HK1028931B - Power control for a mobile terminal in a satellite communication system - Google Patents

Description

Power control of mobile terminals within a satellite communication system

Technical Field

The present invention relates generally to the field of mobile satellite communication systems, and more particularly to a power control method for controlling the transmit power of a mobile terminal within a satellite communication system.

Background

Satellite systems are playing an increasing role in mobile communications by providing coverage within areas where terrestrial-based infrastructures are unable, or are not efficient, to provide mobile services. For example, satellite systems can provide coverage within areas where large areas of sparseness are not cost effective for implementing network infrastructure. Satellite systems can also make services available to aviation and sea-based users. Another use of satellite systems is to provide backup and supplement to land-based networks. The satellite system can continue to provide service to users when different parts of the network infrastructure fail, can also carry additional service during peak transmissions when the terrestrial-based network is overloaded, and can fill in holes within the coverage area of the terrestrial-based system due to man-made or natural obstructions.

Satellite communication systems are usually of GSM origin, using GSM for satellite systems. The European Telecommunications Standards Institute (ETSI) has published GSM specifications, which are hereby incorporated by reference in their entirety. In GSM, the power level of a mobile terminal is controlled by a base station. The algorithms used by the base station are quite complex. To simplify the above algorithm, the base station performs two measurements based on the received signal, namely called RXQUAL and RXLEV. These measurements are used to determine the transmit power level of the mobile terminal. RXQUAL is based on Bit Error Rate (BER) and RXLEV is based on the strength of the received signal. These measurements are typically performed during one complete SACCH time. Once the base station determines the appropriate power level for the mobile terminal, it sends a signaling message to the mobile terminal to tell the mobile terminal to adjust its transmit power level. The mobile unit has no means to determine its transmit power level. Although this method of power control works effectively in land-based cellular systems for developing GSM, it has drawbacks when applied in mobile satellite communication systems.

In terrestrial-based cellular systems, the propagation time of the signal transmission is relatively short. In satellite systems, signals travel a large distance before reaching the intended receiver. The result is a significant propagation delay within the satellite communication. The power control method for GSM does not take these propagation delays into account. For example, if the base station sends a message to the mobile terminal to tell the mobile terminal to increase its power, the mobile station does not receive the message until after approximately four SACCH cycles. At which time the mobile station may move out of an unfavorable position. The mobile unit will then transmit at a power level greater than the minimum required to ensure useful signal quality, which will result in unnecessary drain on the mobile unit battery. Conversely, if the base station commands the mobile terminal to reduce its transmit power because of favorable conditions, the specified transmit power level may be too low to ensure the desired signal quality when the mobile terminal receives the command.

Therefore, there is a need for a new power control method that is specifically designed for use within a satellite system and that takes into account the long propagation delay of the satellite system.

Disclosure of Invention

The present invention is a method for controlling the transmit power of a mobile wireless communications device within a satellite communications system. The power control method of the present invention includes both closed loop and open loop units. For a closed-loop element, the base station calculates power control constants for the mobile terminal based on the strength of the signal received from the mobile terminal. The base station takes into account the propagation delay of the satellite system when determining the transmit power level of the mobile terminal. The mobile terminal calculates its transmit power using the power control constant such that changing the power control constant causes a corresponding increase or decrease in the mobile terminal transmit power.

The open loop element of the power control method enables the mobile terminal to adjust its own transmit power level based on the strength of the signal received from the base station while waiting for a new transmit power setting from the base station. It is assumed that the path loss on the uplink is the same as the loss on the downlink. If the power of the received signal on the downlink is reduced without any change in the transmit power level, then it is assumed that the path loss is increasing. The mobile unit monitors the received signal from the base station and varies its transmit power level inversely according to the observed change in signal strength of the received signal within the previous frame.

When intermittent transmission (DTX) is employed in order to save power, transmission is stopped in the direction that is not currently speaking. According to one aspect of the invention, one frame in every 13 TDMA frames always contains an active transmission of SACCH information, even if DTX is used. The SACCH information is preferably transmitted at a constant power level to provide a stable signal strength criterion point. The occurrence of a SACCH within thirteen frames does not allow for a fast control of the uplink power level because twelve alternating frames do not receive respective power control information. According to another aspect of the invention, the mobile terminal may receive the SACCH signal within a time slot other than its assigned time slot in order to decide whether the path loss is changing. The further time slots used for this purpose are preferably as close in time as possible to the transmission time slot assigned by the mobile terminal, whereby no difficulties arise in switching from reception to transmission fast enough.

