handoff. In a mobile cellular system which sets up a queue for
mobile stations waiting to make a handoff to a free channel,
problems arise when the speed of the mobile station is not taken
into account in determining which mobile station receives the
first available free channel. For instance, two mobile stations
may be waiting in a handoff queue for a free channel to become
available. The first mobile station may be traveling very fast
and the second mobile station may be moving very slow. If the
W093/~777 2 PCT/SE92/~30
second mobile station has requested a handoff before the first
mobile station has requested a handoff, the second mobile station
will be given the first available free channel even though the
signal strength between the base station and the first mobile
station is rapidly decreasing. As a result, the first mobile
station may lose its signal entirely breaking the call before it
ever receives an available channel. This problem will only
increase as the size of the individual cells decrease and the
capacity of the sys_em increases.
In a mobile telephone system, the variations in a received
signals phase and strength is due to the movement of the mobile
station. Short term variations depend upon multipath propagation
and is called short term fading, multipath fading or just fading.
The maximum frequency of the fading is called doppler frequency
and is proportional to the velocity of the mobile station.
Another variation in a received signals phase and strength is
long term variations due to the change in the propagation lose
between the mobile station and the base station. The rate of the
variation also depends upon the speed of the mobile station,
e.g., the doppler frequency.
In cellular mobile radio systems, detecting algorithms are used
to detect data in a signal received from a mobile station.
However, some detecting algorithms work better for slow moving
mobiles while other detecting algorithms work better for fast
moving mobile stations. In most prior art systems, the systems
uses detecting algorithms that usually optimize a received signal
from a fast moving mobile station. As a result, a non-optimum
solution results for slow moving mobile stations.
8ummarY of t~e Invention
The present invention overcomes the shortcomings of the prior art
by taking into account the speed of a mobile station during a
call handoff and when detecting data in a received signal. In
the present invention, the speed of a mobile station is deter-
mined when the mobile station requests a handoff. As a result,
W093/~777 3 ~ ~c)~ n ~ ~ PcT/sEs2~0~30
a handoff queue can be prioritized according to the speed of the
mobile station. For example, a fast moving mobile station will
be given a higher priority than a slow moving mobile station. As
a result, the faster moving mobile stations, which are losing
their signals faster than the slower moving mobile stations,
receive the first available free channels. As a result, fewer
mobile stations will lose their calls.
In addition, the speed of a mobile station can be used when
deciding which detecting algorithm should be used for detecting
data in a received signal. As a result, the cellular mobile
radio system can select detecting algorithms which give the best
results for each individual mobile station.
In one embodiment of the present invention, an estimate of the
doppler frequency is determined by first determining an estimate
of the channel between the transmitter and the receiver. The
channel is considered to be a linear transfer function with an
impulse response which creates all the variations in phase and
signal strength between the transmitter and the receiver.
Several channel estimates are made at different times and the
difference between the channel estimates is used to determine the
approximate doppler frequency of the mobile station.
Brief Descrlpt~on of the Drawina
For a detailed description of the preferred embodiments of the
present invention reference will now be made to the accompanying
drawings wherein;
Figure 1 illustrates a portion of a cellular mobile radio system
having cells, a mobile switching center, base stations and mobile
stations.
Figure 2 is a block diagram of the relationship between the
transmitter and the receiver.
Figure 3 is a block diagram of the circuit arrangement used in
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the method according to the present invention.
Figure 4 is a flow chart illustrating the calculation of the
doppler frequencyO
Deta~lea Description
Figure 1 illustrates 10 cells Cl-C10, in a cellular mobile radio
system. Normally, a cellular mobile radio system according to
the present invention would be implemented with more than 10
cells. However, for the purposes of simplicity, the present
invention can be explained using the simplified representation
illustrated in Figure 1.
For each cell Cl-C10 there is a base station Bl-B10 with the same
reference number as the corresponding cell. Figure 1 illustrates
the base stations as situated in the vicinity of the cell center
and having omni directional antennas. The cells Cl-C10 are,
therefore, schematically represented as hexagons. The base
stations of adjacent cells may, however, be co-located in the
vicinity of the cell borders and have directional antennas as is
well known to those skilled in the art. Figure 1 also
illustrates 9 mobile stations Ml-M9 movable within a cell and
from one cell to another. In a typical cellular radio system
there would normally be 9 cellular mobile stations. In fact,
there are typically many times the number of mobile stations as
there are base stations. However, for the purposes of explaining
the invention, the reduced number of mobile stations is suf-
ficient.
