METHOD AND APPARATUS FOR DETECTING A MINIMUM TRANSMISSION

POWER STATE IN A CLOSED LOOP POWER CONTROLLED CHANNEL OF

WIRELESS COMMUNICATIONS SYSTEMS

DESCRIPTION

TECHNICAL FIELD OF THE INVENTION

This invention relates to the field of wireless communications and, more specifically, to the closed loop power controlled channels of a cellular telephony network.

BACKGROUND OF THE INVENTION

It is highly advantageous in wireless communications system to know when the transmitter is at the lowest possible power level. This information can be useful for power control systems, Radio Access Bearer reconfiguration diversity management, etc., to name but a few among other systems involved in a cellular telephony network using Code Division Multiple Access (CDMA) . A brief explanation on such systems is given below:

i) The power control in a CDMA system comprises two differentiated loops: the open loop power control and the closed loop power control. The base and mobile stations use the open loop power control at the start of a connection to set the initial uplink and downlink transmission power, based on the estimation of the path loss in the complementary link. The closed loop power

control, in turn, consists of two mechanisms: the inner loop mechanism and the outer loop mechanism. The outer loop power control determines the target desired signal to interference ratio (SIR _ta rget) which is required to achieve a certain quality of service for the specific link. The inner loop power control maintains the received desired signal to interference ratio (SIR _rec ) as close to the SIRtarget as possible. In order to accomplish this function, the inner loop sends power control commands instructing the transmitter to increase or decrease the transmitted power.

ii) In the case of data communications, different radio bearers are used depending on the application requirements and the available resources. For the Radio Access Bearer (RAB) reconfiguration, different RAB parameters or attributes can be allowed to be negotiated in order to better adapt the bearer performance to the radio propagation conditions. One of these negotiable parameters is the bit rate of the RAB, which can be adjusted to improve the Quality of Service (QoS) for the specific services supported on the cellular telephony network.

iϋ) In wireless communications, antenna diversity that exploit the multipath propagation, resulting in a diversity gain that combats fading and co-channel interference and avoids error bursts, can be applied by using multiple receiving antennas. Thus, multiple versions of the same signal are received (this is called diversity reception) and then combined in the receiver by

a diversity combining technique.

The needing to detect whether the transmitter is at the minimum power level in order to, for example, vary the diversity reception has been satisfied in previous work

(see patent application WO03088521). However, this and other known mechanisms for the detection of when the transmission power reaches its lowest level involve the active participation of the transmitter itself in the process. These solutions that call for the transmitter to announce the receiver that the power level of said transmitter is at its minimum imply several inconveniences :

• It requires the implementation of additional signalling functions, since the transmitter must inform the receiver about being at the minimum power state .

• It requires changing the standards to include the new signalling information. • The results may be different for different transmitter types, even in the same conditions.

This patent application solves the problem of determining when a transmitter is at the minimum power level by using the power control means only at the receiver, without needing participation of the transmitter, which directly corrects the commented drawbacks.

SUMMARY OF THE INVENTION

This invention describes a method and apparatus to detect

from a receiver, in a mobile communication system based on CDMA, when the power level of a transmitter in a closed loop power controlled channel reaches the minimum value. When this situation occurs, the method and apparatus of the present invention send a minimum power command, generated at the device comprising said receiver

(mobile or base station, which is the receiver end in the closed loop power controlled channel) and indicating such status of the transmitter comprised in another remote device (base or mobile station correspondingly, which is the transmitter end) .

The minimum power command is generated at the receiver end and sent from the here so-called minimum transmission power detection subsystem of this receiver end to the interested subsystems of said mobile communication system; for example, to the power control subsystem, or to the Radio Network Controller (RNC) for the Radio Access Bearer (RAB) reconfiguration, or to the diversity subsystem in the receiver, etc. In some applications of the invention, this minimum power command can be sent locally to another subsystem or module of the same receiver; then, there is no radio but a local transmission of said minimum power command. In other applications, for example when reconfiguring the RAB bit rate, a configuration parameter can be sent to a remote subsystem depending on the specific mobile communication system.

