|Title of Invention||
A RADIO NETWORK CONTROLLER IN A CODE DIVISION MULTIPLE ACCESS (CDMA) RADIO COMMUNICATION SYSTEM
|Abstract||A radio network controller in a Code Division Multiple Access (CDMA) radio communication system having a plurality of base stations and a mobile station capable of establishing a simultaneous communication link with multiple sectors within the base stations, said radio network controller comprising: a receiver for receiving a requested service from the mobile station; a processor in communication with the receiver that determines that a handoff between the sectors is required in order to continue the communication link, and adaptively optimizes radio performance during the handoff based on the requested service and radio quality measurements that include signal pathloss, pathloss and uplink interference, downlink received signal strength (RSSI), and downlink interference (Eb /I0).|
|Full Text||FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
THE PATENTS RULES, 2003
[See Section 10; rule 13]
"A radio network controller in a Code Division Multiple Access (CDMA) radio communication system"
TELEFONAKTIEBOLAGET LM ERICSSON (PUBL), a Swedish company, of S-126 25 Stockholm, Sweden,
The following specification particularly describes the invention and the manner in which it is to be performed:
FIELD OF THE INVENTION
The present invention relates generally to wireless radio telecommunication systems, more particularly, it relates to the use of adaptive handoff algorithms for mobiles operating in CDMA systems.
BACKGROUND OF THE INVENTION
Code division multiple access (CDMA) telecommnication systems, especially those designated as the so-called third generation -wideband systems, such as CDMA (WCDMA) for example, were designed to provide access services mat include circuit switched and packet switched data services having substantially improved bit rate performance. Such high bit-rale performance supported by third generation wideband systems gives rise to a multitude of services such as wireless multimedia and video, wireless data services such as simultaneous voice, and enhanced internet services.
The operating requirements of the various services may be considerably
different thereby demanding correspondingly different performance levels from the system. By way of example, the characteristics of a circuit switched voice call rypically are delay sensitive (i.e no excessive breaks in the conversatian which disrupts flow), high mobility, relatively short call duration, and symmetrical service (i.e. same amount of data is sent in both uplink and downlink directions). In contrast,
packet based services such as interact web browsing are typically characterized by being delay insensitive (errant packets may be retransmitted), low mobility, relatively long call duration, and asymmetrical service i.e. downlink direction is typically more heavily used when loading web pages, for example. There are numerous factors that may affect the performance of various services operating within a CDMA system. One such factor that is known to affect the performance of typical services is mobile station handoffs.
Fig. 1 illustrates a basic cellular teleconmmnication system having a radio
network controller (RNC) linked to a network of base stations by a series of digital
transmission links115. The base stations are geographically dispersed to form an area
of coverage*for the system. Each.base station (BS) is designated to cover a specified
area, known as a cell, in which a two way radio communicarion connection can take
place between a mobile station and the BS in the associated cell. The boundary
between the cells is indicated by line 110. In this simplified exemplary depiction, only
two base stations are shown but in practice, a substantial multiplicity of base stations
will form the functional coverage area for the system. It is understood by those skilled
in the art that other components and devices axe typically included in the system that
are not shown in the exemplary illustration. In general, as the MS moves throughout
the network, communications are maintained with die network by transferring the
connection to a neighboring base station in an event referred to as a handoff. For
simplicity, the term mobile station will henceforth be referred to as the mobile.
In telecommunication systems operating in accordance with code division multiple access (CDMA), macro diversity is typically employed where a mobile simultaneously communicates with more than one BS prior to a handoff from an origmating BS to a neighboring BS. This is referred to in the art as "soft handofr in that the mobile will commence communication with the neighboring BS before terminating coiminunication with the originating BS. This "make before break" procedure is made possible by operating all traffic on a common spread spectrum waveform frequency. A variant of me soft handoff is what is referred to as "softer
handoff in which fhe mobile simultaneously communicates with multiple sectors of the same BS. There are several advantages associated with soft handoffs such as reduced rale of dropped calls, no interruption in speech upon handoff, and increased gain in downlink signal-to-noise ratio. Another important advantage of soft handoff/softer handoff is that of macro diversity during mobility i.e. greater protection from log.normal and multi-path fading since, on average, the convergence from the effects of fading or multi-paths do not occur at the same time.