Drawings

Fig. 1 is a diagram illustrating major elements in a satellite communication system.

Fig. 2 is a diagram illustrating TDMA frames for use within the satellite system of the present invention.

Fig. 3 is a graphical illustration of propagation delay within a satellite communication system.

Fig. 4 is a block diagram illustrating components of a mobile terminal.

Fig. 5 is a flow chart illustrating the closed loop portion of the power control method.

Fig. 6 is a flow chart illustrating the open loop portion of the power control method.

Fig. 7 is a graph comparing the combined closed loop/open loop power control method of the present invention with the closed loop power control method alone.

Fig. 8 is an illustration of a TDMA frame.

Fig. 9 is a graphical illustration of relative frame timing on different channels in a TDMA system.

Detailed Description

Referring now to the drawings, a power control method for controlling transmission power of a mobile terminal within a satellite communication system will be described. Fig. 1 illustrates a mobile satellite communications system utilizing the power control method of the present invention and is generally referred to by the numeral 10. The satellite system 10 includes a space-based satellite 12 having an antenna 14 that projects one or more beams directed toward the surface of the earth to provide a communications medium for communications between earth stations. A ground station within a satellite communication system includes a plurality of mobile stations 16 and at least one base station 18. The base station 18 includes a Mobile Switching Center (MSC)22, and the MSC22 provides a connection to a Public Switched Telephone Network (PSTN)28 or other terrestrial network. The MSC22 includes a control processor 24 and memory 26 to control its operation and to process signaling as will be described later herein. Since the basic structure of a satellite communication system is well known to those skilled in the art, the same additional discussion is omitted.

Fig. 2 illustrates a TDMA transmission format applied within the satellite system of the present invention. In satellite applications, the GSM frame structure is modified to include 13 frames of 16 slots, where each 13 th frame is a SACCH frame. The multiframe, generally designated 50, includes 26 TDMA frames 52. Within each multiframe 50, the first 12 frames 52 carry traffic information. The fourteenth to twenty-fifth frames 52 also carry traffic information. The thirteenth frame and the twenty-sixth frame 52 each carry one eighth of a Slow Associated Control Channel (SACCH) message. Four multiframes 50 are required to complete the delivery of a SACCH message, while sixteen 20ms blocks of speech vocoder data are delivered per multiframe 50. Each 20ms block of encoded data representing a segment of a speech waveform is spread between three successive TDMA frames 52 in accordance with the well-known block-diagonal interleaving method. Each TDMA52 frame can be transmitted over a different frequency using frequency hopping to obtain the well-known advantages of interference averaging or interference diversity.

The SACCH messaging facility is used to send power control information to and from the mobile terminal. In a typical GSM system, the base station determines an appropriate transmit power level for the mobile terminal based on the average received signal strength over a complete SACCH time period and sends a SACCH message to the mobile terminal to command the mobile terminal to set its transmit power level to a specified level. The timing diagram illustrated in fig. 3 illustrates the time delay that results when the power control method is applied within a satellite system. This takes approximately 960ms to send the SACCH message and 240ms to propagate the signal. Once the base station has received the SACCH signal, it takes some time to calculate a new power setting for the mobile terminal based on the signal strength of the received SACCH message. The base station starts to transmit a SACCH message containing the new power setting within the next SACCH cycle. This takes another 960ms to send a SACCH message and 240ms to propagate the signal before the mobile terminal receives the signal. The mobile terminal begins transmitting at the new power level at the beginning of the next transmission slot.

By the time the mobile station receives the transmit power setting from the base station, the situation may have changed drastically from the pre-existing conditions when the original SACCH signal on which the power setting is based was sent from the mobile terminal to the base station in the past. Due to the time delay, no adjustment may be necessary until the mobile terminal receives a new power setting. For example, assume that the base station notifies a great drop in RSSI from the mobile terminal, thereby reflecting an increase in path loss between the mobile terminal and the base station. The base station calculates a new power setting based on the average RSSI over a SACCH time period and then sends a message to the mobile terminal telling the mobile terminal to increase its transmit power. By the time the mobile terminal receives the new transmit power setting, it may have not moved outside of the unfavorable location. This will cause the mobile terminal to transmit at a power level greater than that required to maintain the minimum required signal quality, thereby wasting power. In addition, the excess power increases the amount of co-channel interference experienced by other users within the system. Conversely, if the base station informs of an increase in the RSSI of the mobile terminal due to favorable conditions, this will direct the mobile terminal to reduce its transmit power level. If the favorable conditions no longer exist when a new power setting is received at the mobile terminal, the mobile terminal may transmit at a power that is insufficient to maintain the minimum required signal quality.