The present invention is applicable to any wireless communications system with a cellular infrastructure,

supporting one or more standards of the CDMA protocol, such as WCDMA, IS-95, CDMA2000, specification HDR, etc.

The goal of this invention to determine whether the transmitter is really at the minimum power level, or on the contrary the power level of said transmitter is enable to be lowered, is achieved by using information exclusively from the receiver. As mentioned before, the fact of performing the detection of the minimum transmission power level from the very receiver constitutes a very important advance over previous work.

The receiver uses the knowledge about the transmitter being at the minimum power level to improve the reception performance in different situations. It is important to note that these improvements are independent of the transmitter type used since no specific transmitter data are needed in the present invention. The existence of a closed loop power controlled channel is the only requirement.

This invention is based on the averaged values of the respective target desired signal to interference ratio (SIRtarget) and the received desired signal to interference ratio (SIR _rec ) that are estimated over a determined time interval, which is long enough to avoid a false detection (for example, the mean of the SIR _rec and SIR _ta rget calculated for the last 100 milliseconds) . The reason to determine a sufficient time interval is that the inherent variability of the SIR _rec around the target SIR _ta rget and the potential existence of fast fading effects could lead

to detect an erroneous status when comparing the SIR _rec and SIRtarget magnitudes, if the instantaneous values were used.

In accordance with a first aspect of the invention, a method for detecting a minimum transmission power state in a closed loop power controlled channel, set between a transmitter end and a receiver end of a wireless communication system, is described here, being the minimum transmission power state determined when the transmitter end operates at the minimum power level previously established or assigned to its means of transmission (a minimum power level limit) . The transmitter end can be a base station and the receiver end can be a mobile station, or vice versa. Both ends are transceivers so the method can be applied in any of the two communication links: uplink or downlink. The method comprises the following steps performed at the receiver end of the closed loop power controlled channel: estimating a received desired signal to interference ratio (SIRrec) ; using an outer loop power control mechanism to estimate the target desired signal to interference ratio (SIRtarget) required to maintain the target quality of the communication service; calculating the mean values of the SIR _rec and SIR _ta rget estimated for a determined time interval; comparing the two mean values calculated. generating a minimum power command when the SIR _rec mean value exceeds the SIR _ta rget mean value by a certain quantity or difference magnitude, and additionally said

minimum power command can be sent from the receiver end to inform the interested modules or subsystems that the transmitter end is at the lowest power level, i.e., the minimum transmission power state is detected.

Otherwise, if in the result of the comparison step, the difference between the calculated mean values of SIR _rec and SIRtarget is equal or lower that the certain quantity considered in said comparison, normal condition in the transmitter (not minimum transmission power state) is detected.

In case that the minimum transmission power state is detected as explained before, the minimum power command can be used to block the outer loop power control mechanism in order to keep the SIR _ta rget value fixed until the SIRrec changes and is able to follow said SIR _ta rget value. If a receive diversity scheme is being used at the receiver end, the minimum power command can prompt to turn off or disconnect one or more radiofrequency (RF) chains as benefit to save energy. Also, the minimum power command can trigger a renegotiation of the radio bearer access which is being used in order to increase the data bit rate without varying the transmitted power.

Hence, the proposed method contributes to optimize performance and power resources in the closed loop power controlled channel.

Another aspect of the invention refers to an apparatus for the detection of the aforementioned minimum

transmission power state in a closed loop power controlled channel of wireless communications systems, comprising at least one programmable electronic device that operates according to the method described above. The programmable electronic device may be a general purpose processor, a digital signal processor (DSP) , an application-specific integrated circuit (ASIC) and a programmable card (FPGA) or any combination thereof. The general purpose processor is preferably a microprocessor or other possible alternatives: a conventional processor, a microcontroller or any state machine in general. Moreover, the programmable electronic device can comprise a combination of multiple microprocessors, one microprocessor and one or more DSP devices, or any other configuration in which the execution of the various phases is distributed in series or in parallel, comprised within the method described above.

