Mobile station assisted forward link open loop power and rate control in a CDMA system

Establishment of a forward link traffic channel between one or more candidate base stations and a mobile station considers forward link quality such that the initial transmit power is set to a level that achieves a desired received signal quality at the mobile station. With this approach, the initial transmit power is set to a mobile-specific power level rather than to a default initial transmit power level. That is, the open loop transmit power control that sets traffic channel power at the outset of a call, or under certain handoff scenarios, uses knowledge of forward link channel loss and interference to set initial transmit power for the traffic channel to the level needed to achieve the target signal quality at the mobile station. Such knowledge is gained from receiving pilot signal measurements for the candidate base stations from the mobile station, which measurements may be improved in accuracy by compensating them for base station loading.

BACKGROUND OF THE INVENTION

The present invention generally relates to wireless communication systems, and particularly relates to determination of the initial transmit power required to reliably establish a forward link traffic channel between a network and a mobile station.

Transmit power control is an essential element of many wireless communication systems, and is particularly needed in Code Division Multiple Access (CDMA) systems, where potentially large numbers of users share a common frequency or frequencies and rely on unique spreading codes to differentiate between users' signals. Such systems employ relatively sophisticated power control mechanisms for both forward link and reverse link transmissions.

In the typical CDMA system, forward and reverse link power control rely on closed-loop transmit power control during active communication between the network and a given mobile station. For example, during an active call, the network controls forward link traffic channel transmit power based on power control commands returned from the associated mobile station. If received signal quality at the mobile station falls below a desired threshold, it commands the network to increase transmit power; otherwise it commands the network to decrease transmit power. Thus, forward link transmit power is generally maintained at the minimum level required to maintain acceptable received signal quality at the mobile station. Reverse link power control employs similar closed-loop techniques, but with the network providing power control commands to the mobile stations, such that supporting base stations receive transmissions from the mobile stations at minimum required levels.

While the above power control schemes offer advantageous operation during active calls, they generally require the existence of a traffic channel or other data channel on the forward and reverse links. That is, these closed-loop power control schemes represent an approach to maintaining transmit powers in support of a communication over one or more traffic channels, but they do not, for example, provide a mechanism for establishing or determining an initial transmit power level.

Initialization of forward link traffic channel power arises in several scenarios including, but not limited to, call origination, soft/softer handoff, and hard handoff. Reliable establishment of forward link traffic channels aids network efficiency by reducing the signaling overhead arising from repeated connection attempts, and measurably influences users' perceptions of service quality. Thus, a conventional approach to setting initial traffic channel transmit power embraces a “more is better” philosophy, and simply sets the initial transmit power at a level high enough to ensure reliable mobile station reception under many circumstances.

Of course, once the forward link traffic channel is established, power control feedback from the mobile station generally reduces transmit power to the minimum level required. Still, using a high default initial transmit power means that statistically the initial transmit power is set higher than actually needed by the typical mobile station. As such, opportunities exist for improving overall network efficiency and capacity based on a more intelligent approach to power initialization.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus enabling a wireless communication network to set the initial transmit power of a forward link traffic channel based on a determination of the power required to achieve a target received signal quality for the traffic channel at a target mobile station. Determination of the required transmit power considers forward link quality as inferred from signal measurements made by the mobile station. In an exemplary embodiment, the mobile station feeds back measurement information for pilot channel signals from one or more base stations that are candidates for serving the mobile station on the forward link traffic channel to be established, i.e., base stations in the mobile station's “active set.”

Generally, the target received signal quality for the traffic channel is pre-determined based on the service option negotiated between the mobile station and the network. That is, the received signal quality to achieve a given frame error rate (FER), or to maintain some other reception quality metric at the mobile station, is known to the network a priori based on the target data rate selected for serving the mobile station on the forward link traffic channel. Note that the network might adjust the target received signal quality based on the number of serving base stations to reflect the possible benefits of transmit diversity. The mobile station may also autonomously update the target signal quality, based, for example, on the rate of frames received in error, within a range established by the network.

