Methods and arrangements in a radio communications system

Details Methods and arrangements in a radio communications system Methods and arrangements in a radio communications system

1998-06-12

2002-09-10

Olms, Douglas (Department: 2661)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S324000, C370S350000, C370S509000, C370S512000, C370S516000, C375S356000, C455S013200

Reexamination Certificate

active

06449290

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method for synchronising communication of framed data via asynchronous base stations in a cellular communications system, e.g. a CDMA-system (Code Division Multiple Access). The synchronisation method is performed continuously, but in particular at connection establishment and during execution of soft handover.
The invention is also directed to an arrangement for performing the above mentioned method.
STATE OF THE ART
Today there is an increasing interest in using CDMA or spread spectrum systems in commercial applications. Some examples include digital cellular radio, land mobile radio, satellite systems, and indoor and outdoor personal communications networks referred to herein collectively as cellular systems.
CDMA allows signals to overlap in both time and frequency. Thus, CDMA signals share the same frequency spectrum. In the frequency or the time domain, the multiple access signals appear to be on top of each other.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are high. This is a result of the properties of a wide band CDMA system, such as improved interference diversity, voice activity gating, and reuse of the same spectrum in interference diversity.
In principle, in a CDMA system the informational data stream to be transmitted is superimposed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data are binary, providing a bit stream. One way to generate this signature sequence is with a PN-process (pseudo-noise) that appears random, but can be replicated by an authorised receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or −1. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique spreading code. The ratio between the signature sequence bit rate and the information bit rate is called the spreading ratio.
A plurality of coded information signals modulate a radio frequency carrier, for example by QPSK (Quadrature Phase Shift Keying), and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. If the receiver is authorised, then the composite signal is correlated with one of the unique codes, and the corresponding information signal can be isolated and decoded.
In CDMA, also referred to as DS-CDMA (direct sequence-CDMA) to distinguish it from FH-CDMA (frequency hopping-CDMA), the “information bits” referred to above can also be coded bits, where the code used is a block or convolutional code. One or more information bits can form a data symbol. Also, the signature sequence or scramble mask can be much longer than a single code sequence, in which case a sub-sequence of the signature sequence or scramble mask is added to the code sequence.
In a CDMA cellular communications system, each cell has several modulator-demodulator units or spread spectrum modems. Each modem consists of a digital spread spectrum transmit modulator, at least one digital spread spectrum data receiver and a searcher receiver. Each modem at the base station BS can be assigned to a mobile station as needed to facilitate communications with the assigned mobile station MS. In many instances many modems are available for use while other ones may be active in communicating with respective mobile stations. A soft handover scheme is employed for a CDMA cellular communications system in which a new base station modem is assigned to a mobile station while the old base station modem continues to serve the call. When the mobile station is located in the transition region between the two base stations, it communicates with both base stations. Similarly, if one base station is responsible for more than one geographical sector handover may be carried out between different sectors belonging to the same base station.
When mobile station communications are established with a new base station or a new sector, for instance, the mobile station has good communications with the new cell or sector, the old base station/modem discontinues serving the call. This soft handover is in essence a make-before-break switching function. The mobile station determines the best new base station, or sector, to which communications are to be transferred to from an old base station, or sector. Although it is preferred that the mobile station initiates the handover request and determines the new base station, handover process decisions may be made as in conventional cellular telephone systems wherein the base station determines when a handover may be appropriate and, via the system controller, request neighbouring cells, or sectors, to search for the mobile station signal. The base station receiving the strongest signal as determined by the system controller then accepts the handover.
In the CDMA cellular communications system, each base station normally transmits a pilot carrier signal in each of its sectors. This pilot signal is used by the mobile stations to obtain initial system synchronisation and to provide robust time, frequency and phase tracking of the base station transmitted signals during a so called air interface chip synchronisation phase. The RNC (Radio Network Control node) maintains its synchronisation with the PSTN (Public Switched Telephone Network).
An active set for a specific mobile station is a listing of sectors via which the mobile station communicates. Adding and/or dropping sectors from the active set is called an ASU (active set update). Thus, a regular handover from a first base station (serving a first sector) to a second base station (serving a second sector) can be defined as the active set before handover containing only the first sector and after the handover containing only the second sector. Handover from the first to the second base station may, of course, also be defined as the active set originally containing several sectors i.a. the first sector, but not the second sector and after handover the active set containing several sectors i.a. the second sector, however not the first sector. Furthermore a handover may be performed either between identical frequencies, a so called intra radio frequency handover (intra RF-HO) or between different frequencies, a so called inter radio frequency handover (inter RF-HO). The exact definition of handover is nevertheless irrelevant for the present application, since the invention only concerns active set update and in particular adding one or more sectors to the active set.
The active set. may also be different for the up- and the downlink connection for a particular mobile station. For instance, it is possible that the active set contains many different sectors of one and the same base station for the uplink and only one of these sectors for the corresponding downlink connection.
During macro diversity the active set contains sectors, which are served by more than one base station. Macro diversity must be used during a soft handover, while a hard handover implicates that the active set never contains more than one sector during the procedure.
Radio frequency synchronisation is accomplished through detection and selection of a particular chip sequence, which is associated with the strongest radio frequency carrier received by the mobile station. This allows identification of the “best serving” base station. Said chip sequence is referenced to a system time that is used, for instance, to set the air interface frame transmit time.
In a CDMA system, overlap of time-slots as in TDMA (Time Division Multiple Access) systems is not a problem since a mobile station transmits continuously, and thus does not need to synchronise to other mobile

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