Adaptive power control for spread spectrum transmitter

Abstract: The application relates to power control in a mobile station, particularly in a CDMA cellular system. The base station transmits an APC code, indicative of its transmission power, which the mobile station decodes. The mobile compares the received power to the APC code, and adjusts its own power level upwards or downwards accordingly.

Description: This invention relates to a mobile station for use in a cellular communications network using spread-spectrum modulation wherein a base station transmits spread-spectrum signals which are received by the mobile station and the mobile station includes a transmitter and an associated antenna for transmitting spread-spectrum signals to the base station, the mobile station including an adaptive-power control. A spread-spectrum signal typically is generated by modulating an information-data signal with a chip-code signal. The information-data signal may come from a data device such as a computer, or an analog device which outputs an analog signal which has been digitized to an information-data signal, such as voice or video. The chip-code signal is generated by a chip-code where the time duration, T c , of each chip is substantially less than a data bit or data symbol. Spread-spectrum provides a means for communicating in which a spread-spectrum signal occupies a bandwidth in excess of the minimum bandwidth necessary to send the same information. The band spread is accomplished using a chip-code which is independent of an information-data signal. A synchronized reception with the chip-code at a receiver is used for despreading the spread-spectrum signal and subsequent recovery of data from the spread-spectrum signal. Spread-spectrum modulation offers many advantages as a communications system for an office or urban environment. These advantages include reducing intentional and unintentional interference, combating multipath problems, and providing multiple access to a communications system shared by multiple users. Commercially, these applications include, but are not limited to, local area networks for computers and personal communications networks for telephone, as well as other data applications. A cellular communications network using spread-spectrum modulation for communicating between a base station and a multiplicity of users, requires control of the power level of a particular mobile user station. Within a particular cell, a mobile station near the base station of the cell may be required to transmit with a power level less than that when the mobile station is near an outer perimeter of the cell. This is done to ensure a constant power level at the base station, received from each mobile station. A representative power level control system adapted to address power control requirements within a particular cell is that discussed in a scientific paper by R.F. Ormondroyd entitled "Power Control for Spread-Spectrum Systems", pages 109 to 115 of the Proceedings of the Conference on Communications Equipment and Systems, held on April 20-22, 1982 in Birmingham, U.K. The Ormondroyd system is a closed loop system requiring feedback, namely, the power level of a mobile unit transmission is measured at the base station and, responsive to this measurement, the base station directs an increase or decrease in the mobile unit's transmitter power. To implement such a system in a jamming environment, frequency hopping may be employed, as taught in an article by Munday and Pinches, entitled "Jaguar-V Frequency-Hopping Radio System", IEE Proceedings, Vol. 129, Part F, No. 3 (June 1982). Another similar radio transceiver including a control section is disclosed in document EP-A-0 392 079 (MITSUBISHI), 17 October 1990. In a first geographical region, such as an urban environment, the cellular architecture within the first geographical region may have small cells which are close to each other, requiring a low power level from each mobile user. In a second geographical region, such as a rural environment, the cellular architecture within the region may have large cells which are spread apart, requiring a relatively high power level from each mobile user. A mobile user who moves from a first geographical region to the second geographical region must adjust the power level of his transmitter, for meeting the requirements of a particular geographical region. Otherwise, if the mobile user travels from a sparsely populated region with few and spread out cells using the relatively higher power level with his spread-spectrum transmitter, to a densely populated region with many cells without reducing the power level of his spread-spectrum transmitter, his spread-spectrum transmitter may cause undesirable interference within the cell in which he is located and/or to adjacent cells. An object of the invention is to provide an apparatus according to claim 1. Preferred embodiments of the invention are given in the subclaims. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention, and together with the description serve to explain the principles of the invention. Fig. 1 shows an adaptive power control receiver and transmitter according to the present invention; Fig. 2 shows an adaptive power control receiver and transmitter according to the present invention; Fig. 3 shows the relative effect of a multiplicity of users communicating with spread-spectrum in a cellular environment with and without using adaptive power control; and Fig. 4 is a flow chart of the method of the present invention. Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals indicate like elements throughout the several views. The present invention assumes that a plurality of mobile stations operate in a cellular communications network using spread-spectrum modulation. The cellular communications network has a plurality of geographical regions, with a multiplicity of cells within a geographical region. The size of the cells in a first geographical region may differ from the size of the cells in a second geographical region. In a first geographical region such as an urban environment, a cellular architecture within the region may have small cells which are close to each other. In a second geographical region such as a rural environment, a cellular architecture within the region may have large cells which are spread apart. A mobile station while in the first geographical region may be required to transmit a lower power level than while in the second geographical region. This requirement might be due to a decreased range of the mobile station from the base station. Within a particular cell, a mobile station near the base station of the cell may be required to transmit with a power level less than that when the mobile station is near an outer perimeter of the cell. This is done to ensure a constant power level at the base station, received from each mobile station. In the present invention, a base station within a particular cell transmits a generic spread-spectrum signal and an APC-data signal. The APC-data signal may be transmitted with or separate from the generic spread-spectrum signal. For example, a spread-spectrum signal using a first chip codeword may be considered a first channel having the generic spread-spectrum signal, and a spread-spectrum signal using a second chip codeword may be considered a second channel. The ACP-data signal may be transmitted in the same channel, i.e., the first channel, as the generic spread-spectrum signal, or in a second channel which is different from the generic spread-spectrum signal. In the exemplary arrangement shown in Fig. 1, an apparatus for adaptive-power control of a spread-spectrum transmitter is provided. Each mobile station comprises acquisition means, detector means, decoder means, difference means, transmitter means, an antenna, and variable-gain means. The apparatus, by way of example, may have the acquisition means embodied as a first product device 101, a first chip-code generator 102, and a bandpass filter 103. The detector means may be embodied as envelope detector 104. Other type detectors may be used with the present invention, such as square law detectors or coherent detectors, as is well known in the art. The decoder means may be embodied as decoder 105. The difference means may be embodied as differential amplifier 106, the transmitter means as transmitter 112 and the variable-gain means as variable-gain device 111. The variable-gain device 111 may be a variable-gain amplifier, a variable-gain attenuator or any device which performs the same function as the variable-gain device 111 and described herein. The first product device 101 is coupled to the first chip-code generator 102 and to the bandpass filter 103. The envelope detector 104 is coupled between the bandpass filter 103 and the differential amplifier 106. The decoder 105 is coupled between the bandpass filter 103 and the differential amplifier 106. The decoder 105 alternatively may be coupled to the output of the envelope detector 104. The variable-gain device 111 is coupled to the differential amplifier 106 and between the transmitter 112 and antenna. The decoder 105, as illustratively shown in Fig. 2, alternatively may be coupled between a second bandpass filter 203 and differential amplifier 106. In this particular embodiment, the second bandpass filter 203 is coupled to a second product device 201. The second product device 201 is connected to a second chip-code generator 202. The embodiment of Fig. 2 could be used where the APC-data signal is transmitted over the second channel simultaneously while the generic spread-spectrum signal is transmitted over the first channel. The second channel uses a spread-spectrum signal with a second chip codeword, and the second chip-code generator 203 generates a second chip-code signal using the second chip codeword. The product device 101, Figures 1 or 2, using a first chip-code signal from the first chip-code generator 102 acquires the generic spread-spectrum signal transmitted from the base station. The first chip-code signal has the same chip codeword as the generic spread-spectrum signal. The envelope detector 104 detects a received power level of the generic spread-spectrum signal. The received power level may be measured at the detector. The decoder 105 decodes the APC-data signal from the generic spread-spectrum signal as a threshold. More particularly, data which establish or set the threshold for differential amplifier 106 are sent with the APC-data signal. The differential amplifier 106 generates a comparison signal by comparing the received power level of the generic spread-spectrum signal to the threshold. The differential amplifier 106 may employ a differential amplifier or other circuits for performing the comparison function. The transmitter 112 transmits a transmitter spread-spectrum signal. The variable-gain device 111, using the comparison signal, adjusts a transmitter-power level of the transmitter spread-spectrum signal from the transmitter. The variable-gain device 111 may be realized with a variable-gain amplifier, a variable-gain attenuator, or an equivalent device which can adjust the power level of the transmitter spread-spectrum signal. In operation, a base station in a cell may transmit the generic spread-spectrum signal on a continuous basis or on a repetitive periodic basis. Mobile stations within the cell receive the generic spread-spectrum signal. The received generic spread-spectrum signal is acquired and despread with the first chip-code signal from first chip-code generator 102 and first product device 101. The despread generic spread-spectrum signal is filtered through bandpass filter 103. The mobile station detects the despread generic spread-spectrum signal using envelope detector 104, and measures the received power level of the generic spread-spectrum signal. The APC-data signal may be transmitted on the same channel as the generic spread-spectrum signal using the same chip codeword as the generic spread-spectrum signal. In this case, the APC-data signal is transmitted at a different time interval from when the generic spread-spectrum signal is transmitted. This format allows the mobile station to acquire synchronization with the first chip-code, using the generic spread-spectrum signal. As an alternative, the APC-data signal my be transmitted on a different coded channel using a second chip codeword. In the latter case, the second spread-spectrum signal having the APC-data signal would be acquired by the second chip-code generator 202 and second product device 201. In either case, the APC-data