Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2000-08-08
2003-06-10
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C370S503000, C370S350000, C375S356000
Reexamination Certificate
active
06577872
ABSTRACT:
BACKGROUND AND FIELD OF THE INVENTION
The present invention pertains to cellular telecommunications, and particularly to synchronization of nodes in a cellular network such as a code division multiple access (CDMA) cellular network. In particular, certain embodiments of this invention relate to a system and corresponding method for adjusting the oscillator of a slave node in order to avoid and/or reduce potential phase jumps or steps associated with directly adjusting a frame counter of that slave node.
RELATED ART AND OTHER CONSIDERATIONS
This application is related to commonly owned U.S. patent application Ser. No. 09/095,585, filed Jun. 11, 1998 (atty. ref. 2380-5), U.S. Ser. No. 09/257,233, filed Feb. 25, 1999 (atty. ref. 2380-94), and U.S. Ser. No. 09/443,208, filed Nov. 18, 1999 (atty. ref. 2380-113), the disclosures of which are all hereby incorporated herein by reference.
In mobile telecommunications, a mobile station (MS) such as mobile cellular telephone, communicates over radio channels with base station(s) (BS or BTS). Typically a plurality of base stations are connected by an upper node, such as a radio network controller (RNC), to a mobile switching center (MSC). The mobile switching center (MSC) is usually connected, e.g., via a gateway, to other telecommunication networks, such as the public switched telephone network (PSTN).
In a code division multiple access (CDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a code (such as channelization and/or scrambling codes) to distinguish it from information for other mobile stations which are utilizing the same radio frequency band(s). Thus, in CDMA, individual radio links are discriminated on the basis of codes.
In CDMA systems, on the downlink (e.g., from the base station(s) to a MS) the same baseband signal with suitable codes is typically sent from several base stations with overlapping coverage at approximately the same time to a particular MS(s). In other words, frames with equal user data are sent from different base stations at approximately the same time on the downlink to the mobile station. The MS can thus receive and use signals from several base stations simultaneously. Since the radio environment changes rapidly, an MS likely has radio channels to several base stations at the same moment, e.g., so that the MS can select or combine the best channel and, if necessary, use signals directed to the MS from various base station(s) in order to keep radio interference low and capacity high. This selection procedure in a MS among frames from different base stations enables optimization of the quality of the MS-BS connection.
As for the uplink (e.g., from the MS to BS(s)), user data sent in frames from the MS is often received in multiple BSs. Frame identities are typically attached to uplink frames that are received by base station(s), so as to enable selection by an upper node (e.g., RNC) of one of a plurality of signals received from an MS at approximately the same time at different base stations. While interference is kept low, the BS having the best quality reception of the signal of the MS may randomly change during the course of the connection. Therefore, an upper node such as a radio network controller (RNC) may select the best quality one(s) of the comparable number of frames received from the mobile station by the different base stations diversity combining.
This utilization of radio channels between multiple base stations and a MS in a CDMA scheme, as summarized above, is known as “soft handover” in that different frames may be selected as a function of reception quality, strength, or the like.
The principles of diversity combining and soft-handover require that the base stations participating in a particular connection be synchronized relative to the upper node, e.g., to the RNC. Synchronization is required since, among other things, the plural base stations participating in a connection must send the same frame information at the same time to the MS involved in the connection.
A way to identify frames in the downlink and uplink directions is to attach a sequence number to each frame. In the downlink, sequence numbers can be correlated in the base stations against a reference timing
umbering in order to align frames to intended transmission time (air interface). In the uplink, sequence numbers (related to base station reference timing
umbering) can be attached to frames in each base station before transfer to the RNC (where diversity combining/selection is done based on these numbers).
As can be seen from the above, there exists a need in the art for efficient alignment of timing in different base stations and/or RNC(s) in order to enable reliable soft handover operations.
Typically, timing systems at nodes such as RNCs and BSs have frame counters driven by respective oscillators. Unfortunately, oscillators tend to experience “drift” over time (e.g., how many seconds, or fractions of seconds, they drift over a month or year). Drift may be caused by aging, temperature, voltage instability, or the like. Drift is unitless and tends to be described in terms of “ppm” (parts per million) or “ppb” (parts per billion). For example, 20 ppb means that the frequency uncertainty is +/−20 cycles per one billion nominal cycles. Oscillator drift is one reason why timing systems in respective BS nodes need to be periodically adjusted so as to fulfill air-interface radio requirements and phase drifting requirements so as to stay in sync with a master timing unit such as at a controlling RNC node.
In some systems such as WCDMA or DS-CDMA cellular networks, an upper node (e.g., RNC) has a master system frame counter which is locked to an external reference or clock source. It is desirable for such systems to have a procedure capable of synchronizing base stations so that base stations are substantially phase stable in time (i.e., to avoid substantial phase drifting by base stations). Avoiding/reducing substantial BS phase drift reduces the likelihood of BSs drifting apart in phase during a connection with a particular MS. In other words, it is desirable to keep BSs phase stable enough to prevent one or more BSs from substantially drifting apart during a connection with a MS (when the MS is connected via several BSs). It is thus desirable to achieve a substantially common frame counter and/or a substantially phase stable frame counter in the entire cellular system to make it easier to determine offset values which are to be used in securing radio frames. In situations where a network (e.g, WCDMA network) need not have an absolute phase in the entire system but instead desires substantially phase stable nodes, only substantially phase stable frame counter(s) are desired. This achieves short delays and ensures that the same frames are sent on the downlink in macro-diversity and that the same frames are combined on the uplink in a diversity handover unit (DHT).
It is known that frame counters in base stations may need to be adjusted periodically. In a CDMA system where each user channel includes a number of chips per coded information bit (different numbers of chips may be used depending upon the spreading factor used and/or on the number of users), the chip sequence must be adjusted whenever the frame counter is adjusted. Unfortunately, as discussed more fully below, phase jumps tend to occur upon directly adjusting a frame counter which in turn can lead to dropped call(s).
Commonly owned WO 99/33207 discloses a synchronization system for a cellular telecommunications network. A master timing unit is provided at a control node and a slave timing unit at a base station. When it is determined that an adjustment is necessary, the master timing unit transmits a synchronization adjustment signal to the slave timing unit so that the “counter” at the base station is adjusted accordingly, while keeping the oscillator at the base station untouched.
Unfortunately, the direct frame counter adjustment in WO 99/33207 is problematic in t
Konradsson Per
Lundh Peter
Cong Le Thanh
Harry Andrew T
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
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