HK1032861A

HK1032861A - Method and apparatus for providing ternary power control in a communication system

Info

Publication number: HK1032861A
Application number: HK01103437.4A
Authority: HK
Inventors: R‧帕多瓦尼
Original assignee: 夸尔柯姆股份有限公司
Priority date: 1997-12-04
Filing date: 1998-11-30
Publication date: 2001-08-03

Description

Method and apparatus for providing ternary power control in a communication system

Technical Field

The present invention relates to data communication. More particularly, the present invention relates to improved new methods and apparatus for providing power control in a communication system.

Background

The use of Code Division Multiple Access (CDMA) modulation techniques is one of several techniques that facilitate communications in which a large number of system users are present. Other techniques such as Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA is significantly superior to other modulation techniques of these multiple access communication systems. The use of CDMA techniques in multiple access communication systems is disclosed in U.S. patent No. 4,901, 307, entitled "spread spectrum multiple access communication system using satellite or terrestrial repeaters," which is assigned to the assignee of the present invention and the contents of which are incorporated herein by reference. The use of CDMA techniques in multiple access communication systems is further disclosed in U.S. patent No. 5,103, 459, entitled "system and method for generating signal waveforms in a CDMA cellular telephone system," which is also assigned to the assignee of the present invention and is hereby incorporated by reference. In addition, CDMA systems may be designed according to the "TIA/EIA/IS-95 Mobile station-base station compatibility Standard for Dual-mode wideband spread-spectrum cellular systems," which IS hereinafter referred to as the IS-95 Standard or TIA/EIA/IS-95.

Since CDMA is an inherent property of wideband signals, CDMA provides a form of frequency diversity by spreading the signal energy over a wide bandwidth. Thus, selective fading of frequency affects only a small portion of the CDMA bandwidth. Space or path diversity is achieved by providing multiple signal paths through a synchronous link to a mobile user or remote station through two or more base stations. In addition, path diversity can be obtained by exploiting the multipath environment through spread spectrum processing by separately receiving and processing signals arriving with different propagation delays. Examples of path diversity are shown in U.S. patent No. 5,101,501 entitled "method and system for providing soft handoff in communications in a CDMA cellular telephone system" and U.S. patent No. 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 reverse link refers to transmission from the remote station to the base station. On the reverse link, each transmitting remote station becomes interfering with other remote stations in the network. The capacity of the reverse link is limited by the total interference transmitted from other remote stations. CDMA systems increase the capacity of the reverse link by transmitting fewer bits, thereby using less power and reducing interference when the user is not speaking.

To minimize interference and maximize reverse link capacity, the transmit power of each remote station is controlled by three reverse link power control loops. The first power control loop adjusts the transmit power of the remote station by setting the transmit power inversely proportional to the power of the received forward link signal. In IS-95 systems, the transmission power of a remote station IS defined by P_out=-73-P_inGiven here P_inReceived power, P, of a remote station given in dBm_outThe remote station's transmit power, given in dBm, -73 is a constant. This power control loop is also called open loop.

The second power control loop adjusts the transmit power of the remote station such that the signal quality of the reverse link signal received at the base station is increased by adding noise per bit energyInterference E_b/I_oMeasured) is maintained at a predetermined level. This level is called E_b/I_oA set point. Base station measuring E of reverse link signal received at base station_b/I_oAnd in response to measured E_b/I_oA reverse power control bit is transmitted to the remote station on the forward traffic channel. For an IS-95 communication system, the reverse power control bits are transmitted 16 times per 20 ms frame, or one power control bit per power control group for an effective rate of 800 bps. The forward traffic channel transmits the reverse power control bits to the remote station along with data from the base station. This second loop is also called a closed loop.

CDMA communication systems typically transmit data packets in discrete data frames. Thus, the required performance level is typically measured by the Frame Error Rate (FER). The third power control loop so regulates E_b/I_oSet point to maintain a desired level of performance as measured by FER. Required E to maintain a given FER_b/I_oDepending on the propagation conditions. This third loop is also called an outer loop. The power control mechanism for the reverse link is disclosed 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 the contents of which are incorporated herein by reference.

The forward link refers to transmission from the base station to the remote station. On the forward link, the transmit power of the base station is controlled for several reasons. High transmit power from the base station may cause excessive interference to other signals received at the remote station. On the other hand, if the transmit power of the base station is too low, the remote station may receive an erroneous data transmission. Terrestrial channel fading and other well-known factors can affect the quality of the forward link signal received by the remote station. As a result, each base station test adjusts its transmit power to maintain a desired level of performance at the remote station.

Power control for the forward link is particularly important for data transmission. Data transmission is typically asymmetric, with the amount of data transmitted on the forward link being greater than the amount of data transmitted on the reverse link. The overall forward link capacity is increased by the presence of an efficient power control mechanism on the forward link that controls the transmit power to a performance level required for protection.

A method and apparatus for controlling forward link transmit power is disclosed in U.S. patent application No. 08/414, 633 entitled "method and apparatus for fast forward power control in a mobile communication system" filed on 31/3/1995, which is assigned to the assignee of the present invention and the contents of which are incorporated herein by reference. In the method disclosed in U.S. patent application No. 08/414, 633, a remote station transmits an Error Indication Bit (EIB) message to a base station when a transmitted data frame is received in error. The EIB may be a bit contained in a reverse traffic channel frame or a separate message sent on the reverse traffic channel. In response to this EIB message, the base station increases or decreases its transmit power to the remote station.