To avoid power control problems due to long delays within the satellite communication system, the present invention uses a dual method of power control, which includes both closed-loop and open-loop elements. Closed loop power control is a function of the base station. The base station sends a power control message to the mobile terminal that causes the mobile terminal to either increase or decrease its transmit power. Open loop control is performed by the mobile terminal. The mobile terminal is allowed to change its own transmit power based on changes in the strength of the signal received from the base station. It is assumed that the path loss on the uplink is the same as the path loss on the downlink. If the mobile terminal observes a decrease in received signal strength (indicating an increase in path loss between the base station and the mobile terminal), it will increase its transmit power. Conversely, if the mobile terminal observes an increase in received signal strength (indicating a decrease in path loss between the base station and the mobile terminal), it will correspondingly decrease its transmit power.

In the closed loop element of the power control method, the base station determines the value of the power control constant for the mobile terminal based on the quality of the signal received from the mobile terminal. The base station uses the SACCH messaging facility to send power control constants to the mobile terminal. The mobile terminal receives a new power control constant once per SACCH cycle and uses the power control constant to calculate its transmit power setting according to the following equation:

TX transmit power level-constant-received signal level (RECEIVED SIGNAL LEVEL)

The quantities are here expressed in logarithmic scale (i.e. decibels). This equation indicates that the mobile terminal will vary its own transmit power inversely with the observed change in received signal strength in order to provide open loop control of the transmit power. This equation also indicates that a change in the power control constant will result in a corresponding change in the transmit power of the mobile terminal for all values of received signal strength.

Measuring (RECEIVED SIGNAL LEVEL), by the mobile terminal, the received signal level as an average performed between each received time slot; and these slot averages may be subject to other averages depending on the time constant modified by the SACCH command from the base station.

The open loop control unit allows the mobile terminal to change its own transmit power during the time between receiving two complete SACCH messages from the base station (approximately one second).

Description of Mobile terminal

Referring now to fig. 4, there is illustrated a schematic diagram of a mobile terminal utilizing the power control method of the present invention. The mobile terminal is generally indicated by the reference numeral 100. The mobile terminal 100 includes a burst receiver 102 that receives a transmitted signal and a burst transmitter 104 that transmits a signal. The signal processing unit 106 processes signals communicated by the receiver 102 and prepares transmitted signals by the transmitter 104. The signal processing unit 106 includes a received signal processor 108, a transmitted signal processor 114, and a channel codec 110. The receive signal processor 108 includes a demodulator and demultiplexer (collectively referred to by the numeral 108) to extract the transmit bit stream from the received burst and to classify the received information from different time slots and frames into its appropriate logical channels.

The received signal processor also includes a power measurement circuit that measures the received signal strength and a bit error decision circuit that determines the received signal quality. The power and quality measurements are passed to the control and signaling unit 112. The channel codec 110 decodes the bit sequence from the received signal processor 108. If the decoded bit stream is a speech frame, the channel codec passes it to the speech codec 118. If the decoded bit stream is a signaling frame, it is passed to the control and signaling unit 112.

The control and signaling unit performs all control functions of the mobile terminal. These functions include power control and channel selection. To perform these control functions, the control and signaling unit 112 exchanges signaling information with a base station or network. These signaling messages are prepared or processed within the control and signaling unit 112 and passed to the channel codec 110 or received from the channel codec 110.

The burst setup unit, multiplexer and modulator (generally indicated by the numeral 114) prepare the transmitted signal. The burst setup unit places the coded bit sequence received from the channel codec 110 in the appropriate burst structure and then adds the necessary training sequence bits, tail bits, and padding bits. The multiplexer assigns each respective burst to each time slot within the numbered frame. After sorting and ordering the bit sequence, the modulator modulates the bit sequence onto a carrier frequency for transmission by the transmitter 104.

The frequency synthesizer 116 provides an internal time reference for the bit and frame clocks and the RF sources within the receiver and transmitter. The voltage controlled oscillator is assumed to be at a stable operating frequency as controlled by the control and signaling unit 112.

The control and signaling unit 112, as mentioned above, performs the power control functions of the mobile terminal. The power control function is performed by a software program within the control and signaling unit 112. As will be apparent to those skilled in the art, power control functions may be performed within a time-shared processor that performs other functions at different times. The power control algorithm processes the signaling message along with the received signal strength and quality measurements to determine the power level for transmitting the next signal burst.

