Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-08-14
2004-05-11
Nguyen, Steven H. D. (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S519000
Reexamination Certificate
active
06735222
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to the mobile communications field and, in particular, to a method and system for determining time slot offsets in a non-synchronized Time Division Multiple Access (TDMA) network.
2. Description of Related Art
TDMA mobile communications systems can be either inter-cell synchronous or inter-cell asynchronous systems. In other words, the base stations (BSs) in an inter-cell synchronous system are accurately synchronized with one another, and the BSs in an inter-cell asynchronous system are not. More specifically, asynchronous BSs do not share a common time reference, and their transmissions, therefore, have arbitrary timing relative to each other. An example of an inter-cell synchronous system is the North American IS-95 system. Examples of inter-cell asynchronous systems include the Wideband Code Division Multiple Access (WCDMA) systems proposed in the CODIT, ETSI SMG2 Group Alpha, and ARIB technical specifications and Ericsson's GSM system protocols.
A number of disadvantages exist with inter-cell synchronous systems. One prerequisite for such systems is a high level of synchronization among the BSs with the degree of synchronization generally measured in microseconds (&mgr;s). Also, it is believed that a loss in frequency reuse efficiency is realized in an inter-cell synchronous network, at a cost of valuable bandwidth.
However, an area in which the synchronous network has shown particular advantage over non-synchronous networks is in mobile positioning applications. Synchronous networks have a distinct advantage over non-synchronous networks since, by design, the synchronous networks share a reference clock. Triangulation algorithms may then be utilized to measure delays in the time of arrival of specific time slot numbers from a particular mobile station (MS) to a base station. With, in general, measurements from a MS to three BSs, the position of an MS within a telecommunications system may be accurately determined.
When BSs are operating asynchronously, however, the task of location calculation is complicated by the fact that each BS is operating on clocks independent from one another. For example, while one BS is receiving data on Time slot Number
1
(TN
1
), another BS might be simultaneously receiving on TN
3
. To make an effective location determination, the respective delay of the MS transmission to the BS reception must be ascertained along with the relative TN offset with respect to the other BSs participating in the location calculations.
A better understanding of the problems associated with positioning in asynchronous networks may be had with reference to
FIGS. 1 and 2
.
FIG. 1
illustrates a typical framing scheme as employed in a digital TDMA communications system. The numbering scheme depicted is specific to the European GSM system and is used only for illustrative purposes. Shown are contiguous frames, denoted F
1
through F
2715647
, the span of which is often referred to as a hyperframe, and one single frame of the hyperframe, specifically frame
2
(F
2
), is shown in more detail. In GSM, the transmission of such frames occurs on carrier frequencies with an approximate bandwidth of 200 kHz separating adjacent transmissions.
As illustrated by the expanded frame F
2
, each single frame contains eight individual time slots numbered TN
0
through TN
7
therein. One time slot of a TDMA frame on one carrier is referred to as a physical channel. Therefore, a mobile station transmitting on a particular carrier will occupy a specific time slot, or physical channel, on contiguous frames thereby allowing up to eight such “simultaneous” communications on the carrier frequency depicted on the respective time slots TN
0
to TN
7
.
Mobile stations are, in general, within reception range of a number of BSs with a traffic channel maintained between the mobile station and that BS exhibiting the best communication characteristics, i.e., signal to interference ratio. As is understood in the art, however, when RF characteristics decline below a specified level or when RF characteristics from another BS increase beyond a specific threshold relative to the current BS with which the mobile station is maintaining the traffic channel, a handover is initiated where a traffic channel is setup between the mobile station and the BS exhibiting the better communication characteristics and, concurrently, the traffic channel between the mobile station and BS previously in use is broken. However, in specific situations, i.e., MS positioning, a number of BSs may concurrently tune to the same transmitting MS in order to make time delay of arrival measurements between the MS and the BSs.
FIG. 2
illustrates a network containing three BSs
50
,
60
, and
70
, in a telecommunications network, generally designated by the reference numeral
90
, operating asynchronously and sharing a common node, or Base Station Controller (BSC)
80
. Each of the BSs are transmitting data in a TDMA format consistent with the aforedescribed data frames such as shown in FIG.
1
. Since the BSs are operating asynchronously, however, their frame transmission times will have no time correspondence other than that by coincidence.
For convenience of discussion, BS
50
is taken as a reference in FIG.
2
and the beginning of its current frame transmission is designated as occurring at a time of zero seconds (t
11
=0). Here, the first digit of the double subscript indicates both the number of the BS, e.g., BS
50
or the first (“1”) BS, while the second subscript refers to the reference base station from which corresponding times are measured, i.e., BS
50
, again the first (“1”) BS. As is understood in the art, the time span of a single GSM frame transmission is approximately 4.615 ms. With reference now to the frame transmission corresponding to BS
60
, which as discussed is asynchronous to the other BSs and offset from the other frame transmissions such as that of BS
50
, BS
60
(the second “2” BS) begins its current frame transmission, F
5000
, at t
21
=1.026 ms, or 1.026 ms after BS
50
began transmitting its frame F
3
, the base reference time in this example. As discussed, BS
60
completes the transmission of frame F
5000
about 4.615 ms after commencing transmission, i.e., at time t
21
=5.641 ms, at which point frame F
5001
commences. Likewise, BS
70
(the third “3” BS) begins transmission of its current frame F
11358
at time t
31
=3.969 ms after BS
50
began transmitting its frame F
3
and 2.666 ms after BS
60
began transmitting its frame F
5000
. Completion of transmission of frame F
11358
by BS
70
arrives one frame time length later, i.e., t
31
=8.307 ms, as illustrated in FIG.
2
.
Since the frame lengths and time slot lengths are of constant durations, a single time slot has a span of 0.577 ms. Lines
100
and
110
may then be constructed to gain further insight into the time slot offsets between particular BSs within the telecommunication network
90
. Line
100
, drawn from the start point of frame F
5000
ofBS
60
to intersect the corresponding time point within frame F
3
of the reference BS
50
, indicates that BS
60
began transmitting its time slot zero (TN
0
) within frame F
5000
at a point in time where BS
50
has completed transmission of its TN
0
and additionally about 78% of its TN
1
. Similarly, line
110
, drawn from the start point of frame F
11358
to intersect the corresponding time points within frame F
5000
of BS
60
and frame F
3
of the reference BS
50
, indicates that BS
70
began transmitting its TN
0
at a point in time where BS
60
is in progress of transmitting its TN
4
and, where BS
50
is in progress of transmitting its TN
6
.
The transmission offsets of the aforedescribed network are simply the result of non-synchronous operation. The BSs are free to begin transmissions when needed and without correspondence between ongoing transmission in nearby BSs. Thus, any BS frame transmission in an asynchronous network may beg
Holmring Anders T.
Kingdon Christopher H.
Duong Duc
Nguyen Steven H. D.
Telefonaktiebolaget LM Ericsson (publ)
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