One disadvantage of this approach is the long response time. The processing delay comprises the time interval from the time the base station transmits a frame at insufficient power to the time the base station adjusts its transmit power in response to an error message from the remote station. This processing delay includes the time it takes (1) for the base station to transmit a data frame at insufficient power, (2) for the remote station to receive the data frame, (3) for the remote station to detect a frame error (e.g., a frame erasure), (4) for the remote station to send an error message to the base station, and (5) for the base station to receive the error message and adjust its transmit power appropriately. Forward traffic channel frames must be received, demodulated and decoded before an EIB message is generated. The reverse traffic channel frame carrying this EIB message must then be generated, encoded, transmitted, decoded and processed before the bits can be used to adjust the transmit power of the forward traffic channel.

Typically, the desired performance level is one percent FER. Thus, on average, the remote station transmits an error message indicating a frame error every 100 frames. According to the IS-95 standard, each frame IS 20 milliseconds long. Such an EIB based on power control is suitable for adjusting the forward link transmission power to handle shadowing (shadowing) situations, but is not suitable for fading other than the slowest fading situation because of its slow speed.

A second case for controlling forward link transmit power utilizes E of the signal received at the remote station_b/I_o. E due to FER and received signal_b/I_oIn relation thereto, the power control mechanism can be designed to be E_b/I_oMaintained at the desired level. This design encounters a challenge if data is transmitted on the forward link at a variable rate. On the forward link, the transmit power is adjusted depending on the data rate of the data frame. At lower data rates, each data IS transmitted over a longer period of time by repeating the modulation symbol (symbol) as described in TIA/EIA/IS-95. Energy per bit E_bIs the accumulation of the received power over a one bit time period, which is obtained by accumulating the energy in each modulation symbol. For equal number of E_bEach data bit may be transmitted at a transmit power that is proportionally reduced at a lower data rate. Typically, the remote station does not know the transmission rate in advance, and it is not possible to calculate the energy per bit E received_bUntil the entire data frame has been demodulated, decoded and the data rate for the data frame has been determined. Thus, the delay of this approach is approximately that described in the aforementioned U.S. patent application No. 08/414, 633, where the rate is one power control message per frame. This IS different from the reverse link power control mechanism described above (where one power control message (bit) IS sent sixteen times per frame as specified by TIA/EIA/IS-95).

Other methods and apparatus for fast forward link power control are disclosed in the aforementioned us patent application No. 08/414, 633, 08/559 entitled "method and apparatus for fast forward power control in a mobile communication system" filed on 15.11.1995, us patent No. 386, us patent No. 08/722 entitled "method and apparatus for measuring link quality in a spread spectrum communication system" filed on 27.9.1996, us patent application No. 08/710 entitled "method and apparatus for distributed forward power control" filed on 16.9.1996, us patent application No. 335, and us patent application No. 08/752,860 entitled "adjusting power control thresholds/measurements by presetting power control commands that have not yet been executed" filed on 20.11.1996, all of which are assigned to the assignee of the present invention, the contents of which are incorporated herein by reference.

For IS-95 systems, the basic difference between the forward and reverse links IS that the transmission rate on the reverse link need not be known. As described in U.S. patent No. 5,056,109, above, at lower rates, the remote station does not transmit continuously. When a remote station is transmitting, the remote station transmits at the same power level using the same waveform structure, regardless of the transmission rate. E of base station according to received reverse link signal_b/I_oThe value of the power control bit is determined by measurement and transmitted to the remote station 16 times per frame. The base station may ignore the power control bits corresponding to when the remote station is not transmitting. This enables fast reverse link power control. However, the effective power control rate varies with the transmission rate. For TIA/EIA/IS-95, this rate IS 800bps for full rate frames and 100bps for 1/8 rate frames.

Additional reverse link architectures are described in U.S. patent application No. 08/654, 443 entitled "high data rate CDMA wireless communication system" filed on 28.5.1996, which is assigned to the assignee of the present invention and is hereby incorporated by reference. According to U.S. patent application No. 08/654, 443, secondary pilots are introduced on the reverse link. The level of the pilot is independent of the transmission rate on the reverse link. This allows the base station to measure the pilot level and transmit the reverse power control bits to the remote station at a constant rate.

Various prior art methods of providing power control for the forward and reverse links utilize a one-bit power control command to direct the source unit (remote station or base station) to rely on the measured E of the signal received at the receiving unit (base station or remote station)_b/I_oTo increase or decrease its transmit power. The one-bit command is used for workThe number of bits transmitted by the rate control function is minimized, thereby minimizing the overhead required by the system and reserving more resources for data transmission. However, this one-bit command inherently causes toggling (toggle) (or limit cycling) of the power control because the transmit power is increased or decreased at each power control group depending on the value of the received power control bit. Furthermore, due to processing delays, the transmit power of several power control groups may be adjusted in the wrong direction before correction is made, thereby amplifying the effect of the limit cycles. Limiting the cycles can reduce the efficiency and performance of the communication system. There is a need for a method of controlling the transmit power of a source with a minimum number of bits while reducing or eliminating the transmit power limiting cycle inherent in a one-bit power control mechanism.

Summary of The Invention

The present invention is an improved new method and apparatus for providing power control for a communication system utilizing a ternary signaling scheme. The present invention improves the performance of a communication system by reducing or eliminating the limiting cycles inherent in binary signaling schemes. In an exemplary embodiment, the power control values (each having one of three possible values) are punctured (processed) onto the data to improve the response time of the power control loop and to allow dynamic adjustment of the transmit power. The power control mechanism of the present invention may be utilized on the forward link and/or the reverse link. However, for simplicity, the present invention is described in terms of reverse link power control.

It is an object of the present invention to provide a ternary power control signaling scheme. In the exemplary ternary signaling scheme, a power up (power up) command is represented by a positive value (e.g., +1), a power down (power down) command is represented by a negative value (e.g., -1), and a no operation command is represented by zero. The ternary signaling scheme minimizes the number of bits allocated for the power control function, thereby reserving more resources for data transmission.

