Extended range concentric cell base station

Details Extended range concentric cell base station Extended range concentric cell base station

1998-08-31

2001-04-03

Eisenzopf, Reinhard (Department: 2682)

Telecommunications

Transmitter and receiver at same station

Radiotelephone equipment detail

C455S443000, C455S444000, C455S446000, C455S448000, C370S337000

Reexamination Certificate

active

06212405

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to wireless communications systems and, in particular, to extending access ranges of wireless communications systems.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a wireless communications system
10
employing Code Division Multiple Access (CDMA) techniques based on the well-known IS-95 standard of the Telecommunication Industrial Association. The wireless communications system
10
comprises a mobile switching center (MSC)
12
and a plurality of base stations (BS)
14
-
i
connected to the MSC
12
. Each of BS
14
-
i
provides wireless communications services to mobile-telephones (MT), such as mobile-telephones
16
-
k,
within an associated geographical coverage area referred to herein as cell
18
-
i
with a radius R
i
. For illustrative purposes, cells
18
-
i
are depicted as circular in shape with base stations
14
-
i
centrally positioned. It should be understood that cells
18
-
i
may also be non-circular in shape (e.g., hexagonal) with the base stations positioned non-centrally, and that the term “radius R
i
” should be construed to define a distance between the base station and a point on the circumference of cell
18
-
i
(which will vary depending on the particular point on the circumference).
Each base station
14
-
i
includes radios and antennas for modulating and transmitting base station signals to mobile-telephones, and for receiving and demodulating mobile-telephone signals from mobile-telephones within its associated cell
18
-
i
. Each base station
14
-
i
further includes a receiver for receiving timing information using the well-known Global Positioning Satellites (hereinafter referred as a “GPS receiver”).
Signals are transmitted by base stations
14
-
i
and mobile-telephones in accordance with a timing protocol aligned with GPS time using the GPS receiver.
FIG. 2
depicts a timing schedule
20
incorporating an implementation of a timing protocol based on the IS-95 standard. The timing schedule
20
comprises a series of frames
22
-
n
, wherein each frame
22
-
n
spans a time interval t. The beginning of each frame
22
-
n
is marked by a frame boundary at time T
n
aligned to GPS time. In accordance with the timing protocol, base stations
14
-
i
are configured to begin transmitting base station signals at the frame boundaries, wherein the base station signals include zero or more information bearing signals and a pilot signal for coherent demodulation of the information bearing signals by the mobile-telephones and system access operations. By contrast, mobile-telephones
16
-
k
are configured to begin transmitting mobile-telephones signals at some multiple x of a frame time period (i.e., tx) after mobile-telephones
16
-
k
began receiving base station signals, where x is some integer greater than or equal to zero. Unlike base station signals, mobile-telephone signals include one or more information bearing signals and no pilot signal, and are encoded using a set of orthogonal codes (referred to as Walsh codes) combined with a pseudo-noise (PN) sequence (or a known code) such that the information bearing signal may be non-coherently demodulated. The PN sequence comprises random
0
and
1
digital signals, wherein the duration for a
0
or
1
to transmit is referred to herein as a PN chip.
The above described timing protocol will now be discussed in reference to
FIG. 3
, which depicts a time chart
28
illustrating a sequence of transmissions and receptions by base station
14
-
i
and mobile-telephone
16
-
k
. At time T
1
, BS
14
-
i
begins transmitting base station signal S
1
to MT
16
-
k
, which may be located anywhere in cell
18
-
i
. MT
16
-
k
begins receiving signal S
1
at time T
1
+d
BS→MT
, where d
BS→MT
is a propagation delay from BS
14
-
i
to MT
16
-
k
. Note that the term propagation delay shall be construed to include line-of-sight and non-line-of-sight propagation delays.
MT
16
-
k
will wait a time interval tx from when MT
16
-
k
began receiving signal S
1
before it begins transmitting mobile-telephone signal S
2
. Thus, MT
16
-
k
will begin transmitting signal S
2
at time T
1
+d
BS→MT
+tx (or time d
BS→MT
after some frame boundary). For example, if x=2, then MT
16
-
k
transmits signal S
2
at time T
3
+d
BS→MT
(or two frames after receiving the base station signal S
1
).
Due to a propagation delay d
MT→BS
from MT
16
-
k
to BS
14
-
i
, BS
14
-
i
will begin receiving signal S
2
at time T
1
+d
BS→MT
+tx+d
MT→BS
. For ease of discussion, it is assumed that the propagation delay d
MT→BS
from MT
16
-
k
to BS
14
-
i
is the same as the propagation delay d
BS→MT
, and both will hereinafter be referred to individually as a one way propagation delay d
ow
, i.e., d
ow
=d
MT→BS
=d
BS→MT
, or collectively as a round trip propagation delay 2d
ow
. Thus, BS
14
-
i
will begin receiving signal S
2
at time T
1
+tx+2d
ow
.
In order to demodulate the received signal S
2
, BS
14
-
i
must first detect signal S
2
. Each radio includes a correlator, which is a device that detects mobile-telephone signals. For example, the correlator detects mobile-telephone signal S
2
by multiplying an incoming signal by the PN sequence, where the PN sequence is time shifted in discrete steps over a period or time interval (referred to herein as a search window W
n
) until the resulting product (of the PN sequence and the incoming signal) exceeds a threshold indicating the detection of mobile-telephone signal S
2
. If BS
14
-
i
does not begin to receive signal S
2
within the confines of a search window W
n
, BS
14
-
i
will not be able to detect signal S
2
(using the timing protocol incorporated in FIG.
2
).
To ensure that BS
14
-
i
begins receiving signal S
2
within the confines of search windows W
n
, search windows W
n
should span time intervals that include possible arrival times for signal S
2
(traveling a straight line or line-of-sight path between the mobile-telephone and the base station) regardless of the position of mobile-telephone
16
-
k
in cell
18
-
i
. Based on the above described timing protocol, base station
14
-
i
can expect to receive signal S
2
no earlier than the frame boundary and no later than time 2d
ow-radius
after the frame boundary, where d
ow-radius
is the one way propagation delay (or 2d
ow-radius
is the round trip propagation delay) for a signal traveling a distance equal to the radius R
i
. Thus, search windows W
n
should span a duration of at least 2d
ow-radius
beginning at time T
n
and ending no earlier than time T
n
+2d
ow-radius
. In effect, the duration of search windows W
n
restricts the effective radius (or size) of cell
18
-
i
, which is also referred to herein as the access range of a base station.
The duration of search windows W
n
depends on the implementation of the correlator. Typically, correlators are implemented in the form of an Application Specific Integrated Circuit (hereinafter referred to as an “ASIC correlator”) having a predetermined number of bits (also referred to herein as a “bit limitation”) for representing a round trip delay (of a signal traveling from the base station to the mobile-telephone and back to the base station). Such bit limitation limits the duration of the search windows which, as discussed above, limits the effective size of cell
18
-
i
or access range of the base station
14
-
i
. As long as the bit limitation does not limit search windows W
n
to a duration of less than 2d
ow-radius
, base station
14
-
i
should be able to detect signal S
2
transmitted by any mobile-telephone located anywhere within its cell
18
-
i
(assuming that R
i
is the same for all points on the circumference).
Typical implementations of base stations in an IS-95 based CDMA wireless communications system include an ASIC correlator having a 12-bit limitation for representing the round trip delay. In order to have fine resolution of delay, a typical value

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