WO2000079700A2

WO2000079700A2 - Method and apparatus for controlling transmission power in a cdma communication system

Info

Publication number: WO2000079700A2
Application number: PCT/US2000/016778
Authority: WO
Inventors: Brian K. Butler; Paul E. Bender
Original assignee: Qualcomm Incorporated
Priority date: 1999-06-23
Filing date: 2000-06-16
Publication date: 2000-12-28
Also published as: CA2375824A1; WO2000079700A3; AU5877400A; KR20020014813A

Abstract

The present invention is a novel and improved method and apparatus for controlling the transmission power in a closed loop power control system. In the present invention, power control commands are issued in response to the reception of portions of a frame. An adaptive threshold is generated based on the energy of all other portions of the received frame and the energy of the current power control group is compared to this adaptive threshold.

Description

METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA COMMUNICATION

SYSTEM

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for controlling the power of a CDMA transmitter.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system 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 has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Patent No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system 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", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.

CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Patent No. 5,101,501 entitled "METHOD AND SYSTEM FOR PROVIDING A 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 by reference herein.

A method for transmission of speech in digital communication systems that offers particular advantages in increasing capacity while maintaining high quality of perceived speech is by the use of variable rate speech encoding. The method and apparatus of a particularly useful variable rate speech encoder is described in detail in U.S. Patent No. 5,414,796, entitled "VARIABLE RATE VOCODER", assigned to the assignee of the present invention and incorporated by reference herein.

The use of a variable rate speech encoder provides for data frames of maximum speech data capacity when said speech encoding is providing speech data at a maximum rate. When a variable rate speech coder is providing speech data at a less that maximum rate, there is excess capacity in the transmission frames. A method for transmitting additional data in transmission frames of a fixed predetermined size, wherein the source of the data for the data frames is providing the data at a variable rate is described in detail in U.S. Patent No. 5,504,773, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. In the above mentioned patent application a method and apparatus is disclosed for combining data of differing types from different sources in a data frame for transmission.

In frames containing less data than a predetermined capacity, power consumption may be lessened by transmission gating a transmission amplifier such that only parts of the frame containing data are transmitted. Furthermore, message collisions in a communication system may be reduced if the data is placed into frames in accordance with a predetermined pseudorandom process. A method and apparatus for gating the transmission and for positioning the data in the frames is disclosed in U.S. Patent No. 5,659,569, entitled "DATA BURST RANDOMIZER", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.

A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Patent No. 5,056,109, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.

In a communication system that provides data using a QPSK modulation format, very useful information can be obtained by taking the cross product of the I and Q components of the QPSK signal. By knowing the relative phases of the two components, one can determine roughly the velocity of the mobile station in relation to the base station. A description of a circuit for determining the cross product of the I and Q components in a QPSK modulation communication system is disclosed in U.S. Patent No. 5,506,865, entitled "PILOT CARRIER DOT PRODUCT CIRCUIT", assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.

There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in copending U.S. Patent Application Serial No. 08/886,604, entitled "HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and apparatus for controlling the transmission power in a closed loop power control system.

The receive system measures the signal energy (E_n) for a portion of a frame. An adaptive target threshold for the current power control group is computed. A power error (e_n) is defined as: e_n = T- E_n , (1) where e_n is power error at n^th power control group, and E_n is the measured energy in the n^th power control group. The adaptive energy threshold ( f_n ) against which the energy of the current power control group energy (E_n) is compared prior to generating the appropriate power control command. The adaptive energy threshold ( T_n ) is generated according to equation as follows:

where e_; is power error for the previous power control groups, and α is a gain factor to adjust this effect, and T is a fixed energy value that bears relation to the target energy of an individual power control group. The energy of the current power control group (E_n) is compared to the adaptive threshold( f_n ).

if E , < T„ , then PC , indicates turn "Up" , and

¹ (3) if E, ≥ T„ , then PC, indicates turn" Down" .

The power control command PCj is then issued from the receive system to the transmitter system requesting an increase or decrease in transmitted energy.