It is another object of the invention to reduce or eliminate the power in the power control loopThe loop is limited to improve the performance of the communication system. In example embodiments, the power control value comprises a power up, a power down, or a no operation command. In an example embodiment, if the quality of the received signal (e.g., by the energy per bit to noise plus interference ratio E)_b/I_oMeasured) is within a predetermined range, the base station transmits a no operation command. The do nothing command minimizes the restriction cycles inherent in the binary signaling scheme. The do not operate command is also due to E received at the base station_b/I_oThe variation in the transmission power of the remote station due to uncertainty in the measurements of (a) is minimized.

It is a further object of the invention to improve the response time of the power control loop. In an example embodiment, the power control value is transmitted to the remote station without encoding. In addition, power control values are punctured onto the encoded data. At the remote station, the power control value can be quickly demodulated and detected without having to endure a long decoding process. The fast response time improves the performance of the power control loop and may improve the performance of the communication system and increase capacity.

It is a further object of the present invention to provide a power control mechanism that supports hand-off (handoff). The remote station may perform soft handoff with multiple base stations and receive the same or different power control values from these base stations. At the remote station, the transmitted power control value is received, demodulated, and filtered. The same power control values from multiple base stations or multiple signal paths are combined to produce a measure of improved power control value. Each individual power control value is compared to a set of thresholds to produce a corresponding received power control value. The power control values received from all base stations in communication with this remote station are then logically combined so that the remote station reduces its transmit power when any base station transmits a power down command, does not operate when no base station transmits a power down command and at least one base station transmits a do not operate command, and increases its transmit power when all base stations transmit a power up command.

It is a further object of this invention to provide a reliable power control mechanism. The use of reverse power control bits in the power control loop that are deemed unreliable may be omitted by, for example, maintaining transmit power.

Although the present invention is described in terms of reverse link power control, the concepts of the present invention are fully applicable to forward link power control.

Brief description of the drawings

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 is a diagram of the communication system of the present invention showing a plurality of base stations communicating with remote stations;

FIG. 2 is an example block diagram of a base station and a remote station;

fig. 3 is an example block diagram of a forward traffic channel;

FIG. 4 is an exemplary block diagram of a demodulator within a remote station;

FIG. 5 is an example block diagram of a power control processor within a remote station;

FIG. 6 is an exemplary block diagram of a power detector within a base station; and

fig. 7 is an example timing diagram of reverse link power control signals.

Preferred embodiments of the invention

In the present invention, the base station transmits a reverse power control value along with data on a forward traffic channel. The remote station uses the reverse power control value to control its transmit power to maintain a desired level of performance (e.g., a predetermined frame error rate, FER) at the base station while minimizing interference to other remote stations in the network. In an example embodiment, each power control value includes a power up command (e.g., +1), a power down command (e.g., -1), or a no operation command (e.g., 0). In an exemplary embodiment, to minimize processing delay, the power control values are not encoded and punctured onto the data (see fig. 3). In this sense, puncturing is the process of replacing one or more code symbols with a power control value.

In an exemplary embodiment, the base station measures the quality of the received reverse link signal according to the method described in U.S. patent No. 5,506,109, above. In an example embodiment, from the measured E_b/I_oTo indicate the quality of the received reverse link signal. In another embodiment, the quality of the reverse link signal received at the base station can be determined by measuring the amplitude of the reverse link pilot signal transmitted by the remote station or a forward power control bit (if one is utilized). In this alternative embodiment, the quality of the data bits is not measured directly, but is inferred from the measured amplitude of the reverse link pilot signal or the forward power control bits. This is reasonable because the forward power control bits are equally affected by the broadcast environment changes as the reverse link signal. This further embodiment is adapted to maintain the amplitude of the data bits at a known proportion of the amplitude of the pilot signal or the amplitude of the forward power control bits. Other methods of measuring the quality of the reverse link signal at the base station may be used and are within the scope of the invention.

In an exemplary embodiment, the base station measures E_b/I_oCompared to a set of set points comprising the first and second set points. If measured E_b/I_oIf the first set point is exceeded, the base station issues a power down command. If measured E_b/I_oBelow the second set point, the base station issues a power increase command. Finally, if measured E_b/I_oBetween the first and second set points, the base station issues a no operation command. The set point may be adjusted based on a set of parameters including the performance requirements of the system and the uncertainty of the received reverse link signal measurements.

In the exemplary embodiment, a power control value is transmitted to each remote station for each power control group. For the exemplary IS-95 communication system, each power control group IS 1.25 milliseconds in duration. Transmitting the power control values at evenly spaced intervals may result in the base station transmitting the power control values to multiple remote stations simultaneously. This may result in a peak in transmit power that may reduce capacity. To avoid this phenomenon, the power control values may be pseudo-randomly located in the power control groups. This may be accomplished by dividing the power control groups into a predetermined number of locations (e.g., 24 for IS-95 systems) and pseudo-randomly selecting (e.g., with a long PN sequence) the locations of the power control values to be punctured. For the IS-95 system, only one of the first 16 positions IS selected as the starting position of the power control value, and the last 8 positions are not selected.

In an example embodiment, the power control value is transmitted using a ternary signaling scheme having the above-described example values (e.g., +1, 0, -1). By using a ternary signaling scheme, it may not be necessary to randomize the location of the power control values. Preferably, by placing the power control value in an earlier portion of the power control group, processing delay of the power control loop is reduced, thereby improving performance. However, it is within the scope of the present invention that this power control value may be placed in various locations in the power control group to satisfy other system considerations.

