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METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to cellular mobile telephone systems. More specifically, the present invention relates to a novel and improved method and apparatus for controlling transmitter power in a code division multiple access (CDMA) cellular mobile telephone system.
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. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. application Ser. No. 06/921,261, filed Oct. 17, 1986, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", now U.S. Pat. No. 4,901,307 assigned to the assignee of the present invention, the disclosure thereof incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations (also known as cell-sites stations, or for short cellsites) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques. In a CDMA system, increases in system capacity may be realized by controlling the transmitter power of each mobile user so as to reduce interference to other system users.
In the satellite application of the CDMA communication techniques, the mobile unit transceiver measures the power level of a signal received via a satellite repeater. Using this power measurement, along with knowledge of the satellite transponder downlink transmit power level and the sensitivity of the mobile unit receiver, the mobile unit transceiver can estimate the path loss of the channel between the mobile unit and the satellite. The mobile unit transceiver then determines the appropriate transmitter power to be used for signal transmissions between the mobile unit and the satellite, taking into account the path loss measurement, the transmitted data rate and the satellite receiver sensitivity.
The signals transmitted by the mobile unit to the satellite are relayed by the satellite to a Hub control system earth station. The Hub measures the received signal power from signals transmitted by each active mobile unit transceiver. The Hub then determines the deviation in the received power level from that which is necessary to maintain the desired communications. Preferably the desired power level is a minimum power
level necessary to maintain quality communications so as to result in a reduction in system interference.
The Hub then transmits a power control command signal to each mobile user so as to adjust or "fine tune" 5 the transmit power of the mobile unit. This command signal is used by the mobile unit to change the transmit power level closer to a minimum level required to maintain the desired communications. As channel conditions change, typically due to motion of the mobile unit, both 10 the mobile unit receiver power measurement and the power control feedback from the Hub continually readjust the transmit power level so as to maintain a proper power level. The power control feedback from the Hub is generally quite slow due to round trip delays through the satellite requiring approximately I of a second of propagation time.
One important difference between satellite or terrestrial base stations systems are the relative distances 2q separating the mobile units and the satellite or cell-site. Another important different in the satellite versus the terrestrial system is the type of fading that occurs in these channels. Thus, these differences require various refinements in the approach to system power control 25 for the terrestrial system.
In the satellite/mobile unit channel, i.e. the satellite channel, the satellite repeaters are normally located in a geosynchronous earth orbit. As such, the mobile units are all at approximately the same distance from the 30 satellite repeaters and therefore experience nearly the same propagation loss. Furthermore, the satellite channel has a propagation loss characteristic that follows approximately the inverse square law, i.e. the propagation loss is inversely proportional to the square of the 35 distance between the mobile unit and the satellite repeater in use. Accordingly, in the satellite channel the variation in path loss due to distance variation is typically on the order of only 1-2 dB.
In contrast to the satellite channel, the terrestrial/mo40 bile unit channel, i.e. the terrestrial channel, the distance between the mobile units and the cell sites can vary considerably. For example, one mobile unit may be located at a distance of five miles from the cell site while another mobile unit may be located only a few feet 4^ away. The variation in distance may exceed a factor of one hundred to one. The terrestrial channel experiences a propagation loss characteristic as did the satellite channel. However, in the terrestrial channel the propa50 gation loss characteristic corresponds to an inverse fourth-power law, i.e. the path loss is proportional to the inverse of the path distance raised to the fourth power. Accordingly, path loss variations may be encountered which are on the order of over 80 dB in a cell 22 having a radius of five miles.
The satellite channel typically experiences fading that is characterized as Rician. Accordingly the received signal consists of a direct component summed with a multiply reflected component having Rayleigh fading 50 statistics. The power ratio between the direct and reflected component is typically on the order of 6-10 dB, depending upon the characteristics of the mobile unit antenna and the environment about the mobile unit. Contrasting the satellite channel with the terrestrial 65 channel, the terrestrial channel experiences signal fading that typically consists of the Rayleigh faded component without a direct component. Thus, the terrestrial channel presents a more severe fading environment
than the satellite channel where Rician fading is the dominant fading characteristic.
The Rayleigh fading characteristics in the terrestrial channel signal is caused by the signal being reflected from many different features of the physical environ- 5 ment. As a result, a signal arrives almost simultaneously at a mobile unit receiver from many directions with different transmission delays. At the UHF frequency bands usually employed for mobile radio communications, including those of cellular mobile telephone sys- 10 terns, significant phase differences in signals traveling on different paths may occur. The possibility for destructive summation of the signals may result, with on occasion deep fades occurring.
Terrestrial channel fading is a very strong function of 15 the physical position of the mobile unit. A small change in position of the mobile unit changes the physical delays of all the signal propagation paths, which further results in a different phase for each path. Thus, the motion of the mobile unit through the environment can 20 result in a quite rapid fading process. For example, in the 850 MHz cellular radio frequency band, this fading can typically be as fast as one fade per second per mile per hour of vehicle speed. Fading on this order can be extremely disruptive to signals in the terrestrial channel 25 resulting in poor communication quality. However, additional transmitter power can be used to overcome the problem of fading.
