HK1073941B

HK1073941B - A power control subsystem

Info

Publication number: HK1073941B
Application number: HK05106502.3A
Authority: HK
Inventors: S．维伦格; 周渔君
Original assignee: 高通股份有限公司
Priority date: 1997-02-13
Filing date: 2005-07-29
Publication date: 2009-05-15

Description

Power control subsystem

This application is a divisional application of an invention patent application having an application date of 13/2/1998, an application number of 98802509.4, and an invention name of "power control subsystem".

Technical Field

The present invention relates to a communication system. More particularly, the present invention relates to a novel and improved method for independent closed loop power control of subchannels in a spread spectrum communication system.

Background

In a Code Division Multiple Access (CDMA) spread spectrum communication system, a common frequency band is used to communicate with all base stations in the system. An example of such a system IS described in TLA/IEA interim standard IS-96-a entitled "mobile station-base station compatibility standard for dual mode wideband spread spectrum cellular systems," which IS incorporated herein by reference. Generation and reception of CDMA signals is described in U.S. patent nos. 4,401,307 entitled "spread spectrum multiple access communication system using satellite and terrestrial repeaters" and 5,103,459 entitled "system and method for generating waveforms in a CDMA cellular telephone system," both of which are assigned to the assignee of the present invention and are incorporated herein by reference.

Signals occupying the common frequency band are identified at the receiving station by spread spectrum CDMA waveform characteristics based on a high speed pseudo-noise (PN) code. The signals transmitted from the base station and the remote station are modulated with PN codes. Signals from different base stations may be separately received at a receiving station by identifying unique time offsets introduced in the PN codes assigned to each base station. High-speed PN modulation also allows a receiving station to receive a signal from a single transmitting station, where the signal has traveled through different propagation paths. Demodulation of multiple signals is described in U.S. patent nos. 5,490,165 entitled "tunable element arrangement in a system capable of receiving multiple signals" and 5,109,390 entitled "diversity receiver in a CDMA cellular telephone system," both assigned to the assignee of the present invention and incorporated herein by reference.

The IS-95 Over The Air (OTA) interface standard defines a set of RF signal modulation procedures for implementing a digital cellular telephone system. The IS-95 standard, and its derivatives, such as IS95A and ANSI JSTD-008 (collectively referred to as the IS-95 standard), are promulgated by the Telecommunications Industry Association (TIA) to ensure operability between telecommunications equipment produced by different vendors.

The IS-95 standard has gained intense popularity because it uses the available RF bandwidth, which IS more efficient than previous cellular telephone technology. Increased efficiency is provided by using CDMA signal processing techniques in conjunction with large transmit power control to increase frequency reuse in cellular telephone systems.

Fig. 1 depicts a digital cellular telephone system configured in a manner consistent with IS-95 use. During operation, telephone calls and other communications are effectuated by exchanging data between the remote station 1 (typically a cellular telephone) and the base station 2 using RF signals. Communication is also effected from the base station 2 through a Base Station Controller (BSC)4 and a Mobile Switching Center (MSC)6 to a Public Switched Telephone Network (PSTN)8, or to another base station transmitting to another remote station 1. BSCs4 and MSC6 typically provide mobility control, call processing, and call routing functions.

The RF signals transmitted from a base station 2 to a group of remote stations 1 are referred to as the forward link and the RF signals transmitted from remote stations 1 to a base station 2 are referred to as the reverse link. The IS-95 standard requires that the remote station 1 provide telecommunication services in a manner that transmits user data, such as digitized voice data, via a reverse link signal. The reverse link signal is composed of a single traffic channel and is therefore often referred to as a "non-coherent" signal because it does not include a pilot channel and as such cannot be demodulated coherently.

In the reverse link signal, user data IS transmitted at a maximum data rate of 8.6 or 13.35kbps, depending on which rate set IS selected from a set of rate sets provided by IS-95. The use of a single channel, incoherent, reverse link signal simplifies the implementation of an IS-95 cellular telephone system by eliminating the need for synchronization between a group of remote stations 1 communicating with a single base station 2.

As described above, IS-95, in conjunction with extended transmit power control, more efficiently utilizes the useful RF bandwidth. According to IS-95, power control IS performed by measuring the received signal strength and the quality of the reverse link traffic channel at the time of reception at the base station, and generating a command for power control based on the measurement. The power control commands are transmitted to the remote station via a forward link signal. The remote station increases or decreases the transmit power of the reverse link signal in response to the power control command. This method of power control is referred to as closed loop power control. The design of closed loop power control in a CDMA communication system is described in U.S. patent No. 5,056,109, entitled "method and apparatus for controlling transmit power in a CDMA cellular mobile telephone system," which is assigned to the assignee of the present invention and is incorporated herein by reference.