At the remote station, the transmitted power control value is received and demodulated and processed. In particular, the demodulated power control symbols are accumulated for the duration of the power control value. The same power control values from multiple base stations or multipaths are then accumulated. Each obtained independent power control value is then compared to a set of thresholds to provide a corresponding received power control value, which is either +1, 0 or-1. The received power control values (each corresponding to each individual power control value) are then logically combined to provide a single power control command that directs the remote station to reduce its transmit power when any base station issues a power down command, to not operate when the symbol base station issues a power down command and at least one base station issues a do not operate command, and to increase its transmit power when all base stations issue power up commands.

Typically, the reverse power control value is transmitted to the remote station at a low transmit power level. Further, the power control value may be transmitted from a plurality of base stations within the communication system. The amplitude of the power control value can be measured more accurately by increasing the power control value of a multipath received from each base station or the same base station, adjusting the phase and amplitude of the power control value according to the phase and amplitude of the pilot signal from each base station or multipath, and filtering the amplitude of the adjusted power control value. The magnitude of the filtered power control value can be combined (in time) and used to control the remote station's transmit power to maintain the quality of the reverse link signal received at the base station at a desired level.

To improve the efficiency of the power control mechanism, e.g., to combat slow fading in the channel, the power control loop is designed to operate at a high rate. In an exemplary IS-95 system, the power control value IS transmitted at 800 bps. Thus, the transmit power of the remote station may be adjusted up to 800 times per second. However, since the transmitted power control values are uncoded and of little energy, some power control values may not be satisfactorily received at the remote station. The remote station may choose to ignore any power control values it deems insufficiently reliable.

In the rate embodiment, to minimize processing delay, power control values are transmitted without encoding and punctured onto the data. However, for communication systems requiring a higher level of reliability, the power control value may be encoded with data or a separate code provided only for this power control value. It is within the scope of the invention to use coding to improve the reliability of the received power control value.

For simplicity, the present invention is described in terms of reverse link power control, where the base station commands the remote stations to adjust their respective transmit powers. Those skilled in the art will readily appreciate that the present invention is applicable to forward link power control. Thus, forward link power control utilizing the inventive concepts described herein is within the scope of the present invention.

Description of the circuit

Referring to the drawings, FIG. 1 illustrates an exemplary communication system of the present invention that includes a plurality of base stations 4 in communication with a plurality of remote stations 6 (only one remote station 6 is shown for simplicity). The system controller 2 is connected to all base stations 4 in the communication system and to the Public Switched Telephone Network (PSTN) 8. The system controller 2 coordinates communications between users connected to the PSTN 8 and users at the remote stations 6. Data transmission occurs from base station 4 to remote station 6 via signal path 10 and from remote station 6 to base station 4 via signal path 12. The signal path may be a straight path, such as signal path 10a or a reflected path, such as signal path 14. The reflected path 14 is created when a signal transmitted from the base station 4a reflects off the reflection source 16 and reaches the remote station 6 through a path other than the line-of-sight path 10 a. Although shown in the block diagram of FIG. 1, the reflection source 16 is a result of an artifact (e.g., a building or other structure) in the operating environment of the remote station 6.

An exemplary block diagram of a base station 4 and a remote station 6 of the present invention is shown in fig. 2. Data transmission on the forward link originates from a data source 20 that provides data to an encoder 22. An example block diagram of encoder 22 is shown in fig. 3. Within encoder 22, a channel encoder 212 encodes the data in accordance with the encoding format of the system. For the exemplary IS-95 system, channel coding 212 performs CRC coding, code tail bit interpolation (code tail bit interpolation), convolutional coding, and symbol repetition as described in U.S. patent No. 5,103,359, above. The obtained symbols are provided to a block interleaver (interleaver)214, which interleaver 214 records the symbols and provides interleaved data to a Modulator (MOD) 24.

An exemplary block diagram of modulator 24 in accordance with the IS-95 standard IS shown in fig. 3. Within modulator 24, a multiplier 222 scrambles the interleaved data with a long PN code so that the interleaved data can only be received by the remote station 6 to which the data arrives. This long PN code spread data is multiplexed by Multiplexer (MUX)226 and provided to multiplier 228, which multiplier 228 covers the data with a Walsh code corresponding to the traffic channel assigned to the destination remote station 6. The Walsh covered data is provided to multipliers 230a and 230b and further spread with the short PNI and PNQ codes, respectively. The short PN spread data from multipliers 230a and 230b are provided to filters 232a and 232b, respectively, which provide low pass filtering of the data. The I and Q channel data from filters 232a and 232b, respectively, are provided to a transmitter (TMTR)26 (see fig. 2) that the transmitter 26 filters, modulates, upconverts, and amplifies. The modulated signal is routed through duplexer 28 and transmitted by antenna 30 on the forward link via signal path 10. The duplexer 28 may not be used in some base station designs.

In the example implementation shown in fig. 3, the power control value includes a power control bit and a power control enable. In this exemplary embodiment, the power control bit is a one bit command that commands remote station 6 to increase its transmit power when high (e.g., 1) and to decrease its transmit power when low (e.g., 0). In this example embodiment, the power control enable is a one bit command that when high (e.g., 1) allows the power control bit to be processed and provided to the output of filter 232, and when low (e.g., 0) resets the output of filter 232 to the intermediate scalar value (0) of the no operation command. As shown in fig. 7, in which the dotted line shows the low power control enable and the solid line shows the high power control enable. It may be noted that in fig. 7, the transmitted sequences (e.g., I-channel data and Q-channel data) are at an intermediate scale when power control enable is low as shown by the dashed line.

In this example embodiment, the I channel data and Q channel data are modulated by in-phase and quadrature sinusoids, respectively. By equating the non-operation with a zero value, the modulated I and Q signals are zero for the duration of the power control value. Thus, when transmitting the no operation command, base station 4 does not transmit any energy to remote station 6 for the duration of the power control value.