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 an illustration of a simplified wireless communication system;

FIG. 2 is an illustration of power control groups within a frame of data;

FIG. 3 is a flowchart illustrating the method of generating power control commands of the present invention;

FIG. 4 illustrates the exemplary remote station of the present invention; and

FIG. 5 illustrates the exemplary base station of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a simplified block diagram of the elements and signals involved in the present invention. Subscriber station 4 transmits a reverse link signal 8 to base station 2. In the exemplary embodiment, reverse link signal 8 is a code division multiple access communication signal such as that described in the aforementioned U.S. Patent No. 5,103,459 and as proposed in the Telecommunication Industry Association proposal "THE cdma2000 ITU-R RTT CANDIDATE SUBMISSION", which has been proposed to the International Telecommunications Union (ITU) in response to their request for proposals for providing third generation wireless communications systems. The present invention is equally applicable to the UMTS TERRESTRIAL RADIO ACCESS (UTRA) proposal to the ITU by the European Telecommunications Standards Institute (ETSI) SMG2. It will be understood by one skilled in the art that the present invention is not limited to CDMA systems but is applicable to any communication system that employ a closed loop power control and particularly to those that include interleaving, coding or symbol repetition.

Base station 2 receives and demodulates reverse link signal 8. In addition, base station 2 determines the adequacy of the received power of reverse link signal 8. Base station 2 transmits forward link signal 6 to mobile station 4 which includes power control commands requesting that mobile station 4 increase or decrease the energy of reverse link signal 8. A closed loop power control system for controlling the transmission energy of wireless communication signals was described in detail in aforementioned U.S. Patent No. 5,056,109. In the exemplary embodiment, forward link signal 6 is a code division multiple access communication signal. In the exemplary embodiment, the power control commands consist of single bit up /down commands. However, the present invention is equally applicable to multi-bit commands indicative of the amount of transmission energy adjustment and power control command streams that include a commands indicative of a request not to adjust the transmission energy.

Mobile station 4 receives and demodulates forward link signal 6. Mobile station 4 adjusts the transmission energy of reverse link signal 8 in response to the power control commands received from base station 2. FIG. 2 illustrates a frame of data of the present invention. In the exemplary embodiment a frame of data is 20ms in length and is divided into 16 power control groups each of 1.25 ms in length. In the exemplary embodiment, for each power control group time interval, base station 2 transmits a power control command. It will be understood by one skilled in the art that the present invention is equally applicable to other feedback rates for the power control commands and to other frame durations. In current closed loop power control systems such as that found in IS-95 systems, the energy of the power control group is compared to a fixed threshold and in response to that comparison a power control command is generated.

In the exemplary embodiment, the symbols of the frame are encoded and interleaved with repeated versions being introduced during the transmission of data at rates less than a predetermined maximum. As will be appreciated by one skilled in the art of packet based transmission, the error rate of a packet based transmission system that includes forward error correction and symbol redundancy depends upon the energy of the frame received as a whole more than it does upon the energy of any isolated portion of the frame. The reason that the accumulated energy of the frame is of overriding significance is two-fold. First, forward error correction techniques allow the receiver to recover and generate symbols that may have been lost in portions of the frame based on the values of other symbols received reliably. Second, redundancy of the symbols in the frame allow their energy to be accumulated such that although a single version of a symbol may have inadequate energy to reliably receive the symbol, in combination with redundantly transmitted versions of that symbol may have sufficient energy for reliable reception.

Referring to FIG. 3, a flowchart illustrating the method for controlling the transmission energy of mobile station 4 is illustrated. In block 20, base station 2 measures the signal energy (E_n) for the current power control group number n where in the exemplary embodiment, n ranges from 0 to 15. This metric is used to determine the adequacy of the received reverse link signal. In a preferred embodiment, the signal energy is normalized by the interference energy because the limiting factor on signal reception reliability is the signal to noise ratio and not simply the energy of the received signal. However, since the receiver subsystem of most receivers will include an automatic gain control element, this normalization operation is typically transparent to the power control processing operation. The operation then moves to block 22 where the adaptive target threshold ( T_n ) for the current power control group is computed. In current fast closed loop power control systems such as described in IS-95, the energy of a predetermined fraction of a frame is compared to a single fixed threshold (T).

if E < T, then PC , indicates turn" Up" , and

(4) if E ≥ T, then PC _} indicates turn" Down" .