Also illustrated in Figure 1 is a mobile switching center MSC.The mobile switching center MSC illustrated in Figure 1 is
connected to all 10 base stations Bl-B10 by cables. The mobile
switching center MSC is also connected by cables to a fixed
public switching telephone network. All cables from the mobile
switching center MSC to the base stations Bl-B10 and cables to
the fixed network are not illustrated.
W093/~777 2 ~ PCT/SE92tO~30
In addition, to the mobile switching center MSC illustrated,
- there may be another mobile switching center connected by cables
to base stations other than those illustrated in Figure 1.
Instead of cables, other means, for example, fixed radio links
may be used for connecting base stations B1-B10 to the mobile
switching center MSC. The mobile switching center MSC, the base
station Bl-B10 and the mobile stations Ml-M9 are all computer
controlled. Figure 2 illustrates an overview picture of the
link between a transmitter 10 and a receiver 14. In one
embodiment of the present invention, the transmitter 10 is in the
mobile station and the receiver 14 is located at the base
station. Transmitter 10 sends a signal to the receiver 14
through a channel 12. The channel is considered to be a linear
transfer function with an impulse response h(t) which creates all
the variations and phase and signal strength between the
transmitter and the receiver. The transmitted signal s(t) is
affected by the channel impulse response h(t) to form the
received signal r(t) which is fed into a channel estimator 16
which calculates a discrete approximation to h(t) called h(n).
The discrete approximation h(n) is then fed into the fading
frequency estimator 18 which produces an estimate of the doppler
frequency for the mobile station.
Figure 3 provides a detailed description of the components
contained in the fading frequency estimator 18. The fading
frequency estimator circuit contains a delay circuit D for
delaying a channel estimate for D samples. An arrangement 24 is
used for forming the difference between two channel estimates
wherein one estimate has been delayed for D samples. The result
of the arrangement 24 is a signal V~(n) which is equal to the
difference between two channel estimates. The circuit also
contains low pass filters 26, 30 and 34 which reduce the amount
of noise and modulation rests in a signal. The circuit also
contains squaring circuits 28 and 32 which square the V~(n)
signal and the h~(n) signal. In the alternative, magnitude
calculators can be substituted for the squaring circuits 28 and
32 to calculate the amplitude of the signals. Finally, the
circuit contains a comparing component for dividing two signals.
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The operation of the fading frequency estimator shown in Figure
3 will be described in more detail with reference to Figure 4.
When the receiver 14 receives a signal from the transmitter 10,
the channel estimator 16 calculates a channel estimate of the
impulse response h(t) for a time tl. The channel estimator then
calculates a second channel estimate component at a time t2 which
occurs D samples later in Step 52. The arrangement 24 then
calculates the difference between the two channel estimate
components h~(n) and h~(n-D) in Step 52. The result of the
difference between the two signals is the signal V~(n). The
signal V~(n) is then filtered by a low pass filter 26 to reduce
the amount of noise and modulation in the signal. This is
essential to getting a useful doppler frequency estimate in a
noise environment. The V~(n) signal is then squared in Step 58.
The square value of the signal Vj(n) is then passed through a low
pass filter 30 in Step 60. In the meantime, the h~(n) signal is
squared in Step 62 and passed through a low pass filter 34 in
Step 64. Finally, the square value of the signal V~(n) is
divided by the square value of the hj(n) signal to normalize the
signal in Step 56. The result is proportional to the squared
value of the doppler frequency for the mobile station. As a
result, the speed of the mobile station can be estimated by
determining the approximate doppler frequency for the mobile
station.
The present invention, as described above, determines an
approximation of the doppler frequency of a mobile station. As
a result, the cellular mobile radio system can use the doppler
frequency or the speed of the mobile station to prioritize a
handoff queue or to select an appropriate detecting algorithm.
While the invention has been described in its preferred embodi-
ments, it is to be understood that the words that have been used
to words of description rather than of limitation and the changes
within purview of the appended claims may be made without the
party from the true scope and spirit of the invention in its
broader aspects.