In this exemplary embodiment, a reverse power control value is punctured into the data stream for each power control group. The duration of each power control value is predetermined and may be related to the data rate on the traffic channel. In addition, the location where the reverse link power control value is punctured can be fixed or pseudo-randomly selected from the long PN sequence from long PN generator 224 (shown in fig. 3). The MUX 226 is used to interleave the reverse power control bits into the data stream. The output of the MUX 226 includes the encoded data bits and the reverse power control bits. An example definition of power control bits and power control enables is listed in table 1.

TABLE 1

Power control bit	Power control enable	Power control value	Remote station actions
1	1	+1	Power increase
0	1	-1	Power reduction
X	0	0	Do not operate

Alternatively, the power control values (e.g., +1, 0, and-1) may be punctured directly onto the data provided to the filter 232 by a pair of MUXs interposed between the mixer 230 and the filter 232 (not shown in fig. 3). In this embodiment, the short PN spread data is mapped to a new signal space corresponding to the power control value. For example, a high in short PN control data may be mapped to +1 and a low in PN control data may be mapped to-1.

Referring to fig. 2, at remote station 6, the forward link signal is received by antenna 102, routed through duplexer 104, and provided to a receiver (RCVR) 106. Receiver 106 filters, amplifies, demodulates, and quantizes the signal to obtain digitized I and Q baseband signals. The baseband signal is provided to a demodulator (DEMOD) 108. Demodulator 108 despreads the baseband signal with the short PNI and PNQ codes, decovers the despread data with the same Walsh code as used at base station 4, despreads the Walsh decovered data with the long PN code, and provides the demodulated data to decoder 110.

Within decoder 110, a block deinterleaver records the symbols of the demodulated data and provides the deinterleaved data to a channel decoder, which decodes the data according to the encoding format used at channel encoder 212. The decoded data is provided to a data sink (sink) 112.

Detection of power control value

A block diagram of an example demodulator 108 for detecting received reverse link power control values is shown in fig. 4. The digitized I and Q baseband signals from receiver 106 are provided to a bank of correlators 310. Each correlator 310 may be assigned to a different signal path from the same base station 4 or to a different transmission from a different base station 4. Within each assigned correlator 310, multipliers 312a and 312b despread the baseband signal with the short PNI and PNQ codes, respectively. The short PNI and PNQ codes within each correlator 310 have a unique offset that matches the particular offset associated with the base station 4 from which the signal was transmitted and further corresponds to the propagation delay experienced by the signal demodulated by that correlator 310. Multiplier 314 decovers the short PN despread data with the Walsh code assigned to the traffic channel received by correlator 310. The decovered data is provided to a filter 318, and the filter 318 accumulates the energy of the decovered data for one symbol time.

For IS-95 systems that transmit a pilot signal on a separate pilot channel (which IS superimposed on the traffic channel), the short PN despread data from multiplier 312 also contains the pilot signal. For the IS-95 system, this pilot signal IS covered with a sequence of all 0's corresponding to Walsh code 0. Thus, Walsh decovering is not required to obtain the pilot signal. This short PN despread data is provided to a pilot correlator 316, which correlator 316 low pass filters and/or symbol accumulates the despread data to extract a pilot signal from the received signal.

The two complex signals (or vectors) corresponding to the pilot signal from the pilot correlator 316 and the filtered data from the filter 318 are provided to a dot product circuit 320, which circuit 320 calculates the dot product of the two vectors in a manner well known in the art. An example of a dot product circuit 310 is described in U.S. patent No. 5,506,865, entitled "pilot carrier dot product circuit," assigned to the assignee of the present invention and the contents of which are incorporated herein by reference. The dot product circuit 320 projects vectors corresponding to the filtered data onto vectors corresponding to the filtered pilot signal, multiplies the magnitudes of these vectors, and scalar outputs S the signal_m(j) Provide forTo a Demultiplexer (DEMUX) 322. Symbol S_m(j) Representing the output from the low m correlators 320m in the jth symbol period. Remote station 6 knows whether the jth symbol period of the current frame corresponds to a data bit or a reverse power control value. Accordingly, DEMUX322 routes the vector s (j) = { s) associated with its output₁(j)，s₂(j)，…，s_M(j) Is sent to the data combiner 324 or the control processor 120. Data combiner 324 sums its vector inputs, despreads the data using the long PN code, and provides the demodulated data to decoder 110.

The power control data including the demodulated power control symbols is provided to the power control processor 410 shown in fig. 5. Power control processor 410 may be loaded into control processor 120 shown in fig. 2. Within power control processor 410, the demodulated power control symbols are provided to symbol accumulator 412, which accumulator 412 accumulates demodulated power control symbols S for the duration of the power control value_m(j) To generate a demodulated power control value b_m(i) In that respect For example, for an IS-95 system, each power control value has a duration of two modulation symbols or 128 PN chip codes (chips). In this case, symbol accumulator 412 accumulates the demodulated power control symbols over 128 PN chips to produce a demodulated power control value b_m(j) In that respect Symbol b_m(j) Which is used to represent the reverse power control value corresponding to the mth correlator 310m for the ith power control group. Vector of demodulated power control values b (j) = { b =₁(j)，b₂(j)，…，b_M(j) Is provided to the same bit accumulator 414.