If the energy for that portion of the frame (E ) is less than the threshold (T) then the power control command issued in response to that comparison (PC) requesting the transmitter to increase its transmission energy is sent. If the energy for that portion of the frame (E ) exceeds the threshold (T) then the power control command issued in response to that comparison (PC^ requesting that the transmitter decrease its transmission energy is sent.

In a simplified version of the present invention, the power control is generated entirely based upon the received energy accumulated during the current frame. In the exemplary embodiment, a frame consists of 16 power control groups that serve as the basis for the generation of power control commands. The goal under this method whereby the sole goal is that the accumulated energy for the frame is equal to 16*T, results in a power control system wherein the energy accumulated in the current power control group (N) and previous power control groups within the frame are normalized and compared to the fixed threshold (T) as follows:

^« E ∑^ = T , (5)

7=0 n where T is the accumulated energy required to reliably receive a frame, and E is the energy in the nth power control group. A power control command is then issued such that: n £ i 2_ — ~ ^< T> ^tnen PC_j indicates turn" Up" , and

;=0 n n £7 if , — ~ - T , then PC indicates turn" Down" .

,=o n If the normalized accumulated energy for the current power control group is less than the threshold (T) then the power control command issued in response to that comparison (PC_j) requesting the transmitter to increase its transmission energy is sent. If t normalized accumulated energy for the current power control group exceeds the threshold (T) then the power control command issued in response to that comparison (PC) requesting that the transmitter decrease its transmission energy is sent.

In the preferred embodiment of the present invention, the power control commands are based on both the energy of the individual power control groups and the accumulated energy over the frame. Although the energy of the frame is very significant in the reception of frames including repetition and forward error coding, there are limits that require that the received energy for the individual power control groups be tracked. The power error (e_n) for the purposes of the present invention is defined as:

= T- E. (7)

where e_n is power error at n^th power control group, and E_n is the measured energy in the n^h power control group. The receiver of the preferred embodiment computes a new adaptive energy threshold ( f_n ) against which the energy of the current power control group energy (E_n) is compared prior to generating the appropriate power control command. The adaptive energy threshold ( f_n ) is generated according to equation 4 as follows:

f_H = T + a ^, (8) j=0 n where e is power error for the previous power control groups, and is a gain factor to adjust this effect.

Following the generation of the adaptive threshold ( T_n ) in block 22, the operation moves to control block 24. In control block 24, the energy of the current power control group (E_n) is compared to the adaptive threshold( T_n ).

if E < T, , then PC , indicates turn " Up" , and

1 (9) if E ≥ T_n , then PC indicates turn" Down" . If the energy of the current power control group is less than the adaptive threshold, then the operation moves to 26. In block 26, in response to the comparison of block 24, base station 2 generates a power control command indicating the mobile station 4 should increase its transmission energy. If the energy of the current power control group exceeds the adaptive threshold, then the operation moves to 28. In block 28, in response to the comparison of block 24, base station 2 generates a power control command indicating the mobile station 4 should decrease its transmission energy.

It will be understood by one skilled in the art that although described in the text of the common practice of issuing single bit 'up' and 'down' commands with a single threshold as basis for the generation of the commands, the present invention is equally applicable to multiple thresholds and multi-bit power commands. Moreover, it will be understood by one skilled in the art that although described in the context of reverse link power control, the present invention is equally applicable to forward link power control.

