HK1026531A - Pilot based transmit power control - Google Patents

Description

Pilot-based transmit power control

Background

I. Field of the invention

The present invention relates to radio frequency signal communication. More particularly, the present invention relates to a novel and improved method of performing power control.

Description of the related Art

The IS-95 over-the-air (OTA) interface standard defines a set of RF signal modulation procedures that implement digital cellular telephone systems. The IS-95 standard and its derivatives, such as IS-95A and ANSI J-STD-008 (collectively referred to as the IS-95 standard), are promulgated by the communications industry association (TIA) to ensure operability between communications devices made by different vendors.

IS-95 IS popular because the IS-95 standard IS able to utilize the existing RF bandwidth more efficiently than the aforementioned cellular telephone technology. Efficiency may be improved by a combination of Code Division Multiple Access (CDMA) signal processing techniques and a wide range of transmit power control to improve frequency reuse in a cellular telephone system.

Fig. 1 depicts a highly simplified digital cellular telephone system constructed in the manner of IS-95. In operation, telephone calls and other communications are made by exchanging data between the subscriber unit 1 (typically a cellular telephone) and the base station 2 using RF signals. Typically, communication is made by a wireline connection 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 subscriber unit 1. The BSC 4 and MSC 6 typically provide mobile control, call processing, and call routing functions.

The RF signals transmitted from the base station 2 to a group of subscriber units 1 are referred to as forward link signals and the RF signals transmitted from the subscriber units 1 to the base station 2 are referred to as reverse link signals. The IS-95 standard requires that subscriber unit 1 provide communication services by transmitting user data, such as digitized voice data, via a reverse link signal. The reverse link signal is comprised of a single traffic channel and is therefore often referred to as a "non-coherent" signal because it does not include a pilot channel.

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

As noted above, IS-95 encompasses a wide range of transmit powers in order to more efficiently utilize the existing RF bandwidth. According to IS-95, the power control IS performed by measuring the strength or quality of the reverse link traffic channel as it IS received at the base station and generating a power control command based on the measurement. The power control commands are transmitted to the subscriber unit via forward link signals.

The subscriber unit responds to the power control command by increasing or decreasing the transmit power of the reverse link signal in accordance with the power control command. Power control adjustments are repeated at a rate on the order of 800 times per second to maintain the reverse link signal transmit power at the minimum necessary to communicate. In addition, IS-95 also requires adjustment of the transmit duty cycle of the reverse link signal based on changes in voice activity in 20 millisecond increments. Therefore, when the transmit duty cycle is reduced, either the signal is sent at a set point or the signal is gated and not transmitted at all. During the gating of the reverse link signal, the base station generates an incorrect power control increase command because the reverse link signal is not detected. However, the subscriber unit can ignore these spurious up commands because it knows when the reverse link signal is being transmitted and when it is not, and therefore also knows when the spurious up command was generated.

In order to meet the ever-increasing demand for the transmission of digital data generated by network technologies such as the world wide web, co-pending U.S. patent application 08/654,443 provides a higher rate ratio and more complex transmission system with multi-channel, coherent, reverse link signals. 08/654,443 entitled "High DataRate CDMA Wireless Communications System," filed 1996, 5/28, assigned to the assignee of the present invention and incorporated herein by reference (application No. 443). In particular, the above-identified patent application describes a reverse link signal that includes at least a traffic channel, a power control channel, and a pilot channel.

The use of a multi-channel reverse link signal has various advantages, including increased flexibility because different types of data can be simultaneously transmitted on a set of channels. Providing a pilot channel in a multi-channel reverse link signal facilitates coherent processing of the reverse link signal, thereby improving processing performance.

Reverse link power control is also required for the high speed links described in the above-mentioned patent applications to continue to utilize the existing RF bandwidth more efficiently. However, in one configuration of the high data rate system described in the above-mentioned patent application, the reverse link signal is transmitted continuously, and the transmit power of the traffic channel is increased in 20 millisecond increments as the data rate changes, which typically results from changes in voice activity. That is, as the data rate decreases, the traffic channel is transmitted at a reduced power level rather than a reduced duty cycle during each 20 ms. In general, the transmit power may be one of four levels that may be used as one of four voice activity increments, although any number of transmit power levels may be used.