In accordance with the IS-95 standard, when more than one base station 4 IS communicating with the same remote station 6, the base stations 4 may be configured to transmit the same or different reverse link power control values. The base station 4 is typically configured to transmit the same power control value when the base station 4 is actually co-located, such as when the base station is a different sector (sector) of a cell. Base stations 4 that do not transmit the same power control value are typically actually in different locations. The IS-95 standard also specifiesA mechanism is provided by which to identify the base station 4 that is configured to transmit the same power control value to the remote station 6. Furthermore, when remote station 6 is receiving transmissions from a single base station 4 over multiple propagation paths, the reverse power control values received on these paths are inherently the same. The identity bit accumulator 414 combines the known identity reverse power control values b_m(i) In that respect Thus, the output of the same bit accumulator 414 is a vector of independent reverse power control values B '(i) = { B'₁(i)，b′₂(i)，…b′_N(i) It corresponds to N independent reverse power control flows. Each independent power control stream b'_n(i) Including the same power control value corresponding to the stream (e.g., from a different sector in communication with remote station 6 or from a different multipath). The independent power control value may be calculated according to the following formula:

here, K is the number of correlators 310 receiving the same power control value for the nth independent reverse power control stream (e.g., from a different base station 4 or a different multipath).

The ternary channel of the present invention is also commonly referred to as an erasure channel. Instead of using one threshold (which is typically zero for a binary +/-communication channel), two thresholds are typically used for the ternary channel of the present invention. The first threshold is set to exceed zero and the second threshold is set to be below zero. If the amplitude of the received signal exceeds a first threshold value +1 is declared, if the amplitude is below a second threshold value-1 is declared, if the amplitude is between the first and second threshold values erasure is declared.

Vector B ' (i) of independent power control values is provided to threshold compare circuit 416, which circuit 416 provides power control values B ' for each circuit '_n(i) Compared to a corresponding set of predetermined thresholds. If b'_n(i) Exceeds a first threshold value th1_nThen will correspond to b'_n(i) Is received power control value b ″)_n(i) Is set to +1, if b'_n(i) Below a second threshold th1_nThen will correspond to b'_n(i) Is received power control value b ″)_n(i) Is set to-1, if b'_n(i) At a first threshold th1_nAnd a second threshold th1_nThen the received power control value b ″' is set_n(i) Is set to zero.

The first and second thresholds corresponding to each independent power control value may be set in accordance with a set of parameters, such as the number of identical power control values combined to produce the independent power control value and the variation in the measured amplitude of the received signal. As an example, the first threshold may be set at 0.5 of the nominal full-scale value and the second threshold may be set at-0.5 of the nominal full-scale value, with the thresholds being adjusted according to the parameters described above. The output from the threshold compare circuit 416 includes a vector of received power control values B "(i) = { B ″"₁(i)，b″₂(i)，…，b″_N(i) }, each received power control value b ″_n(i) Has a value of +1, -1 or zero. The vector of received power control values, B "(i), is provided to power control logic 418.

According to the IS-95 standard, if any one of base stations 4 issues a power down command, remote station 6 decreases its transmit power. This mechanism minimizes interference and increases system capacity while ensuring proper reception of the reverse link signal at the at least one base station 4. In thatIn an example embodiment, the same power control mechanism is utilized in conjunction with the ternary power control signaling scheme of the present invention. In the exemplary embodiment, if any received power control value b ″ "_n(i) Negative, remote station 6 decreases its transmit power. Furthermore, if the received power control value b ″' is received_n(i) Are all not negative, and at least one received power control value b ″_n(i) Zero, the mobile station 6 does not adjust its transmit power. Finally, if all received power control values b ″ ", are received_n(i) Is both positive, remote station 6 increases its transmit power. The power control logic 418 processes the vector of received power control values B "(i) using the logic scheme described above. The output of power control logic 418 is a single power control value (or power control command) that instructs remote station 6 to increase, decrease, or maintain its transmit power. This power control value is provided to transmitter 136 (see fig. 2), which transmitter 136 adjusts the transmit power of remote station 6 accordingly.

In this example embodiment, the reverse power control values are not encoded, and therefore, they are particularly susceptible to errors caused by interference. The fast response time of the closed loop reverse link power control minimizes the impact of these errors on the reverse link power control performance because these mis-adjustments, or non-adjustments, of the remote station's 6 transmit power in subsequent power control groups can be compensated for.

In the example embodiment described herein, reverse link power control IS described as such, and thus IS compatible with the IS-95 standard. The practice of the present invention is not relevant to any particular communication system or implementation. It will be apparent to those skilled in the art that other implementations may be attempted to perform the power control processing described herein and are within the scope of the invention.

Generating a power control value

Referring to fig. 2, at base station 4, the reverse link signal is received by an antenna 30, routed through a duplexer 28, and provided to a receiver (RCVR) 50. Receiver 50 filters, amplifies, demodulates, and quantizes the signal to obtain digitized I and Q baseband signals. The baseband signal is provided to a demodulator (DEMOD) 52. The demodulator 52 despreads this baseband signal with short PNI and PNQ codes. For the IS-95 system, the demodulator 52 signal maps the received Walsh sequences to corresponding Walsh codes. In particular, the despread data is grouped into blocks of 64 chips and the data is assigned a Walsh code having a Walsh sequence closest to the block from which the data was despread. As described in the above-mentioned us patent No. 5,103,459, this signal mapping is performed by improving the fast Hadamard conversion. The Walsh code includes the demodulated data provided to the decoder 54.

Within decoder 54, a block deinterleaver records the symbols of the demodulated data and provides the deinterleaved data to a channel decoder that decodes the data in accordance with the encoding format used at encoder 132. For the IS-95 system, the decoder 54 performs Viterbi decoding and CRC checking on the decoded data. The CRC-checked data is provided to a data sink 56. The functions of the receiver 50 and the demodulator 52 of the IS-95 system are further described in U.S. patent No. 5,103,459.