FIG. 4 is a block diagram of mobile station 4 of the present invention. In the exemplary embodiment, mobile station 4 transmits a pilot channel in order to allow base station 2 to coherently demodulate reverse link signals 8. The generation and transmission of a reverse link pilot channel is known in the art and is described in detail in the aforementioned copending U.S. Patent Application Serial No. 08/886,604. In the exemplary embodiment, the pilot symbols are simply a predetermined sequence of symbols, for example the all zeroes sequence. The pilot symbols are provided to orthogonal covering element 42.

Orthogonal covering element 42 covers the pilot symbols in accordance with a predetermined covering sequence (W_p). The covering sequence (W ) is selected to be orthogonal to the covering sequence used to cover the reverse link traffic data. Orthogonal spreading of the channels transmitted by mobile station 4 is designed to provide maximum separation in code space between the transmissions from mobile station 4 minimizing self interference experienced. The Walsh spread pilot and power control channel signal is provided to a first input (F) of complex spreading element 48.

Frames of fundamental traffic data are processed by methods that are known in the art. The fundamental traffic frame is provided to frame formatter 44. Frame formatter 44 generates a set of cyclic redundancy check (CRC) bits, by methods that are well known in the art and appends those bits along with a set of tail bits to the end of the frame.

The frame, including the CRC and tail bits, is then provided to encoder 46. Encoder 46 encodes the frame in accordance with a predetermined forward error correction coding format. In the exemplary embodiment, the error correction format may be a convolutional encoding format or a turbo coding format the design and implementation of which are well known in the art.

The encoded symbols are provided from encoder 66 to interleaver 68. Interleaver 68 reorders the encoded symbols in accordance with a predetermined interleaving format. In the exemplary embodiment, interleaver 68 is a block interleaver or a bit reversal interleaver. The reordered symbols are provided to orthogonal covering element 49. Orthogonal covering element 49 covers the reverse link traffic data in accordance with a predetermined covering sequence. The covering sequence, W_f, is selected to be orthogonal to the covering sequence used to cover the pilot symbols, W .

The Walsh spread reverse link traffic data is provided to gain element 50 which adjusts the gain of the traffic channel in accordance with a control signal provided by power control command processor 64. The gain adjusted signal is provided to a second input (Q') of complex PN spreading element 48. The operation of complex PN spreading is performed to reduce the peak to average fluctuations on the power amplifier that reduce the battery life and transmission capacity of mobile station 4. The complex spreading operation is described in detail in the aforementioned U.S. Patent Application Serial No. 08/886,604.

The present invention is described in the context of a remote station that transmits a pilot signal and a single traffic channel. It will be understood by one skilled in the art that the teachings of the present invention are easily extended to controlling the transmission energy of remote stations that transmit an arbitrary number of channels. In addition, the present invention, though described in the context of a code division pilot channel, is equally applicable to a time division pilot channel. Moreover, although described in the context of a remote station that transmits a pilot channel, the present invention is equally applicable to remote stations that transmit a single code channel and do not transmit a pilot signal. Complex spreading element 48 spreads the input pilot symbols and the traffic data in accordance with two pseudorandom sequences PN_: and PN_Q . In the exemplary embodiment, the in phase (I) and quadrature (Q) signals are determined in accordance with equations (1) and (2) below: I = PN-I' - PN_Q Q (10)

Q = PN,r + PN_Q Q, (11) where PN, is a first pseudonoise sequence and PN_Q is a second pseudonoise sequence, the generation of which is well known in the art and is described in detail in the aforementioned U.S. Patent No. 5,103,459. Complex PN spreading is described in detail in the aforementioned copending U.S. Patent Application Serial No. 08/886,604.

The two PN spread channels from complex PN spreader 48 are provided to transmitter (TMTR) 52. Transmitter 52 upconverts, filters and amplifies the signals for transmission in accordance with a quaternary phase shift keying (QPSK) format. The QPSK signal is provided through duplexer 54 for transmission through antenna 56 as reverse link signal 8.