The terrestrial cellular mobile telephone system typically requires a full-duplex channel to be provided in 30 order to allow both directions of the telephone conversation to be simultaneously active such as provided by the conventional wired telephone system. This fullduplex radio channel is normally provided by using one frequency band for the outbound link, i.e. transmissions 35 from the cell-site transmitter to the mobile unit receivers. A different frequency band is utilized for the inbound link, i.e. transmissions from the mobile unit transmitters to the cell-site receivers. According, this frequency band separation allows a mobile unit transmitter 40 and receiver to be active simultaneously without feedback or interference from the transmitter into the receiver.
The use of different frequency bands has significant implications in the power control of the cell-site and 45 mobile unit transmitters. Use of different frequency bands causes the multipath fading to be independent processes for the inbound and outbound channels. A mobile unit cannot simply measure the outbound channel path loss and assume that the same path loss is pres- 50 ent on the inbound channel.
It is therefore, an object of the present invention to provide a novel and improved method and apparatus for controlling in the terrestrial channel transmitter power so as to overcome deleterious fading without 55 causing unnecessary system interference which can adversely affect overall system capacity.
SUMMARY OF THE INVENTION
In a terrestrial CDMA cellular mobile telephone 60 system, it is desirable that the transmitter power of the mobile units be controlled so as to produce at the cell site receiver a nominal received signal power from each and every mobile unit transmitter operating within the cell. Should all of the mobile unit transmitters within an 65 area of coverage of the cell-site have transmitter power controlled accordingly the total signal power received at the cell-site would be equal to the nominal receiver
power of the mobile unit transmitted signal multiplied by the number of mobile units transmitting within the cell. To this is added the noise power received at the cell-site from mobile units in adjacent cells.
The CDMA receivers of the cell-site respectively operate by converting a wideband CDMA signal from a corresponding one of the mobile unit transmitters into a narrow band digital information carrying signal. At the same time, other received CDMA signals that are not selected remain as wide band noise signals. The bit-error-rate performance of the cell-site receiver is thus determined by the ratio of the power of the desired signal to that of the undesired signals received at the cell-site, i.e., the received signal power in the desired signal transmitted by the selected mobile unit transmitter to that of the received signal power in undesired signals transmitted by the other mobile unit transmitters. The bandwidth reduction processing, a correlation process which results in what is commonly called "processing gain", increases the signal to noise interference ratio from a negative value to a positive value thus allowing operation within an acceptable bit-error-rate.
In a terrestrial CDMA cellular mobile telephone system it is extremely desirable to maximize the capacity in terms of the number of simultaneous telephone calls that may be handled in a given system bandwidth. System capacity can be maximized if the transmitter power of each mobile unit is controlled such that the transmitted signal arrives at the cell-site receiver at the minimal signal to noise interference ratio which allows acceptable data recovery. If a signal transmitted by a mobile unit arrives at the cell-site receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications. On the other hand if the mobile unit transmitted signal is at a power level that is too high when received at the cell site receiver, communication with this particular mobile unit will be acceptable. However, this high power signal acts as interference to other mobile unit transmitted signals that are sharing the same channel, i.e. bandwidth. This interference may adversely affect communications with other mobile units unless the total number of communicating mobile units is reduced.
The path loss of signals in the UHF frequency band of the cellular mobile telephone channel can be characterized by two separate phenomena, average path loss and fading. The average path loss can be described statistically by a log-normal distribution whose mean is proportional to the inverse fourth-power of the path distance, and whose standard deviation is approximately equal to 8 dB. The second phenomena is a fading process caused by multipath propagation of the signals which is characterized by a Rayleigh distribution. The average path loss, which is a log-normal distribution, can be considered to be the same for both the inbound and outbound frequency bands, as is for the conventional cellular mobile telephone systems. However, as mentioned previously Rayleigh fading is an independent phenomena for the inbound and outbound link frequency bands. The log-normal distribution of the average path loss is a relatively slow varying function of position. In contrast, the Rayleigh distribution varies relatively fast as a function of position.
In the present invention, a CDMA approach to multiple user access in a cellular mobile telephone system is implemented. In such a system all the cell-sites in a region transmit, a "pilot" signal of the same frequency and code. The use of a pilot signal in CDMA systems is
well known. In this particular application, the pilot signal is used by the mobile units for initial synchronization of the mobile unit receiver. The pilot signal is also used as a phase and frequency reference and a time reference for demodulation of the digital speech signals 5 transmitted by the cell-site.