Therefore, the transmit power of a high data rate system varies over a wider range of values than IS-95, which IS transmitted with IS-95 being a set point or full strobe. Also, since IS-95 requires at least some setpoint transmissions in each frame, and several frames of a higher rate system may be non-setpoint transmissions while the data rate remains reduced, the transmit power of the higher rate strobe may remain at a low level for a longer period of time than IS-95. Since the system receiving the high rate link will not know that this decrease is due to an increase in distance and simply as a result of a decrease in data rate, it is difficult to determine the appropriate power control command for transmission. However, since reverse link power control is required in a high-speed system, a new method of power control of the reverse link is required.

Summary of The Invention

The present invention is a new and improved method of providing reverse link power control. The reverse link signal transmitted at the reverse link transmit power includes a traffic channel transmitted at least at a traffic channel transmit power and a pilot channel transmitted at a pilot channel transmit power. At the receiving system, the received power of the pilot channel is measured and a power down control command is generated when the received energy is greater than the received energy threshold. If the received energy is less than the received energy threshold, an increase power control command is generated. The power control commands are communicated to the system that generates the reverse link signals.

Brief Description of Drawings

The features, objects, and advantages of the present invention will become more apparent to the reader after a detailed description of the invention taken in conjunction with the accompanying drawings. In the drawings, the same reference numerals denote the same meanings.

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

FIG. 2 is a block diagram of a subscriber unit or cellular telephone constructed in accordance with one embodiment of the present invention;

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

Detailed description of the preferred embodiments

Fig. 2 is a block diagram of a subscriber unit or cellular telephone constructed in accordance with an embodiment of the present invention. In operation, encoder 12 convolutionally encodes user data 10 to produce encoded symbols 14. User data 10 is typically vocoded voice information provided at variable data rates, although any type of digital data may be transmitted. User data is processed in increments of 20 milliseconds or frames, where the amount of data contained in each frame varies according to changes in the data rate.

Traffic channel modulator 16 modulates code symbols 14 with a traffic channel code to generate traffic channel symbols 18. In addition, traffic channel modulator 16 increases or decreases the gain of the traffic channel in accordance with channel gain adjustment command 62, as described below. The gain of the traffic channel is then adjusted by traffic channel modulator 16 according to changes in the amount of data transmitted by the frame in each 20 millisecond frame.

The pilot channel modulator 70 generates the pilot channel symbols 22 and also adjusts the amplitude of the pilot channel in accordance with the channel gain adjustment commands 62. Similarly, power control channel modulator 72 generates power control symbols 74 in accordance with forward link power control commands 66, and also adjusts the amplitude of power control symbols 74 in accordance with channel gain adjustment commands 62.

Adder 20 adds traffic channel symbols 18 to pilot channel symbols 22 and power control symbols 74 to produce added symbols 24. A spreader 26 modulates the summed symbols 24 with one or more Pseudorandom Noise (PN) spreading codes to produce spread data 28. Transmitter 30 upconverts spread data 28 to a desired RF frequency to generate a reverse link signal 32, which is transmitted from an antenna system 34 through a duplexer 36. In addition, transmitter 30 adjusts the transmit power of reverse link signal 32 in accordance with reverse link gain adjustment commands 64. In a preferred embodiment of the present invention, the bandwidth of the data from expander 26 is 1.2288MHz, in accordance with the high data rate system of the' 443 patent application mentioned above.

In addition, in one embodiment of the invention, the forward link signal E is based on_FLThe power control 60 also performs "open loop" power control by adjusting the reverse link gain adjustment commands 64. In particular, the forward link signal E_FLThe transmit power of the reverse link signal also increases in proportion to the increase adjustment command 64 as the power level of (d) decreases. Since the reverse link signal appears to experience similar transmission conditions, the gain of the reverse link signal increases in accordance with the decrease in the forward link signal, and thus the received power of the reverse link signal will also decrease at the base station. By beginning to change the reverse link transmit power after a change in the forward link power is detected, compensation for the change can begin more quickly than if only power control commands were employed.

Also, when employing the transmit process described above, receiver 40 in subscriber unit 30 receives one or more forward link RF signals through antenna system 34 and duplexer 36. These forward link signals are typically generated at the base station as shown in fig. 1. Receiver 40 digitizes and downconverts the forward link signal to produce digitized baseband data 42. The digitized baseband data 42 is demodulated by a despreader 44 with a Pseudorandom Noise (PN) spreading code to produce despread samples 46. A channel demodulator 48 demodulates the despread samples 46 with a channel code to produce soft decision data 50, reverse link power control commands 52 and strength measurements 53. The decoder 54 decodes the soft decision data 50 to produce user data 56. Various types of decoding techniques are well known in the art, including trellis decoding (trellis decoding) and viterbi decoding.