In IS-95 systems, adjustments in power control are performed repeatedly, at a rate on the order of 800 times per second, in order to maintain the reverse link signal transmit power at the minimum necessary to communicate. In addition, IS-95 also requires that the transmit duty cycle of the reverse link signal be adjusted in response to changes in voice activity, varying the transmit duty cycle in 20 millisecond increments. Thus, when the transmit duty cycle is reduced, the remote station transmits at the set point, or is gated, and the remote station does not transmit. During gated transmission, the base station generates an erroneous power control increase command because the reverse link signal is not detected. Since the remote station knows when to gate its transmissions, it knows that they are erroneous and can ignore the corresponding increment command.

To meet the increasing demand for transmitting digital data generated by network technologies such as the global network, a more complex and higher speed multichannel correlated reverse link signal is provided in U.S. patent application No. 08/654,443(443 application entitled "high data rate CDMA wireless system," filed 5/28/1996), which is assigned to the assignee of the present invention and incorporated herein by reference. The above referenced patent application describes a system in which a set of individually gain adjusted channels are formed by using a set of orthogonal sub-channel codes. Data to be transmitted through one of the transmission channels is modulated by one of the subchannel codes (gain-adjusted) and summed with data modulated using the other subchannel codes. The resulting sum data is modulated with a user long code and a pseudorandom spreading code (PN code), and is transmitted by up-conversion. The above-referenced patent application describes, among other things, a reverse link signal comprised of Walsh sequence modulated subchannels including at least a traffic subchannel, a power control subchannel, and a pilot subchannel.

The multi-channel reverse link allows for increased flexibility by allowing different types of data to be transmitted simultaneously. Providing the pilot subchannel facilitates coherent processing of the reverse link signals at the base station, which improves the performance of the link. To facilitate power control, time tracking, and frequency tracking, it may be desirable to maintain the average received pilot signal power at a constant signal-to-noise ratio (SNR). Note that in CDMA-based systems, efficient power control is important to achieve high system capacity. In general, power control is divided into two parts, open loop and closed loop. In open loop power control, a mobile station measures a received forward link signal for a predetermined time interval and adjusts its transmit power in response to changes in the received forward link power. Open loop power control as performed in IS-95 IS very slow and takes care of long-term channel variations (known as angular effects). The previously described closed loop power control is faster and attempts to compensate for fading effects.

In IS-95 based CDMA systems, closed loop power control IS also used to drive the reverse link to a desired set point. For example, a Frame Error Rate (FER) of 1% may be desired. If the FER is too high, the reverse link power must be increased to reduce the error rate. On the other hand, if the FER is below the desired set point, the reverse link power may be reduced. Reducing the reverse link power reduces the interference generated, thereby having a direct positive effect on other users in the system. When each user transmits at the setpoint, maximum capacity is achieved in the CDMA system, thereby requiring minimum power to achieve the desired error rate.

The operating set point of the system can be changed by changing the power control decision threshold at the base station. As a result, the total average received power of the reverse link will converge to a new value. The power control mechanism affects the total transmit power. However, if this technique is applied to a system using a plurality of sub-channels (as specified in the 433 application), the relative strength of each sub-channel does not change when the total transmission power changes. For example, any subsequent change in the transmit power of the received FER that changes the data subchannel will affect the pilot power when it meets the required power value with respect to the received pilot subchannel power, and vice versa. Since different data types occupying different sub-channels may have different requirements, it is desirable to separately control each sub-channel of transmit power.

Summary of The Invention

The present invention provides a power control subsystem in a wireless communication system in which a remote station transmits a reverse link signal comprising a plurality of subchannel signals, the power control subsystem being located in a base station for independently adjusting the transmit power of one or more of the plurality of subchannel signals. The power control subsystem includes: a receiver for receiving the reverse link signal; a demodulator for demodulating said reverse link signal to provide said plurality of subchannel signals, comprising: a PN demodulator that demodulates the reverse link signal in accordance with a pseudo-random noise (PN) sequence to provide a remote station signal; and a plurality of quadrature demodulators, wherein each said quadrature demodulator receives said remote station signal and demodulates said remote station signal according to a quadrature demodulation sequence to provide a respective one of said subchannel signals; and message generator means for generating a power control message for adjusting said transmit power of at least one of said plurality of sub-channel signals in dependence on a quality measure or an energy measure associated with a respective one of said sub-channel signals.