Description of Power control Algorithm

Referring now to fig. 5, a flowchart illustrating a closed loop element of a power control method is shown. The base station receives a signal from the mobile terminal 100 (block 200) and measures the RSSI of the signal (block 202). After a complete SACCH period (block 204), the base station averages the RSSI values of the received signal over the SACCH period (block 206). The base station calculates the difference between the average RSSI value of the received signal and the desired received power p, denoted as r (block 208). This difference is referred to herein as a power offset value. The power offset value r is compared to a predetermined threshold power difference (denoted as c) (block 210). If the absolute value of the power offset value r exceeds c, the base station calculates new power control constants for the mobile terminal (blocks 212 and 214) and transmits the new power control constants to the mobile terminal using the SACCH messaging facility (block 216). To obtain a new power constant, the base station divides the power offset value r by a predetermined delay factor k (block 212) and adds the resulting value to the mobile terminal's previous power control constant (block 214). The delay factor k is typically a small integer such as 4. The delay factor k can be left to the system operator decision and can be set e.g. according to the step size.

Due to the presence of the delay factor, the base station will not attempt to correct for large swings in received power within one step. If the received power drops suddenly at the base station due to shadowing, the base station will increase the transmit power by the amount of increase when it suffers from shadowing effects. This increase in the amount of increase prevents large swings in the transmit power over shorter time intervals and accordingly prevents the base station from overreacting to momentary changes in signal strength.

The base station also receives reports from the mobile terminal of the quality of the signal received by the mobile terminal from the base station. If the quality is too poor, the base station will increase the power transmitted by the base station to the mobile terminal during the whole of the next SACCH period. Then, when it is realized that the mobile terminal's open loop power control system will reduce the mobile terminal's transmit power while it detects an increase in the signal level from the base station, the base station must adjust the power control constants that the base station sends to the mobile terminal by the same amount in order to avoid unnecessary interoperation between the downlink power (base station transmit power) control loop and the uplink power (mobile transmit power).

If intermittent transmission (DTX) is employed, the mobile terminal 100 will continue to transmit SACCH signals at a power level (e.g., maximum power level) for one out of every thirteen frames. The base station is able to measure the RSSI of the SACCH signal and use these measurements to calculate the path loss from the mobile terminal since the SACCH is transmitted with a known power. The path loss can be used to calculate an appropriate power level for the mobile terminal to transmit the traffic channel when it is not in DTX mode. Of course, this can be done directly without having to look up an intermediate value equal to the path loss, which is mentioned here to facilitate understanding of the actual system.

The open loop elements of the power control method are illustrated in fig. 6, and the mobile terminal 100 receives a signal on the downlink from the base station (block 300). If the signal completes the SACCH message (block 302), the mobile terminal sets the power control constant accordingly (block 304). The mobile terminal 100 then calculates the RSSI of the downlink signal (block 306) and smoothes the RSSI value of the downlink signal (block 308). The smoothing algorithm gives more weight to the most recently received signal than to the previous signal on the downlink. The smoothed value based on the RSSI of the received signal (referred to as RX _ NEW) is then used to calculate a NEW transmit power level in block 310. RX _ NEW is subtracted from the required received power P to obtain a power correction value Pcor. The power correction value Pcor is then added to the power control constant provided by the base station to obtain the transmit power for the next transmit slot. In block 312, the mobile terminal transmits using the transmit power level calculated in block 310. The power setting is adjusted from frame to frame in such a way that the previously received power control constant is used until a new power control constant is received from the base station. The result is that the transmit power will vary inversely with the RSSI of the received signal. If the RSSI of the received signal decreases, the transmit power of the mobile terminal is increased by a corresponding amount. Conversely, if the RSSI of the received signal increases, the transmit power of the mobile terminal is decreased by a corresponding amount.

The open loop element of the control method allows the mobile terminal to respond to the change in path loss based on the shadowing in a more timely manner. Although shadowing effects are relatively slow, the inherent time delay within the satellite system makes it impossible for the base station to respond in a timely manner. Fig. 7 is a graph illustrating a computer simulation comparing the transmit power level of a mobile terminal using only closed loop power control with a mobile terminal using both closed loop and open loop power control. As shown in this graph, the reduction in received power occurs at the 49 th SACCH cycle of the simulation. Mobile terminals using closed loop power control only do not react until the 53 rd SACCH cycle. A mobile terminal using both open loop and closed loop power control responds almost immediately to a drop in power. It is therefore apparent that the open loop element enables the mobile terminal to better respond to changes in path loss due to shadowing.