Turning to FIG. 5, reverse link signal 8 is received at antenna 100 and provided to receiver (RCVR) 102. Receiver 102 downconverts, filters and amplifies received signal 8 and provides the received signal to complex PN despreader 104. In the exemplary embodiment, receiver 102 RF demodulation the received signal in accordance with a quaternary phase shift keying (QPSK) format. PN despreader 104 despreads the received signal in accordance with a complex PN despreading format in order to remove the spreading that was performed by complex PN spreading element 48 of mobile station 4. The complex PN despread signals are provided to pilot filter 112 and to traffic channel demodulator 114. Pilot filter 112 uncovers the received signal in accordance with the pilot signal spreading code W . In the exemplary embodiment, the pilot signal is spread using the all zeroes Walsh sequence, in which case pilot filter 112 can be implemented using a simple low pass filter. The uncovered pilot signal is provided to traffic channel demodulator 114 in order to allow base station 2 to coherently demodulate the received reverse link signal 8. In the exemplary embodiment, the pilot signal is used to estimate the sufficiency of the energy of reverse link signal 8. It will be understood by one skilled in the art that the teachings of the present invention can be extended to providing an adaptive threshold power control method that uses the received energy of the traffic channels or other channels as the basis for controlling the energy of a remote transmitter.

The uncovered pilot signal is provided to energy calculator 116. The sum of the squares of the signal energy received on the I channel and the Q channel is used to generate the measured energy of the current power control group. The energy of the current power control group (E_n) is provided to power control processor 118. Power control processor 118 in the preferred embodiment generates the adaptive threshold ( f_n ) in accordance with equations (3) and (4) above. The energy of the current power control group is then compared to the adaptive threshold and result of the comparison is provided to message generator (MSG GEN) 120. Message generator 120 then generates a power control command in accordance with the relationship of equation (6) above.

The power control command is provided from message generator 120 to transmission subsystem 122 where the message is modulated upconverted and amplified for transmission through antenna 124 as part of forward link signal 6.

Referring back to FIG. 4, forward link signal 6 is received at antenna 56 and provided through duplexer 54 to receiver subsystem 63. Receiver subsystem 63 downconverts demodulates and decodes forward link signal 6 and provides the power control command portion of forward link signal 6 to power control processor b. Power control processor 64 provides a signal to an amplifier in transmitter 52 indicating the adjustment to the transmission energy of reverse link signal 8.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The 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 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.

WE CLAIM:

Claims

1. A method for generating a power control command for controlling the transmission energy of a frame of data in a wireless communication system in which a power control command is issued in response to the reception of each of a portion of said frame, comprising the steps of: generating an adaptive threshold based in accordance with the energy of previously received portions of said frame; comparing the energy of a most recently received portion of said frame with said adaptive threshold to provide a comparison result; generating a power control command in accordance with said comparison result.

2. The method of Claim 1 wherein said step of generating an adaptive threshold is based on a target accumulated energy for said frame and a target energy for the current power control group.

3. The method of Claim 1 wherein said step of generating an adaptive threshold comprises the steps of: computing a power error value in accordance with the equation: e_n = T- E_n , where e_n is power error at n^th power control group, and E_n is the measured energy in the n^th power control group and T is a threshold value for the current power control group; and. computing said adaptive threshold ( f_n )according to the equation as follows:

f_κ = T + ά∑- , (5) _j=o n where e is power error for the previous power control groups, and α is a gain factor to adjust this effect.

4. The method of Claim 1 wherein said step of generating an adaptive threshold increases the significance of the previously received portions of said frame for the last portions of said frame.

5. The method of Claim 1 wherein said frame is twenty milliseconds in duration.

6. The method of Claim 1 wherein each of said portions is 1.25 milliseconds in duration.

7. A method for generating a power control command for controlling the transmission energy of a frame of data in a wireless communication system in which a power control command is issued in response to the reception of each of a portion of said frame, comprising the steps of: accumulating the energy of each portion of said frame; normalizing said accumulated energy; comparing said normalized accumulated energy with a predetermined threshold; and generating said power control command in accordance with said comparison.

8. The method of Claim 5 wherein said step of normalizing is performed in accordance with the number of said portions of said frame.