This apparatus further comprises a radio-frequency receiver that operates in the receiver end of the closed loop power controlled channel.

Other final aspects of this invention include a Radio Network Controller (RNC in a UMTS network) , a base station (Node-B in UMTS) and a mobile station (remote user or terminal, UE) . Each device comprises the apparatus for wireless communication systems described.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will become apparent in view of the detailed description which follows of a set of preferred embodiments of the invention, given for purposes of illustration only and in no way meant as a definition of the limits of the invention, and made with reference to the accompanying drawings, in which:

Figure 1 shows a global block diagram of part of a mobile communications system, particularly a UMTS system, as known in the prior art, including the components of a cellular infrastructure: user mobile terminal, base station and remote network controller, as related to the object of this invention.

Figure 2 shows a graphical representation of the corresponding behavior of the target desired signal to interference ratio (SIR _ta rget) and the received desired signal to interference ratio (SIR _rec ) t when the transmitter is at the minimum power level.

Figure 3 shows a flow diagram illustrating the steps for detecting whether the transmitter is at the minimum power level and including a minimum power command generation, in accordance with the present invention.

Figure 4 shows a receive diversity system where the present invention is applied.

Figure 5 shows a graphical representation of the application of the present invention in a power control

system, where the minimum power command blocks the target desired signal to interference ratio (SIR _ta rget) with respect to the received desired signal to interference ratio (SIRrec) in a closed loop power controlled channel.

Figure 6 shows a schematic representation of an increment in the bit rate of the radio access bearer when the minimum power command is sent in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Note that in this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

In the context of the present invention, the term "approximately" and terms of its family (such as "approximate", etc.) should be understood as indicating values very near to those which accompany the aforementioned term. That is to say, a deviation within reasonable limits from an exact value should be accepted, because a skilled person in the art will understand that such a deviation from the values indicated is inevitable due to measurement inaccuracies, etc. The same applies to the term "about".

Figure 1 represents one part (100) of a WCDMA mobile

communications system. Aside from the invention, the elements shown in the figure are well known and not described in detail. One element of interest is the user terminal unit or mobile station (104) which is represented by the vehicle icon. The WCDMA mobile communications system comprises several base stations

(102, 103) or B-nodes in the UMTS network, containing processors, memories, interface cards and embedded software programs. The two base stations (102, 103) and the mobile station (104) are representative of end points of the wireless interface. Each base station (102, 103) is associated to an RNC (101) through land lines (105, 106) or fixed radio links. In what follows, it is assumed that the mobile station (104) is in communication with the base station (102) by the downlink data signal (107) and the uplink data signal (108) . In order to transmit/receive the downlink and uplink data signals (107, 108), the mobile station (104) as well as the base stations (102, 103) respectively incorporate a transceiver that contains a transmitter and a receiver. A minimum power level for the transmission power is established, in the mobile station (104) or the base stations (102, 103), that is, the lowest power level the transmitter is enabled to deliver.

This part (100) of the system further includes a radio network controller (101) or RNC which is responsible for governing several base stations (102, 103), to which the RNC is connected or communicates by means of a radio- frequency transceiver comprised of corresponding receiver and transmitter. Among other functions, the radio network controller (101) enables call processing and carries out

radio resource management in the UMTS radio access network. Radio Resource Management functionality includes Outer Loop Power Control, Diversity configuration and Radio Access Bearer configuration, i.e., Radio Access Bearer establishment, reconfiguration and release.

Figure 2 shows a key aspect: in the receiver, when the transmitted power level that is required to match the received desired signal to interference ratio (SIR _rec ) to the target desired signal to interference ratio (SIR _ta rget) is below the minimum power level of the transmitter, the SIRrec value (201) is higher than the SIR _ta rget value (202) . This situation occurs because the transmitter can not perform a further power reduction beyond the established minimum power level and then, the SIR _rec value (201) being higher than the SIR _ta rget value (202) may continue for an extended time, since the radio link quality is above the target and the outage probability is very low. The outage probability parameter is another quality parameter often used in cellular infrastructures and determined in the cellular network planning stage, depending among other parameters upon the characteristics of the corresponding cell, on the area within it and the type of service covered by the link.