Another type of handoff that occurs in CDMA systems is a "hard handoff." A hard handoff is a handoff mat typically takes place, for example, between two channels or when the base stations are not suitably synchronized for a soft handoff. This type of handoff is often characterized as "break before make" since communication on a first frequency is terminated before communication is established on a second frequency. Hard handoffs occuring within the same cell are referred to as terra-cell hard handoffs and those occurring between cells are referred to as inter-cell hard handoffs. Hard handoffs typically occur in situations where vendor equipment lhmrations preclude performing soft handoffs such as, for example, layer changes for moving mobiles, mode switches e.g. in dual mode systems, switching between operator networks, and resource allocation issues that require hard handoffs.
In an exemplary CDMA system, handoff decisions are typically based on the detection by the mobile Of the signal strength of pilot signals transmitted by neighboring base stations. The pilot signals are distinguished by a pseudonoise sequence (PN) such that the mobile is able to determine and allocate the base station within a distinct classification set. By way of example, the sets include, an Active Set which is a set of base stations that the mobile is actively communicating with, a candidate Set which is a set of base stations that have pilot strengths that are sufficient for communications based on system parameters set by the base station, and a Neighbor Set which is a set of base stations in the area that have a pilot strength indicating the potential for sufficient communication with the mobile. However, those skilled in the art will appreciate that the sets referred'to and their functions are
referenced by the CDMA standard known as TS-95 hot that they have analogous counterparts with sinnlat functions in other CDMA standards which may be identified differently. The base station's classification within a set may be changed in accordance with, e.g., the received pilot signal strength by the mobile. Handoff decisions are then made by the system controller which are typically base its decision, at least in pan, on the reported pilot signal strength and other criteria.
In the prior art, handofis, that occur during voice and data services, are
typically based on handoff algorithms that are unchanging and without regard to the
impact on service performance. Accordingly, it is a object of the present invention to
provide technique for utilising handoff algorithms that adapt to the type of service
SUMMARY OF THE INVENTION
To achieve the foregoing and other objectives in accordance with the purpose
of the invention, an adaptive handoff algorithm govemiag mobile handoffs between
sectors of the same or other base stations within a CDMA system is disclosed. In an
embodiment of the invention, the handoff algorithm, using certain quality measures, is
implemented in response to the particular type of service requested by the mobile.
Handofis governed by the handoff algorithm tend to lead to desirable operating
environment for the mobile. By way of example, when a service such as a voice call is
reqnested, the handoff algorithm uses a quality measure dial takes into account the
pathloss and uplink interference. This tends to induce handoffs mat result me mobile
connecting to a base station where output power of me mobile is minimized. This
situation is desirable for conserving battery capacity of the mobile. In a further
example, if the mobile is using a data intensive application such as internet browsing,
the handoff algorithm uses a quality measure that induces the mobile to handoff to
sectors that result in reduced downlink output power levels by the base stations. This
tends to decrease the interference levels in the cell, thereby improving the chances of
receiving the transmitted packets correctly and thus reducing the need for repeat transmissions.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:
Fig. 1 illustrates a simplified exemplary wireless telecormmunication system; and
Fig. 2 illustrates an exemplification of ihe present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In rypical CDMA systems, the mobile plays an active role in handoff procedures. While engaged in a call or packet data transfer, the mobile continuously monitors the pilot signal levels from sectors of the same or neighboring base stations in preparation to perform a softer/soft handoff. In addition to the mobile, the base station (BS) and me radio network controller (RNC) is also involved in the evaluation of the new sectors. The mobile reports to the RNC those sectors or pilots having signal levels above a defined threshold. The threshold may be predetermined or dynamic and is generally based on a given set of measures such as signal strength or signal quality. The RNC, based on this information, directs the mobile to add or remove sectors from its Active Set, i.e, those sectors that the mobile is simultaneously communicating with.