In the exemplary IS-95 system, the transmit power of remote station 6 IS adjusted to maintain a desired reverse link signal quality (e.g., at an energy per bit to noise plus interference ratio E of the reverse link signal received at base station 4)_b/I_oTo be measured). In this exemplary embodiment, the measured E is_b/I_oAnd E_b/I_oThe set point is compared to generate a power control value in response thereto. Then, adjust E_b/I_oSet point to maintain a desired Frame Error Rate (FER).

As shown in fig. 6, the demodulated data is provided to a power detector 430 within the controller 40. Within power detector 430, the demodulated data is provided to power measurement circuit 432, which circuit 432 calculates the power of the received reverse traffic channel and the total received power. Methods and apparatus for measuring the quality of a received signal are described in detail in the above-mentioned U.S. patent No. 5,506,109. In summary, the received can be calculated from the demodulated dataThe power of the reverse traffic channel and the total received power may be calculated from the despread data. The ratio of these two measurements including the measured E_b/I_oThen, this E is added_b/I_oIs provided to a filter 436. The filter 436 measures E over a predetermined interval_b/I_oAveraging and averaging E_b/I_oIs provided to the comparison circuit 438. The filter 436 may be implemented as a Finite Impulse Response (FIR) filter or other filter design known in the art. Furthermore, the filter 436 may be designed such that a compromise between reliable measurements and minimum response time may be obtained for particular system requirements.

In this exemplary embodiment, an indication of the quality of the received reverse link signal (such as the FER) is provided to threshold adjustment circuit 434, which circuit 434 sets two E's in response thereto_b/I_oSet points (including first and second set points). In an example embodiment, if average E_b/I_oExceeding the first set point, E received_b/I_oThe transmit power of remote station 6 is adjusted downward by issuing a power control value of-1 better than necessary. Or, if average E_b/I_oBelow the second set point, then received E_b/I_oBy a difference from what is needed, the transmit power of remote station 6 is adjusted upward by issuing a power control value of + 1. Finally, if average E_b/I_oBetween the first and second set points, then E received_b/I_oThe transmit power of the mobile station 6 is maintained, approximately as required, by issuing a no-operation command with a power control value of zero. The difference between the first and second set points comprises the received E_b/I_oAnd can handle this difference to a particular application. In particular, the difference between the first and second thresholds may be set to account for the received E_b/I_oUncertainty of measurement. For example, if the received E can only be measured with a certainty of + -0.5 dB_b/I_oThen the first and second set points should be set at least 1.0dB apart.

Can be based on the performance of the systemThe first and second thresholds are adjusted (e.g., as determined by the FER of the received reverse link signal). If the received FER is higher than desired, the setpoint may be increased, causing the power control loop to adjust the remote station 6 transmit power upward and increase the received E_b/I_o. Alternatively, if the received FER is lower than desired, the setpoint may be lowered, causing the power control loop to adjust the transmit power of remote station 6 downward to increase capacity.

As described in U.S. patent No. 5,109, 390, above, remote station 6 may be in soft handoff with multiple base stations 4 or in softer handoff with multiple base stations 4 (or sectors). While in handoff, base station 4 may transmit the same or different power control values to remote station 6. Each base station 4 acts independently of the other base stations 4 if a different power control value is transmitted. However, if the same power control values are transmitted, these power control values are transmitted to a central processor, such as the system controller 2, which evaluates the power control values from all base stations 4. In an exemplary embodiment, system controller 2 directs remote stations 6 to reduce their transmit power when any base station 4 issues a power down command, directs remote stations 6 to maintain their transmit power when none of base stations 4 issues a power down command and at least one of base stations 4 issues a no operation command, and directs remote stations 6 to increase their transmit power when all of base stations 4 issue a power up command. The system controller 2 then transmits the same power control value to all base stations 4 in communication with the remote station 6 for transmission on the forward link.

The present invention has been described in detail with respect to reverse link power control for IS-95 communication systems. Those skilled in the art will readily appreciate that the ternary signaling scheme of the present invention may be utilized for reverse link power control for other communication systems. One such other communication system is the exemplary high rate packet data communication system described in U.S. patent application No. 08/963, 386, entitled "method and apparatus for high rate packet data transmission," filed on 3.11.1997, assigned to the assignee of the present invention, the contents of which are incorporated herein by reference. In the packet data communication system, a power control subchannel is assigned to each remote station in communication with a transmitting base station. Each power control subchannel is used to transmit a power control value to each mobile station in each time slot to command the mobile station to power up, power down, or maintain its transmit power. In the packet data communication system, power control values for a plurality of remote stations are transmitted during a power control burst (burst) multiplexed onto a traffic channel at a fixed location within each transmission slot.

The present invention has been described in detail with respect to reverse link power control of a communication system. It will also be apparent to those skilled in the art that the ternary signaling scheme of the present invention may be extended to forward link power control, which is within the scope of the present invention.

The ternary signaling scheme of the present invention can be further extended to send other control signals requiring more than two states. For example, in a communication system capable of transmitting at one of a number of data rates, a base station may transmit a rate control value to a remote station to inform the mobile station of a rate increase, a rate decrease, or a rate invariance of upcoming data transmissions. The use of a ternary signaling scheme minimizes the number of bits required to transmit control signals while preventing or eliminating the limiting cycles caused by transmitting binary control values.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein may be applied to other embodiments without the use of the inventive faculty. 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 method for controlling transmit power in a communication system, comprising the steps of:

measuring the quality of the received signal;

comparing the quality of the received signal to a set of setpoints, the set of setpoints including a first setpoint and a second setpoint; and

generating a power control value in response to the comparing step;

wherein the power control value comprises one of three values.

2. The method of claim 1, wherein the quality of the received signal is based on a measured energy per bit to noise plus interference ratio, E, for the received signal_b/I_o。

3. The method of claim 1, wherein the three values correspond to a power up command, a power down command, and a do not operate command.