Reverse link control commands 52 and strength measurements 53 are controlled by powerThe system 60 receives. Power control system 60 responds by generating gain adjustment commands 62 and 64 and forward link control commands 66. In the preferred embodiment of the present invention, reverse link power control commands 52 are received in the form of power control bits in the forward link signal, and strength measurements 53 are the received forward link signal (E)_FL) The measured energy value of (2). Power control system 60 is typically comprised of a microprocessor controlled by a set of software instructions, the use of which is well known.

To generate gain adjustment commands 62 and 64, power control system 60 examines reverse link power control commands 52 to determine whether an up command or a down command is received and whether the command is for a particular reverse link channel and also for a set of reverse link channels. For example, reverse link power control commands 52 may request an increase in the transmit power of the traffic channel. If so, the power control system 60 increases the amplitude of the traffic channel. The increase in amplitude is performed by applying a channel gain adjustment command 62 to traffic channel modulator 16.

Reverse link power control commands 52, on the other hand, may request an increase in the overall reverse link signal transmit power. If so, power control system 60 increases the transmit power of the reverse link signal via reverse link gain adjustment commands 64 applied to transmitter 30. Similarly, reverse link power control commands 52 may request that the pilot channel increase transmit power control. If so, the power control system 60 increases the amplitude of the pilot channel via a gain channel adjustment command 62.

Those skilled in the art will appreciate that the transmit power may be adjusted at other stages of the transmit process in addition to the illustrated stages. For example, the transmit power of the reverse link signal may be adjusted within spreader 26 or other system introduced into the transmit processing sequence.

Power control system 60 also receives a received forward link signal (E)_FL) The measured energy value of (a). Power control system 60 by productThe raw forward link power control commands 66 request an increase or decrease in the transmit power of the forward link signal to the power control channel modulator 72 in response to the measured energy value of the forward link signal. Power control channel modulator 72 modulates the power control commands with a power control channel code, generates power control symbols 74 that are applied to summer 20 and transmitted to the base station in a reverse link signal. In a preferred embodiment of the present invention, forward Link power control commands 66 are generated in accordance with U.S. patent application 08/722,763 entitled "Method and Apparatus for Measuring Link Quality in a spread Spectrum Communication System", filed 9/27 1996, assigned to the assignee of the present invention and incorporated herein by reference.

Fig. 3 is a block diagram of a base station constructed using the present invention. The reverse link signal transmitted from the subscriber unit shown in fig. 2 is received by antenna system 100 through duplexer 104 and applied to receiver 102. Receiver 102 digitizes and downconverts the reverse link signal to produce digitized baseband samples 105. A despreader 106 despreads the digitized baseband samples 105 with a PN spreading code to produce despread data 108. The traffic channel demodulator 108 despreads the data with the traffic channel code to produce traffic channel soft decision data 110. A pilot demodulator 112 decodes the despread data 108 with a pilot channel code to produce pilot data 114. The phase rotation 116 phase rotates the traffic channel soft decision data 110 to produce phase adjusted traffic data 118.

Power control command generator 120 measures reverse link pilot channel energy (E)_P) And compares it with a required pilot channel energy threshold (E)_PT) And (6) comparing. In a first embodiment of the invention, if the reverse link pilot channel energy (E)_P) Exceeding the required pilot energy E_pTThen reverse link power control commands 121 requesting a reduction in transmit power of the entire reverse link signal are generated by power control command generator 120 and applied to multiplexer 122. If the reverse link pilot energy (E)_P) Less than required pilot energyAmount (E)_PT) Then reverse link power control commands 121 requesting an increase in the transmit power of the entire reverse link signal are generated by power control command generator 120 and applied to multiplexer 122. The power control command is typically in the form of a bit or a set of bits.

In other embodiments of the present invention, a more complex set of power control commands may be employed, including indicating that the transmit power should be adjusted by one of a set of possible increments, or no increments, or that only a particular channel in the reverse link signal should be adjusted. For example, a power control command may be generated requesting an adjustment to the transmit power of the traffic channel.