signal is decoded using decoder 105. Further, the APC-data signal may be time division multiplexed or frequency division multiplexed with the generic spread-spectrum signal. The decoder 105 decodes from the APC-data signal the value of the threshold for use by differential amplifier 106. For example, if there were eight levels for which to set the threshold level, then at minimum, a three bit word may be used with the APC-data signal to transmit the threshold to the mobile station. The threshold is used to adjust and/or set the threshold of the differential amplifier 106. Accordingly, the received power level from envelope detector 104 is compared to the threshold of the differential amplifier 106. If the received power level is greater than the threshold, then the variable-gain device 111 would decrement or decrease the transmitter spread-spectrum power level. If the received power level is less than the threshold at differential amplifier 106, then the variable-gain device 111 increases the transmitter spread-spectrum power. The APC circuit 110 of Figures 1 and 2 may be built on a digital signal processor chip. An analog to digital converter located at the output of the bandpass filter 103 would convert the received signal to a data signal. The envelope detector 104, decoder 105 and differential amplifier 106 may be implemented as part of digital signal processing functions on the digital signal processor (DSP) chip. The analog to digital converters may be included on the DSP chip. Fig. 3 shows the advantage of using an adaptive power control apparatus with a mobile station in a personal communications network. The number of adaptive power control mobile stations, N(APC), versus the number of mobile stations which can be used in a similar region, N, is plotted with respect to an inner radius, R i , and an outer radius R o . When the inner radius goes to zero, using spread-spectrum as code division multiplex, the number of users with adaptive power control who can operate within a cell, is approximately ten times the number of users without adaptive power control who can operate within the cell. The present invention also includes a method for automatic-power control of a spread-spectrum transmitter for a mobile station operating in a cellular communications network using spread-spectrum modulation. Referring to Fig. 4, a base station transmits 701 a generic spread-spectrum signal and an APC information-data signal. Each mobile base station performs the steps of acquiring 702 the generic spread-spectrum signal transmitted from the base station, and detecting 703 a received power level of the generic spread-spectrum signal. The steps also include decoding 704 the APC-data signal as a threshold, from the generic spread-spectrum signal, or from a separate signal from the generic spread-spectrum signal. A threshold is set 705 from the APC-data signal. The method further includes comparing 706 the received power level to the threshold, and adjusting 707 a transmitter power level of a transmitter spread-spectrum signal in response to the comparison. It will be apparent to those skilled in the art that various modifications can be made to the method and apparatus for adaptively controlling a power level of a spread-spectrum signal in a cellular environment of the instant invention without departing from the scope of the invention, and it is intended that the present invention covers modifications and variations of the method and apparatus for adaptively controlling a power level of a spread-spectrum signal in a cellular environment provided they come in the scope of the appended claims and their equivalents.

Claims: A mobile station for use in a cellular communications network using spread-spectrum modulation wherein the mobile station includes a transmitter (112) and an associated antenna for transmitting spread-spectrum signals to a base station, the mobile station including an adaptive-power control characterized by : at least one circuit (101, 102, 103; 201, 202, 203) for acquiring and decoding said spread-spectrum signals from said base station; a detector (104) for detecting a received power level of a generic spread-spectrum signal transmitted by the base station; a decoder (105) for decoding a threshold value of an APC-data signal encoded within a spread-spectrum signal transmitted by the base station, the decoder including a data demodulator (105); a differential amplifier (106) coupled to said detector (104) and said decoder (105) for generating a comparison signal by comparing the received power level to said threshold value; and a variable-gain device (111) coupled to said differential amplifier (106) and between the mobile station's transmitter (112) and antenna, responsive to said comparison signal indicating an increase or decrease, for adjusting a transmitter-power level of the transmitter spread-spectrum signal from said mobile station transmitter (112), wherein either the detector (104) and the decoder (105) are coupled to one acquisition circuit (101, 102, 103) or the detector (104) is coupled to a first acquisition circuit (101, 102, 103) and the decoder (105) is coupled to a second acquisition circuit (201, 202, 203). The mobile station as set forth in claim 1, wherein each acquisition circuit includes a chip-code generator (102, 202), a bandpass filter (103, 203), and a product device (101, 201) coupled to said chip-code generator and said bandpass filter. The mobile station as set forth in claim 1 or 2, wherein a first acquistion circuit (101, 102, 103) is coupled to said detector (104), and a second different acquisition circuit (201, 202, 203) is coupled to said decoder (105). The mobile station as set forth in one of claims 1 to 3, wherein said detector (104) and said decoder (105) are coupled to the same acquisition circuit (101, 102, 103). The mobile station as set forth in one of claims 1 to 4, wherein said variable-gain device (111) includes a variable-gain attenuator. The mobile station as set forth in one of claims 1 to 4, wherein said variable-gain device (111) includes a variable-gain amplifier.

Inventors: SCHILLING DONALD L, SCHILLING, DONALD L.
Applicants: INTERDIGITAL TECH CORP, INTERDIGITAL TECHNOLOGY CORPORATION
Application year: 1991
Main classification: H04B7
IPC codes: H04B1, H04B7, H04W52