In order to identify when the transmitter really meets the minimum power condition, this invention makes use of data only from the receiver, more concretely the SIR _rec and SIRtarget values estimated at the receiver. Any method known in the state of the art for the estimation or calculation of the SIR _rec and SIR _ta rget values in a closed

loop power controlled channel can be applied, as for instance the methods disclosed in EP 1575185, ES 2214121, US 6434124 and US 6763244.

As shown in Figure 3, after calculating the mean SIR _rec and the mean SIR _ta rget values for a certain time interval at first steps (301-302), the method of this invention compares both magnitudes: If the SIR _rec is greater than the SIRtarget, a minimum transmission power state (304) is detected, that is, the transmitter is using the lowest power for that closed loop power controlled channel and a minimum power command is generated in a subsequent step

(305) to be sent to the applicable subsystem. Otherwise, if the SIRrec is lower than the SIR _ta rget, this means that the transmitter is still able to low the transmission power because is not at the lowest power condition (306) .

In the comparison (303) , minimum transmission power state (304) is determined if SIR _rec exceeds the SIR _ta rget in a predetermined quantity, for example, being a difference between these two magnitudes approximately of 1-3 dB .

The minimum power command generated only when the minimum transmission power state (304) is detected can be sent to a remote device of the mobile communications system from the device comprising said receiver, by means of transmission associated in a transceiver part of said device, which can be a mobile or a base station, depending on the application case. For example, the invention can be preferably used in the following different ways:

1. If a receive diversity subsystem is used in the mobile and/or the base stations, the invention allows the receiver to turn off some of the radio frequency chains in order to reduce power consumption without impacting the cell capacity. Receive diversity technology uses a plurality of antennas (401-402) with a number of radio frequency chains (403-404) at the receiver to improve the reception performance. The use of less reception chains saves battery life. In the situation detectable by this invention, when the minimum transmission power state (304) is determined and a subsequent minimum power command is sent, the receive diversity scheme does not lead to further benefit because the transmitter is already delivering its lowest possible power level. Then, a disconnect procedure to turn off the selected radio frequency chains for reducing the receive diversity is commanded directly from the receiver through the minimum power command, as Figure 4 illustrates. The output signals from the active radio frequency chains are combined in the combiner module (406) according to any scheme known in the art such as Maximal Ratio Combining (MRC) or Minimum Mean Squared Error (MMSE) . After the combining step, all the remaining processing functions in the receiver, represented by a generic "data sink" box (407), can be performed.

2. The invention also applies to the power control of the channel, because when minimum transmission power state (304) occurs the outer loop power control should not keep updating the SIR _ta rget, as the objective provided

is not correct any longer. In this situation, the SIR _rec value (501) is not following the SIR _ta rget value (502) . Therefore, it is desirable to stop or block the outer loop power control as soon as the minimum transmission power state (304) is detected according to the object of the invention and until the SIR _rec is able again to match the SIRtarget- To get this purpose, the minimum power command triggers the blocking of SIR _ta rget establishment in the closed loop power controlled channel, as depicted in Figure 5.

3. For radio access bearer reconfiguration, see Figure 6: if a low bit rate bearer (601) is assigned, the minimum power command is sent to signal the minimum transmission power state (304), so that the system can decide to improve the transmission performance through selecting a higher rate bearer (602), which translates to more bits (bo-bis) in a transmission time interval (T) . Under these circumstances, the bearer change usually does not imply any increase in transmitted power.

It is important to note that the previous applications are only examples of some preferred embodiments. Indeed, this invention can be used for any situation at any communication subsystem in which detecting when the transmitter gests the lowest power level is useful. An advantage of the invention relies on the fact that this detection is in any case performed at the receiver.

Some preferred embodiments of the invention are described in the dependent claims which are included next.