Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-02-02
2003-02-25
Trost, William (Department: 2683)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S342000, C375S150000, C375S152000, C375S142000, C375S143000, C455S456500
Reexamination Certificate
active
06526039
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method and system for facilitating the timing of base stations in an asynchronous Code-Division Multiple Access (CDMA) mobile communications system.
2. Description of Related Art
Direct-Sequence CDMA (DS-CDMA) 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, not predetermined 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 are the Wideband CDMA (WCDMA) systems proposed in the CODIT, ETSI SMG2 Group Alpha, and ARIB technical specifications.
The main disadvantage of inter-cell synchronous systems is that the BSs have to be very accurately synchronized (down to the &mgr;s level). This high level of accuracy is typically provided through the use of highly accurate time references co-located with the BSs, such as Global Positioning System (GPS) receivers. However, because of the line-of-sight nature of satellite signal propagation, the use of such co-located references are likely not feasible for BSs located underground, in buildings or tunnels. Another related disadvantage is that the GPS system is controlled by a government agency. Consequently, the use of GPS receivers for BS network synchronization may be undesirable in some national regions. These disadvantages are the main reasons why inter-cell asynchronous systems are now being considered.
For inter-cell asynchronous systems to work properly, there are two crucial functional issues that need to be addressed: (1) Soft Handovers (SOHOs); and (2) Cell-Searches. In a state of SOHO, a mobile station (MS) is in communication with more than one BS at the same time. To facilitate the SOHOS, the MS constantly scans for other BSs in the vicinity. The MS can thereby monitor the received signal quality from the multiple BSs and determine the time delay of the BSs. For a SOHO to occur, the MS being handed over has to be able to receive the “target” BS's signal at approximately the same time as the “source” BS's signal, in order to minimize buffering requirements (i.e., a smaller time difference between BS signals requires less buffer area than larger time differences). Also, the target BS has to be able to find the MS's signal without an unreasonable expenditure of processing resources.
These SOHO issues are resolved for asynchronous systems by a “per-call” synchronization technique, which is disclosed in “A Design Study for a CDMA-Based Third-Generation Mobile Radio System,” by A. Baier et al.,
IEEE JSAC
, Vol. 12, pp. 733-743, May 1994. Using this technique, the MS involved in the SOHO calculates and reports to the network the time difference between the target BS and source BS. The network notifies the target BS via the Base Station Controller (BSC) or Radio Network Controller (RNC) about the time difference. The target BS can then adjust its receive and transmit timing for the signal intended for the MS involved, to compensate for the difference.
A similar known SOHO technique is used in which the MS reports the timing difference between the target BS's transmission and its own transmission, rather than the difference between the target BS's transmission and the source BS's transmission. However, since the MS's transmit/receive timing relationship is always fixed, the two above-described SOHO techniques are essentially equivalent. These techniques are referred to as mobile assisted handover (MAHO). In other words, the MS assists the target BS in compensating for the difference in timing between the target BS and source BS.
A cell-search generally refers to a procedure whereby an MS accomplishes chip-, slot- and frame-synchronization with a BS, and detects the BS's downlink scrambling code. This procedure is used both during power on (initial synchronization) and continuously thereafter during the idle or active modes while the MS is searching for SOHO candidate BSs. In a synchronous system, the cell-search can be performed efficiently (i.e., with a relatively low level of complexity) because the same scrambling code can be used by all BSs. As such, the MS can perform the complete search for BSs using only a single matched filter (or a similar functionality). However, this same technique cannot be readily used in an asynchronous system because of the different scrambling codes used by the different BSs. Consequently, a need has arisen for a low-complexity, rapid cell-search procedure for asynchronous CDMA systems.
A rapid, multi-step cell-search procedure for asynchronous CDMA systems has been proposed, whereby each BS transmits one unmodulated symbol. This transmitted symbol is spread by a globally-known short code, without a scrambling code, in each slot of each frame. In one such proposal, this symbol is denoted as a “Perch 1 Long Code Masked Symbol (LCMS)”. In a second proposal, this symbol is denoted as a “Primary Synchronization Channel” or Primary (SCH). With the proposed multi-step procedure, an MS can thus find the chip- and slot-timing of a BS, using a single matched filter which is matched to the Primary SCH. Subsequently, the MS still has to find the BS's frame-timing and downlink scrambling code (which spans one frame in the proposed multi-step procedure). The MS can find the BS's frame-timing by detecting a second regularly transmitted symbol, which is denoted as a “Perch 2 LCMS” or “Secondary SCH”.
This second symbol is transmitted in parallel with the first symbol, but the second symbol is spread by a second short code (again without a scrambling code). The second symbol may also have a unique repetitive modulation pattern per frame, and by detecting this pattern, the MS can determine the BS's frame-timing. The spreading code used for the second symbol indicates to the MS which group of possible scrambling codes an actually-used scrambling code belongs to. The MS can then find the scrambling code used, by correlating with the scrambling codes belonging to the indicated group, at the above-identified frame-timing (or at different possible frame-timings). However, a problem with the proposed multi-step procedure is that the level of complexity of the cell-search is still relatively high, especially in the case of a SOHO candidate search (which the MS has to perform on a regular basis).
Another problem with inter-cell asynchronous systems is that the timing difference between BSs makes it difficult to determine the position of the MSs. Mobile communications systems capable of determining the position of MSs in the system are becoming increasingly desirable. Currently, mobile positioning is generally performed by the use of external systems, such as a GPS system. Preferably, however, mobile positioning would be performed by the cellular system itself without the need for such external systems. To perform such cellular positioning, a method is needed to accurately determine the absolute or relative distances between an MS and each of several different BSs. The distances can be calculated using propagation time, time of arrival (TO), or time difference of arrival (TDOA) measurements on the signals transmitted between the MSs and each of several different BSs. Once these measurements are available, a number of algorithms exist to calculate the geographical position of the MS. For example, according to the TOA method, the distance from an MS to each of the BSs is obtained using TOA measurements. Each of these distances can be conceptualized as the radius of a circle with the respective BS in the center. In other words, the TOA measurement can be u
Cedervall Mats
Dahlman Erik Bengt Lennart
Jamal Karim
Lundqvist Patrik
Nyström Johan Anders
Jenkens & Gilchrist PC
Perez-Gutierrez Rafael
Telefonaktiebolaget LM Ericsson
Trost William
LandOfFree
Method and system for facilitating timing of base stations... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and system for facilitating timing of base stations..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and system for facilitating timing of base stations... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3157179