Determination of the required initial transmit power reflects the desire to calculate a mobile-specific transmit power value that achieves the target received signal quality for the traffic channel but does not transmit excessive power which otherwise reduces system capacity. To do so, the network bases its transmit power calculation on the pilot signal measurements made by the mobile station. These measurements reflect the attenuation (path loss) and interference experienced by the mobile station, and thus may be used to determine the actual traffic channel transmit power required of each serving base station such that the mobile station receives the traffic channel at the target received signal quality.

However, the pilot signal measurements made by the mobile station may be biased by base station loading. Interference at the mobile station comprises in-cell interference, out-of-cell interference, and general interference arising from innumerable other sources. Here, the extent of in-cell interference depends on base station loading and the non-orthogonality of the spreading codes used for the in-cell transmissions from the cell's serving base station or sector. Nominally, the cross-correlation between the unique spreading codes is zero such that the mobile station can despread its intended signal with essentially no interference from coded signals intended for other mobiles in the cell. However, such zero cross-correlation depends on spreading code time alignment, which is compromised by multipath propagation of transmitted signals within the cell.

Thus, calculation of the initial transmit power preferably uses compensated pilot signal measurements from which base station loading bias is removed, and further uses estimated or default forward link multipath characteristics such that the effects of multipath propagation are considered in the calculation. In an exemplary embodiment, the result of these considerations is a transmit power calculation that determines transmit power on a per connection basis. That is, the power calculation represents a joint calculation that determines the required power from each of the base stations that will be used to support the forward link traffic channel. While exemplary embodiments calculate per connection transmit power on a balanced basis, that is, with the total required transmit power equally divided between the connections, the power calculation may be modified for unequal per connection power if so desired.

Where each candidate base station provides at most one traffic channel connection per forward link traffic channel, the maximum number of such connections available for the traffic channel to be established is limited to the size of the mobile station's candidate set. The candidate set comprises of those pilots that are received with sufficient strength and deemed suitable for inclusion in the active set, which is the set of pilots corresponding to dedicated traffic channel connection. The mobile station informs the network of the pilots included in the candidate set by means of a signaling message sent on a control channel at call setup, that is, before the traffic channel is established. Generally, the network uses all of the candidate base stations to establish the traffic channel. That is, a traffic channel connection is established at each candidate base station. However, dynamic conditions such as base station loading and resource availability may cause the network to use less than all active set base stations.

For example, where the transmit power computed exceeds the power available for forward link allocation at one or more of the candidate base stations intended to provide a traffic channel connection, the network generally adopts one of two approaches. First, the network may reduce the traffic channel data rate, which has the effect of reducing the required received signal quality target and thereby reduces the required transmit power. The network then performs the initial transmit power calculation again based on the lowered received signal quality target to determine whether the updated per connection transmit power is available for each planned connection. If not, the network may lower the data rate and repeat the process in iterative fashion.

Second, the network may elect not to use a candidate base station that lacks sufficient reserve power. The network may make this election initially or as part of the above iterative approach, such as when the minimum permitted data rate is reached. With this second approach, the network simply does not use planned connections from the base station or stations that lack sufficient forward link power. Thus, this approach requires the network to re-calculate the required initial transmit power based on a per connection basis using the newly reduced number of connections. Note that the per connection transmit power versus the total required transmit power does not necessarily enjoy a linear relationship because, for example, the planned connection that is dropped from consideration may be associated with the most favorable forward link path relative to the mobile. Thus, omitting that connection from consideration may result in a disproportionate increase in the required total transmit power.

Typically, once the network establishes the forward link traffic channel with the mobile station, the network employs closed-loop power control based on power-control commands received from the mobile station to maintain the required forward link traffic channel power. However, the above techniques for setting the initial transmit power for the forward link traffic channel are applicable to a variety of call scenarios including, but not limited to, call origination, soft and softer handoff, and even hard handoff. More generally, those skilled in the art will recognize that the above techniques are applicable any time forward link traffic channel transmit power must be set in open-loop fashion.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a wireless communication network100supporting wireless communication with a mobile station (MS)102. Network100comprises a plurality of base stations (BSs)104, a base station controller (BSC)108, and a mobile switching center (MSC)110. It should be understood that the illustration represents a simplification of actual network implementations, and that network100might in actuality comprise other network entities such as home location registers (HLRs), access/authentication/authorization (AAA) systems, as well as including additional MSCs110, BSCs108, and BSs104. Further, those skilled in the art will recognize that nomenclature and implementation details may differ between network standards, such as between CDMA systems based on IS-95/2000 and those based on, for example, Wideband CDMA (WCDMA). Thus, it should be understood that network100represents an exemplary framework for explaining the present invention, and that the present invention may be practiced across a variety of network types.