The present invention provides a power control subsystem in a wireless communication system, comprising: a receiver for receiving the reverse link signal; a demodulator that demodulates the reverse link signal to provide the plurality of subchannel signals; message generator means for generating a power control message for adjusting said transmit power of at least one of said plurality of sub-channel signals in accordance with a quality measure or an energy measure associated with a respective one of said sub-channel signals; a plurality of accumulators, wherein each of said accumulators receives a respective one of said subchannel signals and accumulates energy of said subchannel signals over a predetermined time period to provide accumulated subchannel energy; and a comparator receiving said accumulated subchannel energy and comparing each of said accumulated channel energy to a respective one of a plurality of threshold values.

The present invention provides a power control subsystem in a wireless communication system, comprising: a receiver for receiving the reverse link signal; a demodulator that demodulates the reverse link signal to provide the plurality of subchannel signals; message generator means for generating a power control message for adjusting said transmit power of at least one of said plurality of sub-channel signals in accordance with a quality measure or an energy measure associated with a respective one of said sub-channel signals; a plurality of decoders, wherein each of said decoders receives a respective one of said sub-channel signals and determines the presence of a frame error in said sub-channel signal; and a comparator that compares a frame error rate based on the determined frame error with a frame error rate threshold.

In accordance with one aspect of the present invention, a power control subsystem in a wireless communication system in which a remote station transmits a reverse link signal comprising a plurality of subchannel signals is provided, the power control subsystem being located in a base station for independently adjusting the transmit power of each of the plurality of subchannel signals. The power control subsystem includes:

receiver means for receiving said reverse link signal and demodulating said reverse link signal to provide said plurality of subchannel signals;

quality measuring means for receiving each of the plurality of subchannel signals and measuring a quality of each subchannel signal; and

message generator means for generating a power control message based on said measured quality of at least one of said plurality of subchannel signals, said power control message being used to adjust the transmit power of said at least one subchannel signal.

In accordance with another aspect of the present invention, a remote station power control subsystem in a wireless communication system in which a remote station transmits a reverse link signal comprising a plurality of subchannel signals is provided, the remote station power control subsystem independently adjusting the transmit power of each subchannel signal in accordance with a received power control message. The power control subsystem includes:

receiver means for receiving said power control message and for providing a plurality of gain values in accordance with said power control message;

a plurality of gain adjustment devices, wherein each gain adjustment device is configured to receive a corresponding subchannel signal and a corresponding gain value, and adjust a gain of the subchannel signal according to the gain value.

In accordance with yet another aspect of the present invention, a method is provided for controlling the transmit power of a remote station transmitting a reverse link signal comprising a plurality of subchannel signals. The method comprises the following steps:

receiving the reverse link signal;

demodulating the reverse link signal to obtain the plurality of subchannel signals;

generating a power control message for adjusting a transmit power of at least one of said plurality of subchannel signals based on a quality measurement or an energy measurement associated with a corresponding one of said subchannel signals;

transmitting the power control message to the remote station; and

controlling the transmission power of the at least one of the plurality of sub-channel signals according to the power control message.

In accordance with yet another aspect of the present invention, a method is provided for controlling the transmit power of a remote station transmitting a reverse link signal comprising a plurality of subchannel signals, the transmit power of one or more of the plurality of subchannel signals being independently adjusted in accordance with a received power control message. The method comprises the following steps:

receiving the power control message;

obtaining one or more gain values from the power control message; and

at one or more of the plurality of gain adjusters, a respective subchannel signal and a respective gain value are received, and the gain of each subchannel signal is independently adjusted according to the gain value.

Brief description of the drawings

The features, objects, and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings in which like reference characters designate corresponding parts throughout the several views, and wherein:

FIG. 1 is a block diagram of a cellular telephone system;

FIG. 2 is a block diagram of a remote station configured in accordance with an exemplary embodiment of the present invention; and

fig. 3 is a block diagram of a base station configured in accordance with an exemplary embodiment of the present invention.