The quality and user's experience of a requested service, such as voice call, may be affected by a handoff event which is typically triggered by movement the
mobile Away from a BS with which it has an established link. System parameters associated with handoff procedures are typically set by operators and thus issues such as evaluating sectors for the Active Set based on quality measures may impact the service experience. By way of example,- in an exemplary CDMA system, an
evaluation is made by comparing the measured value of a quality estimate, called the
HO--quality_estimate, from each sector, in the mobile's neighbor set where a received
signal is detected. A parameter HO-quality measure represents the quality measure
used for detennining the HO_quaIiry_estimate. A handoff algorithm uses the
HO_quality_estimate to determine the contents of me Active Set, i.e. sectors that
- should be added or removed. Hence, the handoff algorithm is able to use different
quality measures for different types of services.
m accordance winl an exemplary embodiment of the present invention, the various quality measures include:
(I) HO_quatiry_measure = Parhloss,'
~ (IT) HO_quality_measure = Patbloss + Uplink Interference;
(IE) HO_qualiry_measure = Downlink RSSI; and
(TV) HO_quality-measure = Downlink E/I,.
Wherein the parhloss in quality measure (I) is estimated by the mobile by reading a transmitted power level value of me pilot signal that is broadcast by the BS and subtracting the actual received signal strength by the mobile. A handoff algorithm predicated on patbloss tends to result in the mobile connecting to the nearest BS. Quality measure (ID takes into account the parhloss and the uplink interference . measured by the BS. Thus a handoff algorithm predicated on this tends to lead to the mobile connecting to the BS requiring the least amount of transmission power thereby minimizing the mobile output power necessary for the connection. In quality measure (HI), the handoff algorithm is based on the received signal strength (RSSI) measured by the mobile. This tends to result in the mobile connecting to the sector with the
strongest pilot signal. In quality measure (IV), the Downlink E/L is an interference measure which is related to the energy per chip divided by the power spectral density of the interference and is indicative of the quality and strength of coverage. A handoff algorithm based on this tends to result in the mobile connecting to the sector where the BS needs the least amount of power, i.e., the algorithm tends to steer the mobile to the sector having the least amount of downlink interference.
In the exemplary embodiment, the nandoff algorithm uses the most suitable HO_quality_measure for a particular type of service. For a voice call, for example, it would be desirable to minimise the mobile output power as much as possible in order to conserve battery capacity and reduce the overall uplink interference in the cell, __thereby making quality measure (II) more suitable. On the other hand, internet web browsing typically requires much more downlink capacity as compared to the uplink thereby making quality measure (IV) a suitable choice. A downlink intensive service such as web browsing benefits from roirdmizmg the downlink output power which typically lowers overall downlink interference levels in the cell resulting in less repeated packets being retransmitted. Repeated .packets result in more transmissions mat contribute to higher levels of interference in the cell as well as delayed data transactions.
Fig. 2 illustrates an exemplification of the above described embodiment In step 200, a mobile makes an access request to me system radio controller in a CDMA system, wherein the type of service requested by me mobile is determined by the RNC, as shown in step 210. In step 220, if the mobile is performing a voice call, me haudoff algorithm uses both the pathloss and the uplink inference as the quality measure, as shown in step 230. If, as shown in step 240, the mobile is browsing the internet, e.g., the handoff algorithm uses a quality measure that includes the downlink E0/L0 in order to reduce cell interference, as shown in step 250. Another example using the adaptive techniques described is during a multimedia session where the handoff algorithm can be chosen to favor the video portion, since video data generally requires much more capacity as compared to voice data. The general concept being
that when multimedia services are performed, the service (video) wnicn requires the highest grade of service should determine the selection of handoff algorithm.