Each BSC108controls number of BSs104, i.e., BSs104-1,104-2. . .104-N. Each of the BSs104includes radio resources for transmitting to and receiving from pluralities of MSs102. BSs104transmit to MSs102on forward links and receive from the MSs102on reverse links. In CDMA systems such as IS-95/2000, multiple BSs104may be used to simultaneously support communication between the network100and a given MS102. That is, the network may transmit forward link traffic to the MS102from more than one BS104and receive reverse link traffic from the MS102at more than one BS104.

The set of BSs104available at any given time for forward link traffic transmissions to the MS102is referred to as the mobile's “active set.” Typically, the MS102measures the strength of the pilot signal from nearby BSs104and sets its active set based on which pilot signals are received above a given signal threshold. Thus, BSs104in the mobile's pilot set may be referred to as “candidate” base stations because each one is a candidate for serving the MS102, based on the mobile's ability to receive signals from them at relatively good signal levels.

In accordance with one or more exemplary embodiments of the present invention, the initial transmit power selected for a traffic channel to be established between one or more candidate BSs104and the MS102is set based on pilot signal measurements made by the MS102. Essentially, this initialization approach uses pilot signal measurements returned by the MS102to determine what level of transmit power is needed to overcome current path loss and interference conditions as is inferentially determinable from the pilot signal measurements. That is, the network100infers expected transmit signal degradation from the path loss and interference conditions indicated by the pilot signal measurements returned to the network by the MS102, and sets the initial forward link traffic channel transmit power to the level required such that the MS102receives the traffic channel with a required signal quality.

FIG. 2depicts an exemplary, top-level approach to transmit power initialization at the network100. Processing starts with the network100receiving forward link quality indicators from the MS102in advance of establishing the forward link traffic channel (Step200). As noted, such forward link quality indicators may be pilot signal strength measurements transmitted from the MS102to the network100for candidate BSs104.

Because pilot signal measurements may be skewed or otherwise biased somewhat by forward link loading conditions, the network100receives base station loading estimates from the candidate BSs104(Step202), which it uses to remove forward link loading biases from the pilot signal measurements. In exemplary embodiments, the MS102transmits pilot signal measurements to one or more BSs104, which in turn provides those measurements to the BSC108. The BSC108further receives candidate base station loading estimates, and is thus positioned to make initial transmit power computations in accordance with the present invention.

First, the BSC108selects the number of traffic channel connections that will be used to support the contemplated forward link traffic channel (Step204). That is, the BSC108determines which BSs104will be used to support the forward link traffic channel. As is explained in more detail later, the BSC108typically sets the number of traffic channel connections equal to the number of candidate BSs104, i.e., to the number of BSs104in the active set of MS102. However, circumstances might arise that force usage of fewer than all candidate BSs104.

Regardless, the network100fixes or otherwise determines the number of traffic channel connections, and then computes the initial transmit power as the actual transmit power required to achieve a desired traffic channel received signal quality at the MS102. A generalized form of the computation considers the expected forward link path loss and interference as inferred from compensated pilot signal measurements, which considerations provide a basis for determining the required initial transmit power.

More generally, the network100knows or can determine the target received signal quality desired for reception of the forward link traffic channel at the MS102based on the service option negotiated between the network100and the MS102. In a typical CDMA system, a given service option may include one of several possible data rates, and in other instances such as voice service, may include only one possible data rate. Variable data rates are particularly common in packet data service options, such as those available in IS-2000 and WCDMA systems. Regardless, the general maxim is that the higher the data rate, the higher the required received signal quality at the MS102. Of course, for a given selected data rate, the network100might adjust the required signal quality value based on diversity gains expected from the use of multiple traffic channel connections. Such adjustments embody the idea that with a greater number of transmission points comes greater resistance to channel fading, etc.

