US 20060183497 A1
A mobile communications system includes base stations and mobile units. A power control scheme is provided in which a mobile unit can enter into a discontinuous transmission (DTX) mode. During DTX mode, the mobile unit is not transmitting traffic channels that can be monitored to determine frame errors so that the target ratio of energy per bit to noise spectral density (target Eb/No) can be adjusted. Instead, the base station monitors bit errors of bits in a pilot channel communicated by the mobile unit during DTX mode. Using this technique, the target Eb/No can be adjusted even when the mobile unit is not transmitting traffic channels, so that outer-loop power control can be performed. A number of mechanisms can also be used to detect when a mobile unit has entered DTX mode.
1. A method of performing power control in a mobile communications system having a base station and a mobile unit, comprising:
detecting an error in control signaling transmitted over a reverse link between the base station and the mobile unit when traffic channels are not being communicated in the reverse link; and
adjusting a power control element based on the detected error,
wherein adjusting the power control element comprises adjusting a target ratio of energy per bit to noise spectral density based on the detected error in the control signaling.
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14. An article comprising at least one storage medium containing instructions that when executed cause a system to:
detect an error in control signaling transmitted over a reverse link from a mobile unit when traffic is not being communicated in the reverse link; and
adjust a target ratio of energy per bit to noise spectral density based on the detected error in the control signaling.
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18. A system for use in a communications system, comprising:
a receiver to receive control signaling from a mobile unit; and
a controller to:
detect an error in the control signaling received from the mobile unit in the reverse link when traffic is not being communicated from the mobile unit; and
adjust a target ratio of energy per bit to noise spectral density based on the detected error in the control signaling.
19. The system of
This is a divisional of U.S. Ser. No. 09/696,491, entitled “Performing Power Control in a Mobile Communications System,” filed Oct. 25, 2000, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 60/168,845, entitled “Performing Power Control in a Mobile Communications System,” filed Dec. 3, 1999, both hereby incorporated by reference.
The invention relates to performing power control in a mobile communications system.
Mobile communications systems, such as cellular or personal communications services (PCS) systems, are made up of a plurality of cells. Each cell provides a radio communications center in which a mobile unit establishes a call with another mobile unit or a wireline unit connected to a public switched telephone network (PSTN). Each cell includes a radio base station, with each base station connected to a mobile switching center that controls processing of calls between or among mobile units or mobile units and PSTN units. In an alternative arrangement, the base stations may be connected to base station controllers that are in turn connected to the mobile switching center.
One type of protocol used in mobile communications is the code-division multiple access (CDMA) protocol, originally implemented as the IS-95 protocol. CDMA is based on a spread-spectrum technique in which multiple users have access to the same band of carriers. This is accomplished by assigning a unique orthogonal code (referred to as the Walsh code) to each mobile unit. To meet requirements for third generation (3G) wireless communications systems, the CDMA technology has been replaced with the CDMA 2000 family of standards. The standards in the CDMA 2000 family include the IS-2000-1, IS-2000-2, IS-2000-3, IS-2000-4, IS-2000-5, and IS-2000-6 (collectively referred to as the “IS-2000 standard”).
According to IS-2000, a mobile station may discontinue the transmission of traffic channels to preserve power and reduce interference when there is no data to send. Such a feature is referred to as the discontinuous transmission (DTX) mode. The DTX feature is available on fixed-rate traffic channels, such as the dedicated control channel and supplemental channel, in the reverse link between the base station and each mobile unit. The CDMA air-link is based on a forward link and reverse link between base stations and mobile units. The forward link is the link from the base station to the mobile unit, while the reverse link is the link from the mobile unit to the base station. Among the channels used in the reverse link are the dedicated control and supplemental channels, which are employed to carry traffic frames and other signaling from the mobile unit to the base station.
Conventionally, in performing outer-loop power control without DTX, such as in IS-95 CDMA systems, the target ratio of energy per bit to noise spectral density (referred to as the target Eb/No) is adjusted based on whether errors occurred in receiving frames. A target Eb/No may be specified for each mobile unit. The target Eb/No may be increased when the frame error rate (FER) rises above a predetermined threshold and decreased when the FER falls below the predetermined threshold. Alternatively, adjustment of the target Eb/No may be based on occurrence of frame errors rather than the FER crossing a threshold. By coordinating the step sizes of the increases and decreases in the target Eb/No, a target FER can be achieved.