Other possible exemplifications using handoff algorithms may include combinig quality measures daring an individual service, for example.dazing a voice call, it may be desirable to use a quality measure that is inclined to minimise mobile output power when, e.g., 70% of the mobiles in the cell are relatively far from the BS in order to reduce me interference in the cell. likewise, another quality measure specified lower BS output power is desirable when a majority of mobiles are operating close to the BS.
Although the invention has been described in some respects with reference to a specified preferred embodiment variations and modifications w3l become apparent to those skilled in the art In particular, alternative quality measures can be used, e.g., those based on other quality measures (e.g., bit error rate, cumulative: interference affecting cell breathing) or which include non-quality related parameters, e.g., service type availability. Moreover, although exemplary handoff algorithms are described with respect to soft handoff, those skilled in the art will appreciate that the present invention may apply handoff algorithms to hard handoffs within CDMA systems, e.g., those made to improve data rates for packet switched services. It is therefore, the intention mat me following claims should not be given a restrictive interpretation but should be viewed to encompass variations and modifications that are derived from the inventive subject matter disclosed.
1. A radio network controller in a Code Division Multiple Access
(CDMA) radio communication system having a plurality of base
stations and a mobile station capable of establishing a
simultaneous communication link with multiple sectors within the
base stations, said radio network controller comprising:
a receiver for receiving a requested service from the mobile station;
a processor in communication with the receiver that determines that a handoff between the sectors is required in order to continue the communication link, and adaptively optimizes radio performance during the handoff based on the requested service and radio quality measurements that include signal pathloss, pathloss and uplink interference, downlink received signal strength (RSSI), and downlink interference (Eb /I0).
2. The radio network controller as claimed in claim 1, wherein if the requested service is a voice call, the processor utilizes the pathloss and interference to induce handoffs of the mobile station that tend to minimize required mobile station output power.
3. The radio network controller as claimed in claim 1, wherein if the requested service is internet web browsing, the processor utilizes the downlink interference (E0, /I0) to induce handoffs of the
mobile station that tend to reduce required downlink output power, thereby decreasing the overall interference level in a corresponding cell.
4. The radio network controller as claimed in claim 1, wherein the processor optimizes radio performance during both soft handoffs and hard handoffs.
5. A mobile station for operation in a Code Division Multiple Access (CDMA) radio communication system as claimed in claim 1 having a plurality of base stations and one or more radio network controllers, the mobile station being capable of establishing simultaneous communication with at least two sectors within the base stations and performing a handoff between the sectors, the mobile station comprising:
a selector for selecting an optimum handoff algorithm from a number of handoff algorithms, said selected handoff algorithm being configured to optimize handoff performance for a requested type of service, and a processor for applying the selected handoff algorithm to perform the handoff.
6. The mobile station as claimed in claim 5, wherein the requested
type of service is a voice call, and the selector selects a handoff
algorithm that favors minimizing the output power of the mobile.
7. The mobile station as claimed in claim 5, wherein the
requested type of service is a data intensive service, and the
selector selects a handoff algorithm that favors minimizing
downlink output power.
8. The mobile station as claimed in claim 7, wherein the data
intensive service is Internet based browsing.
9. The mobile station as claimed in claim 5, wherein the handoff
algorithm is based on anyone of pathloss, pathloss and uplink
interference, downlink received signal strength (RSSI), and
downlink interference (E0 /Io).
Dated this September 14, 2001.
(RANJANA MEHTA DUTT)
OF REMFRY AND SAGAR ATTORNEY FOR THE APPLICANTS
|Indian Patent Application Number||IN/PCT/2001/01114/MUM|
|PG Journal Number||23/2007|
|Date of Filing||14-Sep-2001|
|Name of Patentee||TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)|
|Applicant Address||S-126 25 STOCKHOLM, SWEDEN|
|PCT International Classification Number||H 04 Q 7/00|
|PCT International Application Number||PCT/SE00/00246|
|PCT International Filing date||2000-02-08|