In more detail, a forward link between a given BS104and the MS102is considered. Meeting a desired received signal quality at the MS102requires that the MS102must receive the traffic channel signal at a high enough level. This requirement may be expressed in terms of signal-to-noise ratio (SNR), which itself can be expressed in terms of received bit-energy to noise power spectral density ratio. Thus, the ratio of traffic channel received bit energy to noise power spectral density,

Traffic⁢EbNt,
at the MS102must be set to a level such that:

required⁢⁢transmit⁢⁢TrafficEb=⁢Traffic⁢EbNt⁢(Total⁢⁢Interference)⁢⁢(Path⁢⁢Loss)(1)
That is, in (1) above, the transmit bit energy to achieve the required target received signal quality at the MS102equals the desired traffic channel bit-energy to noise power spectral density multiplied by the path loss and multiplied by the total received interference at the MS102.

From the mobile's perspective, the total received interference equals other cell interference plus self-interference and path loss equals the base station transmit power spectral density divided by the received power spectral density from said base station. Thus, the required traffic channel transmit bit energy may be expressed as,
required transmit TrafficEb=desired received traffic channel bit-energy to noise power spectral density ratio×(other cell interference+self-interference)×(transmit power spectral density/received power spectral density).  (2)
Equivalently,
required transmit TrafficEb=desired received traffic channel bit-energy to noise power
spectral density ratio×(other cell interference/received power spectral density+self-interference/received power spectral density)×transmit power spectral density.  (3)
Where the other cell interference to received power spectral density ratio can be inferred from:measured base station load, expressed in terms of transmit pilot energy to total transmit power spectral density ratio; andreceived pilot energy to total received power spectral density ratio, as measured by the mobile station and reported to the base station via signaling messages.
That is,
other cell interference to received power spectral density ratio≅transmit pilot energy to total transmit power spectral density ratio/received pilot energy to total power spectral density ratio.  (4)
Where the self-interference to received power spectral density ratio can be inferred from a multipath profile, which, in at least some exemplary embodiments of the present invention, is known a priori by the network100. The network100may store multipath propagation information, including default multipath profile information that defines the number of multipaths to be assumed for in-cell interference calculations, and the relative path strengths of the multipaths. In an exemplary embodiment, the network100uses a default number of same-strength multipaths in its in-cell interference calculations.

From the above expressions, one sees that the network100uses pilot signal measurements from the MS102reported for the BS104of interest, in conjunction with forward link load estimates from the BS104of interest, to determine self-interference and other-cell interference at the MS102. Further, as shown, this determination by the network uses one or more multipath profiles, which typically comprise a set of multipath estimates for the forward link channel between the BS104of interest and the MS102. Multipath estimates might, in an exemplary embodiment, comprise a set of default estimates regarding the number of propagation paths and the relative path delays and attenuations. Of course, the network100might use other-than-default values in some implementations.

In an exemplary embodiment where the network100contemplates the use of a single BS104for supporting the traffic channel to be established between it and the MS102, the above operations may be expressed as,

Traffic⁢⁢PTXPilot⁢⁢PTX
is the ratio of traffic channel transmit power to pilot channel transmit power, Rc/Rb represents a processing gain dependent on the chipping rate over the bit rate, and where a single-path propagation channel is considered.

In an another exemplary embodiment where the network100contemplates the use of multiple BSs104for supporting the traffic channel to be established between it and the MS102, it consolidates the above operations into a joint calculation that may be expressed as,

In another exemplary embodiment where the network100contemplates the use of multiple BSs104for supporting the traffic channel to be established between it and the MS102, it consolidates the above operations into a joint calculation that may be expressed as

Where one or more of the candidate BSs104lack sufficient reserve forward link power to support the above calculated transmit power, the network generally either reduces the targeted data rate and re-computes the per-connection power requirement, or drops the offending BSs104from consideration and re-computes with the reduced set of BS104. Of course, other techniques might be employed, and the two primary approaches might be combined by network100, such as where the network100drops the target data rate to a minimum value defined for the service in attempts to find a supportable initial transmit power, and then begins dropping BSs104from the candidate set in an attempt to eliminate BSs104with low reserve power.

FIG. 3illustrates at least one component of the network's exemplary selection logic. Here, the network100determines whether any BSs within the MS's active set are reserved or otherwise excluded (Step210). If so, the network100reduces the number N of traffic channel connections to be used in establishing the traffic channel based on the number of excluded or reserved BSs104, and initial set reduction processing ends. Where all BSs104in the mobile's active set are available, the network100sets, in a first pass, the number of planned traffic channel connections equal to the active set size of the MS102.