If the transmission of traffic frames is discontinued, however, the conventional technique of controlling the target Eb/No based on the transmission of traffic frames during the period of silence is not possible, even if the receiver in the base station can detect that the transmission of frames has ceased. The reception of traffic frames is no longer available to determine exactly what target Eb/No is required for a given quality of reception. Without a way of adjusting the target Eb/No, reliable transmission of frames may not be possible when the mobile unit exits DTX mode and starts transmitting traffic frames again.
Setting a fixed target Eb/No does not entirely solve the problem, since the Eb/No required for successful transmission of traffic frames may vary during the period of silence, in which case no frames may be successfully received at the end of the silence period and a dropped call may result. The target Eb/No may alternatively be set to a high value to ensure reception at the end of the silence period. Such a high value may be the worst-case value based on the velocity of the mobile unit and the known channel conditions. However, setting a high target Eb/No may result in the transmission of excessive power on the pilot, traffic, and power control channels, which may significantly reduce the power reduction that is supposed to result from the use of DTX.
Thus, an improved method and apparatus for performing power control is needed in a mobile communications system.
In general, according to one embodiment, a method of performing power control in a mobile communications system having a base station and a mobile unit comprises detecting an error in reception of predetermined information in a link between the base station and the mobile unit when traffic channels are not being communicated. A power control element is adjusted based on the detected error.
Some embodiments of the invention may have one or more of the following advantages. Effective power control is provided in a link between a mobile unit and a base station, even during periods of silence, such as during discontinuous transmission (DTX) mode. A balance between energy savings and reliable wireless communications can be achieved so that battery life in a mobile unit is extended while the likelihood of dropped calls is reduced.
Other features and advantages become apparent from the following description, from the drawings, and from the claims.
In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. For example, although reference is made to the code-division multiple access (CDMA) protocol in this description other alternative wireless standards can also be employed.
In accordance with some embodiments of the invention, power control, such as outer-loop power control in a CDMA system, can continue to be performed even though transmission of traffic frames in a wireless link between the mobile unit and base station has been discontinued during discontinuous transmission (DTX) mode. Although traffic signaling is discontinued on some traffic channels in DTX mode, control signaling with a known pattern may still be transmitted. Such control signaling may be monitored to detect for errors so that appropriate power control may be performed. In one embodiment, the power control includes adjusting a power control element such as the target ratio of energy per bit to noise spectral density (Eb/No). As used here, a “power control element” refers to any element (e.g., parameter, component, etc.) that can be adjusted to affect the transmission power of signals in wireless links between mobile units and base stations.
In one embodiment, the mobile communications system 10 is a code-division multiple access (CDMA) system, such as a CDMA 2000 system. The CDMA 2000 family of standards includes the IS-2000-1, IS-2000-2, IS-2000-4, IS-2000-5, and IS-2000-6 standards.
As the mobile unit 16 crosses from one cell segment 12 to another cell segment, a hand-off procedure is performed by the base stations. CDMA provides for a soft hand-off procedure in which a mobile unit is directed to hand-off to the same frequency and is assigned to an adjacent cell or sector without dropping the original radio frequency (RF) link. During the soft hand-off process, the mobile unit may keep two or more RF links active. In a CDMA system, a RF link refers to a link associated with a given pseudorandom noise (PN) code. CDMA systems also perform softer hand-offs in which mobile units perform hand-offs within different sectors of the same cell site.
The base stations 14 and the cell segments 12 are connected to a mobile switching center (MSC) 18 that provides the main control functions of the mobile communications system 10. In other arrangements, base station controllers (BSCs) may be provided between corresponding groups of base stations 12 and the MSC 18. The MSC 18 may be connected to other MSCs that are responsible for other service areas. The MSC 18 may also be connected to a public switched telephone network (PSTN) 20 that is coupled to wireline units to enable communications between the mobile units 16 and the wireline units.
The air link between each base station 14 and each mobile unit 16 includes a forward link 22 (from the base station to the mobile unit) and a reverse link 24 (from the mobile unit to the base station). According to CDMA 2000, the reverse link 24 includes a pilot channel, a power control subchannel, a traffic channel, and other channels. The traffic channel may include a dedicated control channel (DCCH), fundamental channel (FCH), supplemental channel (SCH), and other channels. The DCCH and SCH in the traffic channel are fixed-rate channels on which DTX mode is available. During DTX mode, the fixed-rate channels in the reverse link 24 are not transmitted to preserve power and to reduce interference. However, transmission of the pilot channel and power control subchannel in the reverse link 24 continues.
