1. Field of the Invention
The present invention relates to tracking users in communication systems or networks supporting Broadcast-Multicast services (BCMCS).
2. Related Art
Current second generation and third generation wireless systems are primarily designed to support unicast voice and data services. The support of these unicast services to the end user has been achieved through various advances in wireless and networking technologies. The current emphasis in international standardization bodies such as 3GPP and 3GPP2 is on the design of protocols and procedures that allow the support of Broadcast-Multicast Services (BCMCS) over evolving networks. BCMCS is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to a large number of recipients. Examples of these services include voice dispatch or Press-To-Talk (PTT) type services, broadcast/multicast streaming, etc.
There has been recent industry interest in the Public Safety Wireless Network (PSWN) context, where support of BCMCS may be especially important in terms of radio resource management. Radio resource management procedures may be needed to achieve low delay in establishment and delivery of BCMCS content, to provide scalability in terms of the number of multicast groups and/or number of users per multicast group, and to provide advanced service capabilities (e.g., ability to monitor/extract content from multiple BCMCS streams). Further, radio resource management procedures may be needed to achieve high spectral efficiency, to provide security (authentication, encryption and identity protection), and to improve mobile station battery life. Thus, radio resource management procedures that improve spectral efficiency and/or provide service flexibility are of interest to network operators.
In the unicast context, tracking of mobile stations within a geographic area is used primarily for the purposes of efficiently paging a mobile station. Mobile station tracking may perhaps become perhaps more important when assigning radio resources in the multicast context. In a cellular network, shared multicast radio channels should ideally be activated in a zone of cells or sectors for reliable reception. Depending on the level, or granularity, of tracking (i.e., accuracy of information at the network regarding the whereabouts of a BCMCS/PTT subscriber), radio resources may not be assigned in the proper sectors. As a result, there could be a significant loss in spectral efficiency for the network operator, and/or loss in quality of service perceived by a BCMCS/PTT subscriber.
Mobile stations that have an active uplink (e.g., those that send traffic or control signaling at regular intervals) can be tracked at the granularity of a sector. However, this is not expected to be the predominant operating mode for BCMCS/PTT where mobile stations are mostly idle or actively monitoring BCMCS content on the downlink. In such cases, it may be impractical for each user to maintain an active uplink, due to potentially large numbers of BCMCS subscribers. For such situations where maintenance of an active uplink is not feasible, tracking is typically accomplished through timer-based or event driven registration procedures. In addition to assisting procedures such as tracking, mobile station registration procedures also provide, to the network, information regarding subscription to specific services, radio priorities, etc. Timer-based procedures require mobiles to register with the network at given intervals (e.g. periodic intervals). Event driven triggers require mobiles to send a registration message to the network when an important event occurs (e.g. mobile decodes a change in location area).
Depending on the options employed for registration, either fine tracking of mobiles down to the granularity of a sector, or coarse tracking of mobiles over a large number of sectors, is possible. If coarse tracking is employed, physical broadcast/multicast (BCMC) radio channels may need to be established in a large number of sectors that are not needed for reliable BCMCS content delivery, resulting in a waste of radio resources. Since it may not be possible to derive the benefits of power control in a network supporting BCMCS the power fractions needed to support a BCMC radio channel can be quite high. This limits the number of BCMC radio channels and/or unicast channels that can be simultaneously supported within a sector. Coarse tracking may therefore be unacceptable and may waste potentially scarce radio resources, as the network may have to transmit a BCMC stream within a large area, including sectors where mobile stations may not be present. On the other hand, tracking at the level of a sector (fine tracking) can result in excessive uplink overhead arising from frequent and sometimes unnecessary registrations by the mobile stations, thus overloading the uplink.
Mechanisms currently proposed for BCMCS registration include timer-based registration; registration on occurrence of certain events (e.g., power on, starting or stopping to monitor a multicast group); Registration Required Indication enabled in broadcast system parameter overhead on a new sector; and use of paging zone updates. These registration mechanisms are not well suited to tracking BCMCS subscribers that have any significant mobility (movement) across the network.
With timer based registrations, short timer values (the timer value may depend on the user mobility and desired accuracy) are needed for accurate tracking, otherwise the time elapsed from the previous registration may be quite long, potentially resulting in considerable ambiguity regarding mobile station location at the start of a BCMCS/PTT call. Long timers require radio resources to be assigned over a larger area to account for tracking uncertainty, thus resulting in spectral inefficiency. Although short timer values allow finer tracking, they result in large uplink overhead and reduce mobile battery life. They also result in repetitive unnecessary registrations from static (stationary) users.
Events such as “power on” and “start/stop monitoring” tend to be quite infrequent and dependent on end user behavior. As a result, these registration ‘triggers’ are not likely to be sufficient for the purpose of tracking. Relying on alternative fine tracking mechanisms such as the use of a “Registration Required” indicator bit within the broadcast system parameter overhead (i.e., group polling) of each sector may also give rise to a large number of spurious registrations from relatively static mobiles. Since the total number of BCMCS/PTT groups may be quite large, introducing such group polling mechanisms for all possible groups is inefficient.
Paging zone updates have been proposed to reduce downlink paging load for packet data services in cdma2000®, Revision D. Each paging zone is assigned a paging zone identifier which is indicated in the broadcast overhead by all sectors within the paging zone. Mobile stations are associated with the paging zone corresponding to the sector having the strongest received pilot strength. The mobile stations report back to the network every time the paging zone changes. While this paging zone technique is reasonable in the unicast context, it does not allow efficient operation in the case of BCMCS. In particular, since mobile stations do not report updates regarding soft handoff candidates (i.e., sectors from which the received signal strength is sufficiently high) for a BCMC radio channel unless a sector from a different paging zone becomes the strongest in terms of pilot strength, this approach may not be suited to supporting soft handoff at paging zone boundaries. Moreover, hysteresis timers employed in paging zone updates for registration ensure that users will keep sending paging zone registration update messages upon hysteresis timer expiration, even if the paging zone corresponding to the strongest sector is unchanged.