Those skilled in the art will appreciate that many variations are available for determining the number of traffic channel connections to be used. In general, the BSC108sets the number of planned connections for supporting the traffic channel equal to the number of candidate BSs104in the mobile's active set; thus, the “connection set” size generally equals the active set size. However, the BSC108might, as described above, reduce the connection set size where one or more of the candidate BSs104are reserved or should otherwise be excluded from use in supporting the traffic channel.

WhileFIG. 3illustrates initial selection logic that might be applied by the network100before computing required transmit power,FIG. 4illustrates an exemplary approach for instances where the computed required transmit power exceeds the reserve power available from one or more of the BSs104in the connection set. That is, the following logic might be applied by the network100when the per-connection transmit power exceeds the reserve forward link power at one or more BSs104the network planned to use for establishing the traffic channel.

Thus, processing begins with the network100determining whether any of the selected BSs104lack sufficient reserve forward link power relative to the computed required transmit power (Step220). If not, re-calculation of the required initial transmit power ends, and the network100retains the previously calculated initial transmit power and uses the currently selected BSs104.

However, if one or more of the BSs104in the planned connection set lack sufficient reserve power, the network100optionally follows a data-rate reduction approach (Step222), follows a connection-exclusion approach (Step224), or follows some combination thereof. For example, the network100might determine whether a data rate reduction is permitted for the current service option (Step228) and, if so, reduce the data rate and re-compute the required initial transmit power based on the reduced target received signal quality associated with the lowered data rate (Step230). At that point, processing loops back to the reserve power check (Step220), where the network determines whether the reduced power level falls within available limits. If power limits are met, processing ends. Otherwise, processing repeats with the network either further lowering the data rate and recomputing required transmit power iteratively, or by excluding planned traffic channel connections.

The latter choice reflects an exemplary approach and, indeed, the network100might adopt the exclusion approach with or without benefit of the data-reduction approach. In the exclusion approach, the network100determines which BSs104planned for use lack sufficient reserve power, and excludes them from consideration (Step224). The network100then re-computes the required initial transmit power using the number of connections available from the reduced set of BSs104. Again, processing returns for determination of whether the now-reduced set of BSs104has sufficient power to support the re-computed required initial transmit power (Step220). As with the data-reduction approach, the network100can iterate through the connection-exclusion approach until it finds a satisfactory solution set.

It should be understood that the above exemplary logic might be implemented in the BSC108, or might be implemented as a cooperative effort between BSC108and the involved BSs104. One advantage of BSC-based computation is that the BSC108represents a natural point of consolidation for initial transmit power computations affecting the BSs104operating under its control. Other network architectures or standards may use different terminology or different network entities to handle radio resource management and it should be understood that the term BSC as used herein is broadly construed to cover a variety of entities that provide the inventive functionality.

In general, the present invention provides a basis for determining the initial traffic channel transmit power required to reliably establish a forward link traffic channel between the network100and a given MS102. The invention infers reception conditions at the MS102, and determines the transmit power required to achieve a target traffic channel received signal quality at the MS102. Such inferences rely on the use of channel measurements made by the MS102for BSs104that are candidates for serving the MS102on the forward link. In an exemplary embodiment, the network100receives pilot signal measurements made by the MS102for each of the candidate BSs104. With these measurements, and with loading estimates and multipath profile estimates, the network100determines in-cell and other-cell interference at the MS102, which allows it to properly set initial transmit power for the traffic channel.

Those skilled in the art will appreciate that the present invention represents an approach to initial, open-loop transmit power control that essentially mimics the closed-loop power control used once the traffic channel has been established with the MS102. That is, the present invention uses feedback of pilot channel measurements made by the MS102to determine reception conditions (e.g., path loss and interference) at the MS102, and then sets the initial transmit power to be used in establishing a traffic channel to the MS102to the level needed to achieve a target received signal quality for the traffic channel at the MS102. As such, the above details are exemplary and not limiting. Indeed, the present invention is limited only by the scope of the following claims, and the reasonable equivalents thereof.