The pilot channel in the reverse link 24 (also referred to as a reverse pilot channel) is an unmodulated, direct-sequence spread spectrum signal transmitted continuously by a mobile unit. A reverse pilot channel provides a phase reference for coherent demodulation and may provide a mechanism for signal strength measurement. The reverse power control subchannel is a subchannel on the reverse pilot channel used by the mobile unit to control the power of a base station when operating under certain configurations.
The reverse DCCH is a portion of the reverse traffic channel used in some configurations for the transmission of higher-level data and control information from a mobile unit to a base station. The reverse FCH is also a portion of the reverse traffic channel that carries higher-level data and control information from a mobile unit to a base station. A reverse SCH is a portion of the traffic channel, which operates in conjunction with a reverse FCH or reverse DCCH to provide higher data rate services and on which higher-level data is transmitted. In the forward link 22, the forward traffic channel may also include a forward DCCH, a forward FCH, and a forward SCH.
During active communications of traffic channels between the base station and mobile unit, the base station monitors for occurrence of frame errors in one or more of the reverse DCCH, reverse FCH, and reverse SCH to adjust the target Eb/No in the base station, and the mobile unit monitors one or more of the forward DCCH, forward FCH, and forward SCH to adjust the target Eb/No in the mobile unit. The target Eb/No values determine the frame error rate (FER) on the corresponding channel.
However, when the mobile unit enters DTX mode, the transmitter in the mobile unit 16 can cease transmission of DCCH and SCH traffic frames without warning the base station 14. As a result, the base station 14 is not notified explicitly when transmission of traffic frames has stopped. This results in an outer-loop power control problem, especially if fast power control is being employed.
Once traffic frames stop, conventional techniques of detecting for errors in the reception of traffic frames is not available. As a result, the ratio of energy per bit to noise spectral density (Eb/No) cannot be accurately adjusted by conventional techniques during this period of traffic silence. Typically, the target Eb/No is increased when data is received in error and decreased when data is received correctly. An increase in the target Eb/No results in reduced capacity in a cell due to the likelihood of enhanced interference between mobile units. On the other hand, a decrease in the target Eb/No (and thus transmission power) increases the likelihood that the call may be dropped. To achieve proper outer-loop power control to avoid these issues, in accordance with some embodiments, control signaling instead of the traffic signaling is used to perform adjustment of the target Eb/No value. Also, to enable outer-loop power control based on the control signaling instead of traffic signaling, a mechanism is provided to detect that a mobile unit has entered DTX mode.
In further embodiments, the base station 14 can also enter DTX mode, in which case adjustment of Eb/No by the mobile unit is also based on control signaling rather than traffic channels.
The base station 14 also includes a medium access control (MAC) layer 136 (containing modules for managing communications on the communications link, in this case, RF links); a signaling link access control (LAC) layer 138 (providing a radio-independent interface and containing modules for controlling data flow and transporting the data flow to the MAC layer 136); and an upper layer 140 including signaling control modules, protocol stacks, and the software routines 102.
In the mobile unit 16, one or more control units 110 are coupled to one or more storage units 112 and to a radio interface 114 (including a transmitter 118 and receiver 120) that is capable of communicating with the air links 22, 24. Software modules 116 may also be executable in the mobile unit 16. In another embodiment, instead of software routine 116, the control unit 110 may be configured in hardware or by firmware to perform pre-programmed tasks. The mobile unit 16 similarly includes a MAC layer 130, a LAC layer 132, and an upper layer 134. As with the base station 14, one of the software modules 116 may be programmed to perform power control tasks.
However, if the power control routine determines (at 202) that the mobile unit is in DTX mode, then the base station 14 checks (at 208) for errors in bits received on the pilot channel. As shown in
The predetermined information in the pilot channel 302 includes known values (all ones in one embodiment). If any of the received bits is not at the expected state, then a bit error is recorded (at 210), such as in a storage register 122 in the storage device 104 in the base station 14 (
The power control tasks performed in the base station 14, as illustrated in
The detection and comparison acts at 208 and 210 may be performed in one of several ways. In one embodiment, a fixed number of bits in the pilot channel may be observed in a given time interval. The BER is then computed for that time window. The storage register 122 that records the result of a prior comparison may be cleared in preparation for the accumulation of bit errors in a disjoint time window. In another embodiment, a “sliding window” may be used in which the last N bits in the pilot channel are stored and the BER is updated with the arrival of each new pilot bit measurement. In further embodiments, other methods may be used to derive the BER from the sequence of pilot bit errors.