During the transmit process, encoder 131 convolutionally encodes user data 132 to produce code symbols 134. User data 132 is typically digitized and vocoded voice, although other types of digital data may be employed. A multiplexer 122 multiplexes the reverse link power control commands 121 from the power control command generator 120 with the code symbols. In other embodiments of the present invention, power control command 121 may be punctured (punctured) into code symbols 134 or a second channel coding may be employed to generate a separate control channel on which reverse link power control command 121 is transmitted.

Channel modulator and spreader 128 modulates the data from multiplexer 122 with a channel code and PN spreading code to produce spread data 130. The spread data 130 is added by adder 135 to the spread data from the other forward link channels to produce added data 136. The transmitter 138 upconverts the summed data 136 and the upconverted RF signal is transmitted from the antenna system 100 through the duplexer 104.

By generating power control commands 121 based on the energy of the pilot channel rather than the traffic channel, more accurate power control commands are generated because the pilot channel is transmitted with a relatively constant or slowly varying transmit power. This is in contrast to the traffic channel described above which transmits at varying transmit powers depending on voice activity. Generating more accurate reverse link power control commands enhances the performance of the CDMA cellular telephone system because the transmit power of each reverse link signal is kept closer to the minimum necessary to communicate. Improving the performance of a CDMA cellular telephone system or any other CDMA wireless antenna system allows for more efficient use of the existing RF bandwidth.

There has thus been described an improved method of providing reverse link power control. The present invention can be used for a terrestrial wireless communication system or a satellite wireless communication system, and a wired communication system such as an axial cable system that transmits a sinusoidal signal. Also, the present invention is described with respect to a 1.2288MHz bandwidth, but other bandwidth systems, including 2.5MHz and 5.0MHz systems, may be used with the present invention. Additionally, the present invention is described with respect to reverse link signals, but the present invention may also be used for other types of transmissions, including forward link signals. In a preferred embodiment of the present invention, the various described systems are implemented by semiconductor integrated circuits using coupled conductor, inductive and capacitive elements, the implementation of which is well known to those skilled in the art.

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 aid of a person skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown above, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.

Claims (14)

1. A method for transmit power control of a reverse link signal processed using code division multiple access techniques and transmitted at a reverse link transmit power, wherein the reverse link signal has a traffic channel transmitted at a traffic channel transmit power and a pilot channel transmitted at a pilot channel transmit power, the method comprising the steps of:

measuring the received energy of a pilot channel;

generating a down command when the received energy is greater than an energy threshold; and

an increase command is generated when the received energy is less than the energy threshold.

2. The method of claim 1, wherein the increase command is for increasing reverse link transmit power and the decrease command is for decreasing the reverse link transmit power.

3. The method of claim 1 further comprising the step of transmitting said up command and said down command via a forward link signal.

4. The method of claim 2, wherein the traffic channel is received at a set of power levels and a corresponding set of transmission rates.

5. A system for conducting telecommunications, comprising:

a set of subscriber units for transmitting a set of reverse link signals having a pilot channel and a traffic channel; and

a base station for receiving a reverse link signal from said set of reverse link signals, measuring the energy of said pilot channel, and generating a power control command based on said energy level.

6. The system of claim 5, wherein each subscriber unit in the group of subscriber units comprises:

means for generating the pilot channel;

means for generating said traffic channel; and

means for generating an amplitude adjustment command.

7. The system of claim 5, wherein each base station in the set of base stations comprises:

means for measuring the energy level of the traffic channel; and

means for setting the power control command to increase when the energy is less than a threshold and to decrease when the energy is greater than the threshold.

8. The system of claim 6 wherein said means for generating said traffic channel further comprises gain adjusting said traffic channel based on transmission rate changes.

9. The system of claim 6 wherein said subscriber unit further comprises power control means for adjusting reverse link gain adjustment commands in response to changes in forward link received energy.

10. A system for performing receive processing on a signal, comprising:

a traffic channel processor that demodulates a traffic channel in the signal;

a pilot channel processor for demodulating a pilot channel in the signal and for generating a received energy measurement from the pilot channel; and

a power control system for generating power control commands based on the received energy measurements.

11. The system of claim 10, wherein the power control command is used to request a selected adjustment command from a group of adjustment commands comprising an increase or a decrease.

12. The system of claim 11, wherein the adjustment command is for the signal.

13. The system of claim 12, wherein the adjustment command is for the traffic channel.

14. The system of claim 10 further comprising a transmission system for transmitting said power control commands.