Detailed description of the preferred embodiments

In the following description of exemplary embodiments of the present invention, a subchannel control loop controls the reverse link. Thus, a transmitting station is referred to as a remote station and a receiving station is referred to as a base station. The remote stations may include wireless local loop stations, cellular telephones, data terminals, and the like. It goes without saying that the present invention can also be used on the forward link alone or on both the forward and reverse links.

In a channel with N sub-channels, the total transmission power P of the remote station is adjusted_wtDefined as the sum of the transmit power of each subchannel:

P_wt＝P₀+P₁+...+P_N (1)

the remote station transmits power P by changing corresponding sub-channel_iThe set point for a particular subchannel (i.e., subchannel i) may be changed while the operating points for the other subchannels remain unchanged.

Equation (1) can be passed through any power P_refAnd (6) normalizing.

Pwt＝(F₀+F₁+...+F_N)*P_ref (2)

In an exemplary embodiment, by adjusting the power P_refThe power control is completed. Each subchannel control loop is controlled by adjusting P_iTo work with a particular one or subset of.

Fig. 2 depicts an exemplary remote station. In the remote station 100, a plurality of data signal data 0-data N enter the encoders 110A-110N. The encoded results are interleaved in interleavers 120A-120N and then modulated with unique Walsh sequences in spreaders 130A-130N. The outputs of multipliers 130A-130N are amplified in gain adjustment blocks 140A-140N with a unique gain value provided from gain control processor 180. The gain adjustment blocks 140A-140N may be implemented using digital techniques or may be implemented using variable gain multipliers, both of which are known in the art.

In an exemplary embodiment, Walsh sequence 0 (W)₀) Modulating the constant value to form a pilot signal. Also, in the exemplary embodiment, the data input to multiplier 130A is fixed and does not require encoder 110A and interleaver 120A. The gain adjusted signals are combined in summer 150. The adder 150 may operate as a digital or analog device. Although the adder 150 may be digital if the gain adjustment blocks 140A-140N are digital, the gain adjustment blocks 140A-140N may be analog if they are analog, it is not necessary to do so. The signal consisting of the sum of the individual gain-adjusted digital signals is amplified in the gain adjustment block 160 at the gain value provided by the gain control processor 180. Gain adjustment block 140A is not necessary since pilot gain adjustment can be accomplished by gain adjustment block 160. Alternatively, if the full gain is factored into each subchannel gain, gain adjustment block 160 may be eliminated. In both cases, there is no control penalty since each subchannel gain, and thus the overall signal gain, can still be varied independently. The total signal coming out of the gain adjustment block 160 is modulated and up-converted in the transmitter 170 and then transmitted on the antenna 230 through the duplexer 220. For other gain adjustment blocks, gain adjustment 160 may be performed using digital or analog techniques.

Forward link data (including power control message information) from the base station is downconverted and amplified in receiver 210 through antenna 230 to duplexer 220. The received signal is demodulated in a demodulator 200 and then deinterleaved and decoded in a decoder 190. In an exemplary embodiment, demodulator 220 is a CDMA demodulator as described in U.S. patent nos. 4,401,307 and 5,103,459, supra. The subchannel power control messages from the base station are separated in gain control processor 180 from the forward link data decoded by decoder 190.

These messages independently control the gain values in the gain adjustment blocks 140A-140N and 160. The adjustment of the gain value can be performed in a number of ways. For example, the subchannel power control message may be composed of N bits, where each N bit steers the corresponding subchannel to increase or decrease the transmit power. In response to this message, each gain value is increased or decreased by a predetermined amount, which may be for all subchannels or unique for each subchannel. Alternatively, the subchannel power control message may contain N binary sequences representing the gain values or representing the amount of change in the gain values. The control message may control each gain value or a set of gain values independently, and may employ a combination of techniques for each.

Fig. 3 shows an exemplary base station. In base station 300, a signal comprising the sum of all signals transmitted from remote stations operating in the system enters through antenna 310 and is down-converted and amplified in receiver 320. PN demodulator 330 extracts a set of signals transmitted by a particular remote station, such as remote station 100. The PN demodulated signals are directed to a plurality of Walsh demodulators 340A-340N. Each Walsh demodulator demodulates a corresponding subchannel of a signal transmitted by remote station 100.

In an exemplary embodiment, the subchannels demodulated by Walsh demodulators 340A-340N may be deinterleaved and decoded in decoders 350A-350N. Data from the decoders 350A-350N is passed to a comparator 370. A metric useful for the calculation of comparator 370 is a measure of the Frame Error Rate (FER). The frame error rate for each subchannel may be compared to the FER threshold provided by threshold generator 380. Power in the subchannel may be reduced if the frame error rate of the subchannel is below a value necessary for the desired communication quality. Conversely, if the frame error rate of a subchannel is too high, the subchannel needs to increase its power.