Based on the detection of error or lack thereof, the power control routine in the base station adjusts (at 212) the target Eb/No, once per frame interval. The target Eb/No is increased if the BER reported on each link is greater than the threshold value and decreased if one of them is lower than the threshold value. An “excessive BER event” is one in which the measured BER of a link is larger than the BER threshold. The up and down step sizes of the target Eb/No are derived from a desired probability of an excessive BER event in the same way that the step sizes are derived from the target FER in normal IS-2000 operation. Once the mobile unit comes out of DTX mode, the target Eb/No value that has been updated in the base station is used to generate (at 214) power control commands for the mobile unit, which are communicated to the mobile unit. The power control commands from the base station to the mobile unit controls the output power of the transmitter 118 (
In the illustrated embodiment of
The mechanism for detecting when a mobile has entered DTX mode (at 202 in
With the power detection based mechanism, the receiver 124 in the base station 14 attempts to detect the traffic channel transmission disruption based on the received signal. In one embodiment, the power level (or Eb/No) of the traffic channel on the DCCH is used for detection since the power level is relatively high when traffic bits are sent and low when they are not. To account for power level variations with fading, the fading effect is eliminated by using the ratio of (Eb/No)traffic to (Eb/No)pcb, where pcb stands for power control bits, as a decision variable. The ratio correlates fading over traffic bits with fading over power control bits in the same power control group.
The decision variable V is measured over a frame:
In the alternative mechanism, which is the signaling-based mechanism, one or two power control bits for each frame are used as a traffic channel transmission disruption indicator. In this way, the base station is informed whether the traffic channel in the frame is transmitting or not. For example, the first power control bit in a frame can be used to indicate the traffic channel status of the current frame. A “1” represents the presence of traffic channel, and a “0” stands for the traffic channel being gated off (in DTX mode). If the receiver detects a bit indicating that a normal frame is being transmitted, then the base station 14 determines that the mobile unit is not in DTX mode. However, if the detected bit indicates that the current frame is a DTX frame, then the receiver still tries to detect the frame (in case the detected bit is erroneous). If the frame is decoded correctly, then that means the frame is not really a DTX frame. However, if the detection indicates a bad frame, then DTX mode is detected.
By using embodiments of the invention, more effective power management is provided in each mobile unit. Adjustment of the Eb/No value during periods of traffic silence allows for reduction in power levels (and thereby increase the battery life of a mobile unit) while ensuring reliable communications due to traffic patterns and conditions after the mobile unit exits DTX mode and traffic signaling is started again. Although described in conjunction with the DTX mode of a mobile unit, the procedure described may be used for the base station if it is capable of entering DTX mode or some mode in which the base station stops sending traffic signaling. In further embodiments, the mobile communications system 10 may be a system other than a CDMA or CDMA 2000 system.
Various software routines or modules described herein are executable on control units in corresponding systems or nodes. Instructions of such software routines or modules may be stored in one or more storage units. Each control unit includes a microprocessor, a microcontroller, a processor card (including one or more microprocessors or microcontrollers), or other control or computing devices. As used here a “controller” refers to software, hardware, or a combination of both.
The storage units include one or more machine-readable storage media for storing data and instructions. The storage media include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs), and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; and optical media such as compact discs (CDs) or digital video discs (DVDs). Instructions that make up the various software routines or modules in various systems or nodes and stored in respective storage units when executed by a respective control unit cause the corresponding system or node to perform programmed acts.
The instructions of the software routines or modules are loaded or transported into the system or node in one of many different ways. For example, code segments including instructions stored on floppy disks, CD or DVD media, a hard disk, or transported through a network interface card, modem, or other interface device are loaded into the system and executed as corresponding software routines or modules. In the loading or transport process, data signals that are embodied in carrier waves (transmitted over telephone lines, network lines, wireless links, cables, and the like) may communicate the code segments, including instructions, to the system or node. Such carrier waves may be in the form of electrical, optical, acoustical, electromagnetic, or other types of signals.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.