4. The method of claim 3, wherein said power control value corresponds to said power up command when said quality of said received signal exceeds said first set point, corresponds to said power down command when said quality of said received signal is below said second set point, and corresponds to said do not operate command when said quality of said received signal is between said first and second set points.

5. The method of claim 1, wherein the set of set points is adjusted based on performance requirements of the communication system.

6. The method of claim 5, wherein the performance requirement is based on a frame error rate of the received signal.

7. The method of claim 6 wherein said first set point is lowered if said frame error rate is higher than needed.

8. The method of claim 7 wherein said second set point is increased if said frame error rate is less than desired.

9. The method of claim 1, wherein said first and second set points are set based on a measured uncertainty of said received signal.

10. The method of claim 9, wherein the difference between the first and second set points is set equal to or greater than the uncertainty of the measurement of the quality of the received signal.

11. The method of claim 1, further comprising the steps of:

transmitting the power control value to a destination station; and

wherein the transmit power of the destination station is adjusted in dependence on the received power control value.

12. The method of claim 11, further comprising the steps of:

processing the transmit power control value at the destination station to provide the received power control value.

13. The method of claim 12, wherein said processing step comprises the steps of:

receiving at least one signal path corresponding to the transmit power control value;

demodulating each of the at least one signal path to obtain a pilot signal and filtered data;

calculating a dot product of the pilot signal and the filtered signal to obtain demodulated power control symbols; and

accumulating the demodulated power control symbols over a period of the power control value to obtain the received power control value.

14. The method of claim 13, wherein said processing step further comprises the steps of:

comparing the output from said accumulating step with a set of thresholds to obtain said received power control value.

15. The method of claim 1, wherein the power control value is not encoded.

16. The method of claim 1, wherein the power control value is punctured into a data transmission.

17. The method of claim 1 wherein said power control values are pseudo-randomly positioned within a power control group.

18. A method for adjusting transmit power in a communication system, comprising the steps of:

receiving at least one transmit power control value;

processing the at least one transmit power control value to obtain a power control command; and

adjusting the transmit power in accordance with the power control command;

wherein the power control command comprises one of three values.

19. The method of claim 18, wherein the three values correspond to a power up command, a power down command, and a do not operate command.

20. The method of claim 18, wherein said processing step comprises the steps of:

receiving at least one signal path corresponding to the at least one transmit power control value;

demodulating each of the at least one signal path to obtain demodulated power control symbols;

accumulating the demodulated power control symbols over a period of the power control value;

combining the same power control values from the accumulating step to obtain independent power control values;

logically combining the independent power control values to obtain the power control command.

21. The method of claim 20, wherein said processing step further comprises the steps of:

comparing the individual power control values with respective sets of thresholds to obtain received power control values; and

wherein the logically combining step is performed on the received power control values to obtain the power control command.

22. The method of claim 20, wherein the transmit power is adjusted upward if at least one independent power control value is a power down command.

23. The method of claim 20, wherein the transmit power is maintained if none of the independent power control values are power down commands and at least one of the independent power control values is a do not operate command.

24. The method of claim 20, wherein the transmit power is increased if all of the individual power control values are power increase commands.

25. A circuit for controlling transmit power of a communication system, comprising:

power measurement means for receiving a received signal and providing an indication of the quality of the received signal;

a threshold adjustment circuit for receiving a performance requirement and providing a set of set points in accordance with the performance requirement; and

a comparison circuit connected to said power measurement circuit and said threshold adjustment circuit for receiving said indication of said quality of said received signal and said set of set points, respectively, said comparison circuit providing a power control value;

wherein the power control value comprises one of three values.

26. The circuit of claim 25, wherein the quality of the received signal is based on a measured energy per bit to noise plus interference ratio E for the received signal_b/I_o。

27. The circuit of claim 25, wherein the performance requirement is based on a frame error rate of the received signal.

28. The circuit of claim 25 wherein the three values correspond to a power up command, a power down command, and a do not operate command.

29. The circuit of claim 28 wherein said power control value corresponds to said power up command when said quality of said received signal exceeds said first set point, corresponds to said power down command when said quality of said received signal is below said second set point, and corresponds to said do not operate command when said quality of said received signal is between said first and second set points.

30. An apparatus for controlling transmission power of a communication system, comprising:

first power control loop means for maintaining the quality of a received signal within a predetermined range, said first power control loop means receiving said received signal and a set of set points and providing a power control value in response to said received signal and said set of set points; and

second power control loop means for maintaining a measured performance of said received signal, said second power control loop means receiving said measured performance and a performance requirement and providing said set of set points to said first power control loop means in response to said measured performance and said performance requirement.

31. The apparatus of claim 30 wherein said first power control loop means comprises:

receiver means for receiving said received signal;

demodulator means for demodulating said received signal to provide demodulated data;

power measurement means for receiving said demodulated data and providing an indication of the quality of said received signal; and

comparing means for receiving said indication of said quality of said received signal and said set of set points and providing said power control value.

32. The apparatus of claim 31 wherein said first power control loop means further comprises:

filter means for receiving said indication of said quality of said received signal and providing a filtered quality measurement; and

wherein the comparing means receives the filtered quality measurement and the set of set points and provides the power control value.

33. The circuit of claim 31, wherein the indication of the quality of the received signal is based on a measured energy per bit to noise plus interference ratio, E, for the received signal_b/I_o。

34. The apparatus of claim 30 wherein said second power control loop means comprises:

a threshold adjustment device for receiving the measured performance and performance requirements of the received signal and providing the set of set points.

35. The circuit of claim 34, wherein the performance requirement is based on a frame error rate of the received signal.