In another embodiment, the energy in each subchannel signal is summed in accumulators 360A-360N. The energy sum is passed to comparator 370. Receiver 320 typically contains an automatic gain control circuit (AGC) that normalizes the in-band energy to a predetermined value. Parameters related to AGC may be transmitted to comparator 370 to help normalize the energy values for comparison. Comparator 370 compares the received energy in each sub-channel with the energy threshold for that channel as determined by threshold generator 380. An energy threshold is calculated to ensure the quality of service on each subchannel. The power of each subchannel may be adjusted based on the comparison. The power may be reduced if the threshold is exceeded and reduced if the threshold is not exceeded. In addition, the two embodiments may work in conjunction with each other by allowing the energy threshold to vary in response to a frame error rate or other signal quality metric.

It is envisioned that many other applicable methods may be used for the comparison in comparator 370. When the decoders 350A-350N use a Viterbi algorithm, Viterbi decoder metrics may be provided for the comparison. Other examples include comparison of symbol error rates instead of frame error rates, and Cyclic Redundancy Check (CRC) calculations. May be sent to threshold generator 380 by base station controller 4 as shown in fig. 1, or may be calculated within threshold generator 380.

In an exemplary embodiment, comparator 370 makes a decision whether to increase or decrease the power value of each subchannel based on the received subchannels. Based on this determination, message generator 390 generates a power control message to be sent to the remote station to change any of the subchannels, if necessary. The power control message may be transmitted as signaling data or punctured into the data stream as described by IS-95, or any other signaling means capable of forwarding the message to the mobile station. As discussed previously, the message may be a simple command in the forward or reverse direction for each subchannel, or as complex as sending an accurate gain value for each subchannel. In addition, each subchannel may be controlled independently, or subchannel power control messages may control groups of subchannels. The power control message is modulated in modulator 400, up-converted and amplified in transmitter 410, and transmitted via antenna 420 to remote station 100. As described above, the remote station 100 changes the gain value associated with each subchannel so that the subchannel control loop is closed.

In another embodiment, the gain values of the gain adjustment blocks 140A-140N may be calculated in an open loop manner. Based on the received forward link signal, each gain adjustment value may be calculated using a predetermined gain calculation algorithm in the gain control processor 180. For example, different sub-channels may have different coding for error correction, whereby the error rate will vary for the drop in received power given by fading. The predetermined gain calculation algorithm may be improved using empirical studies.

In another embodiment, if the present invention is used on both the forward and reverse links, the gain of the corresponding reverse link subchannel may be adjusted using an open loop calculation of the energy received on the forward link subchannel, and vice versa. In the case of symmetry or partial symmetry for the forward or reverse link, the energy received in the subchannel may be used to calculate to determine the power value for the corresponding transmit subchannel. A combination of open-loop and closed-loop techniques may also be used.

The previous description of the preferred embodiments of the invention is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the specific details. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A power control subsystem in a wireless communication system in which a remote station transmits a reverse link signal comprising a plurality of subchannel signals, the power control subsystem located in a base station for independently adjusting the transmit power of each of the plurality of subchannel signals, the power control subsystem comprising:

2. The power control subsystem of claim 1, further comprising:

a modulator for modulating the power control message according to a modulation format.

3. A remote station power control subsystem in a wireless communication system in which a remote station transmits a reverse link signal comprising a plurality of subchannel signals, the remote station power control subsystem independently adjusting the transmit power of each subchannel signal in response to a received power control message, the power control subsystem comprising:

4. A method of controlling transmission power of a remote station transmitting a reverse link signal comprising a plurality of subchannel signals, comprising the steps of:

receiving the reverse link signal;

transmitting the power control message to the remote station; and

5. The method of claim 4, wherein said generating step comprises generating said power control message for adjusting transmission power of said plurality of subchannel signals; and is

The controlling step includes independently controlling the transmission power of the plurality of subchannel signals according to the power control message.

6. A method of controlling transmission power of a remote station transmitting a reverse link signal comprising a plurality of subchannel signals, the transmission power of one or more of the plurality of subchannel signals being independently adjusted in accordance with a received power control message, the method comprising:

receiving the power control message;

obtaining one or more gain values from the power control message; and