In the present invention, each mobile unit estimates the path loss in signals transmitted from the cell-site to the mobile unit. In order to make this signal path loss estimate, the power level of the cell-site transmitted 10 signals, as received at the mobile unit, are measured. The mobile unit thus measures the pilot signal power as received from the cell-site to which the mobile unit is communicating. The mobile unit also measures the power level sum of all cell-site transmitted signals as 15 received at the mobile unit. The power level sum measurement as described in further detail later herein, is necessary to handle cases in which the mobile unit might temporarily obtain a better path to a more distant cell-site than to a normally preferred closest cell-site. 20
The outbound link path loss estimate is filtered using a non-linear filter. The purpose of the non-linearity in the estimation process is to permit a rapid response to a sudden improvement in the channel while only allowing a much slower response to a sudden degradation in 25 the channel. The mobile unit in response to a sudden improvement in the channel thus suddenly reduces mobile unit transmitter transmit power.
Should the channel for one mobile unit suddenly improve, then the signal as received at the cell-site from 30 this mobile unit will suddenly increase in power. This sudden increase in power causes additional interference to all signals sharing the same wide band channel. A rapid response to the sudden improvement will thus reduce system interference. 35
A typical example of a sudden improvement in the channel occurs when a mobile unit is moving through an area that is shadowed by a large building or other obstruction and then drives out of the shadow. The channel improvement, as a result of the vehicle move- 40 ment, can take place on the order of a few tens of milliseconds. As the mobile unit drives out of the shadow, the outbound link signal as received by the mobile unit will suddenly increase in strength.
The outbound link path loss estimate at the mobile 45 unit is used by the mobile unit to adjust the mobile unit transmitter power. Thus, the stronger the received signal, the lower the mobile unit transmitter power will be. Reception of a strong signal from the cell-site indicates that the mobile unit is either close to the cell-site or else 50 an unusually good path to the cell-site exists. Reception of a strong signal means that a relatively smaller mobile unit transmitter power level is required in order to produce a nominal received power at the cell-site in transmissions by the mobile unit. 55
In the case where there is a temporary but yet sudden degradation in the channel it is desirable that a much slower increase in mobile unit transmitter power be permitted. This slow increase in mobile unit transmitter power is desired so as to prohibit an unnecessarily rapid 60 increase in mobile unit transmit power which increases the interference to all other mobile units. Thus a temporary degradation in one mobile unit channel will be tolerated in order to prevent a degradation of all mobile unit channels. 65
In the case of a sudden degradation in the channel, the non-linear filter prevents the mobile transmitter power from being increased at a high rate in response to
the sudden decrease in signal power in signals received at the mobile unit. The rate of increase of the mobile unit transmitter transmit power must generally be limited to the rate of a closed loop power adjustment command transmitted from the cell-site, as described below, can reduce the mobile unit transmitter transmit power. Using the cell-site generated power adjustment commands, the mobile unit transmitter power will be prevented from being increased to a level significantly higher than the level required for communications, particularly when a sudden channel degradation occurs in only the outbound link path and not in the inbound link path.
It should be noted that it is undesirable to simply use a slow response on the mobile unit transmitter power control in an attempt to separate the fast Rayleigh fading from the slow fading due to distance and terrain. A slow response in the mobile unit transmitter power control is undesirable because the possibility of sudden improvements and fades that affect the inbound and outbound channels equally. If the response to a sudden improvement were to be slowed down by a filter, then there would be frequent occasions when the mobile unit transmitter power would be quite excessive and cause interference to all other mobile users. Thus the present invention uses a two time constant, non-linear approach in estimating the path loss.
In addition to measuring the received signal strength in the mobile unit, it is also desirable for the processor in the mobile unit to know the cell-site transmitter power and antenna gain (EIRP), the cell-site G/T (receive antenna gain G divided by receiver noise level T), the mobile unit antenna gain, and the number of calls active at this cell-site. This information allows the mobile unit processor to properly compute the reference power level for the local power setting function. This computation is done by calculating the cell-site to mobile link power budget, solving for the path loss. This path loss estimate is then used in the mobile cell-site link budget equation, solving for the mobile unit transmit power required to produce a desired signal level. This capability allows the system to have cell-sites with differing EIRP levels to correspond to the size of the cells. For example, a small radius cell need not transmit with as high a power level as a large radius cell. However, when the mobile unit is a certain distance from a low power cell, it would receive a weaker signal than from a high power cell. The mobile unit would respond with a higher transmit power than would be necessary for the short range. Hence, the desirability of having each cell-site transmit information as to its characteristics for power control.
The cell-site transmits information such as cell-site EIRP, G/T and number of active calls on a cell-site setup channel. The mobile unit receives this information when first obtaining system synchronization and continues to monitor this channel when idle for pages for calls originated within the public telephone switching network intended for the mobile unit. The mobile unit antenna gain is stored in a memory in the mobile unit at the time the mobile unit is installed in the vehicle.
As mentioned previously, mobile unit transmitter power is also controlled by a signal from the cell-site. Each cell-site receiver measures the strength of the signal, as received at the cell-site, from each mobile unit to which the cell-site is in communication with. The measured signal strength is compared to a desired signal strength level for that particular mobile unit. A power