If the base station detects a lack of correlation between the signal quality reported by the mobile terminal and the signal level received from the mobile terminal, this may indicate that there is no correlation between the uplink and downlink fading. When this is detected, the base station can send another SACCH command to change the smoothing algorithm in block 310 to use a longer smoothing time constant. Otherwise, the base station may send a SACCH message to reduce the time constant used in block 310, at which point it estimates that the fading of the uplink and downlink are more correlated to accelerate the open loop portion of the power control algorithm.

In the discussion so far, it has been assumed that the base station continuously transmits signals to the mobile station. This assumption is not always correct. When intermittent transmission is employed, transmission is stopped in the direction of no call. The reason for using intermittent transmission is to save power when transmission is not needed. The power control method of the present invention can be applied to the DTX mode.

To enable open loop power control to be applied to DTX mode, one frame within each thirteen frame always contains an active transmission of slow associated control channel information. The SACCH signal is transmitted at a known power level to provide a reference point for signal strength. For example, the SACCH information may be transmitted at a constant level known to the mobile terminal, which may also be the upper transmit power of the SACCH power. Alternatively, a report of the downlink power can be sent to the mobile terminal as part of a SACCH message to inform the mobile terminal of the transmit power level used by the base station for SACCH transmission. When the transmit power level of the base station is known, the mobile terminal can calculate the path loss from the base station or satellite and then use the path loss to calculate the appropriate level to send traffic information. Of course, it is not necessary to calculate an intermediate value equal to the path loss but this can be calculated.

The one-frame SACCH occurring within each thirteen frame does not allow fast control of the transmit power level of the mobile terminal because the twelve alternating frames do not receive any respective power control information. Thus, the mobile terminal may choose to receive signals within a time slot other than its own assigned receive time slot in order to decide whether the path loss is changing. Such slot flipping is illustrated in fig. 8.

As shown in fig. 8, the mobile station assigns the first time slot in each frame to receive transmissions from the base station (i.e., downlink communications). If the base station does not send information during the mobile terminal's assigned receive slot due to the user at the other end not talking, the mobile terminal can switch to an alternate slot (e.g., slot 4), which is the receive slot assigned to some other mobile terminal. It is not necessary to demodulate the signal in its entirety, since the other time slots contain information that is not useful for the mobile terminal in question. It is sufficient to perform a measurement of the RSSI of the received signal strength within the alternate time slot. The time slot used for this purpose is preferably a time slot that is as close in time as possible to the mobile station's transmit time slot, thereby reserving sufficient time to switch from receive to transmit mode.

The signal within the alternate time slot is transmitted at an unknown power. Thus, the first frame received within the alternate slot does not provide enough information to implement the power adjustment. Data from at least two frames, preferably consecutive, is needed to determine whether the path loss changes. As an alternative to receiving another time slot on the same carrier frequency up, the mobile terminal can use its frequency agile synthesizer to receive time slots on a different carrier frequency where an unrelated call is ongoing. The carrier frequencies available within a cell or satellite beam have been read in advance from the system broadcast control channel. In particular the mobile terminal is able to select from these available frequencies either the carrier frequencies which are colliding with another call SACCH slot or the reception time slots which are close in time to the mobile terminal itself. The chance of finding such SACCH slots is enhanced by intentionally alternating over the frames in which the SACCH is transmitted between different carriers, which in any case needs to avoid that the entire SACCH transmission collides with the resulting high peak power requirement from the satellite transponder. Thus, when a cell or beam contains 13 carrier frequencies, a carrier will always carry SACCH sent to a certain mobile terminal and preferably at a known power level.

In one embodiment, the RSSI value of the first frame received in the alternate time slot is compared to the immediately preceding SACCH burst in the mobile terminal's own time slot, and the difference is determined and stored as a calibrated value of the power level of the alternate time slot relative to the known SACCH power level. The difference is then used to correct the RSSI measurement on each successive frame within the alternate time slot. The corrected RSSI value for each successive frame is compared to the immediately preceding frame to determine whether the path loss has changed. The adjusted RSSI based on the signal of the alternate channel inversely adjusts the transmit power of the mobile terminal. The mobile terminal only needs to measure the RSSI and the alternate slot and does not decode the alternate slot and the mobile terminal therefore reserves its decoding resources for continuing processing the signal in its own slot in order to detect when DTX has ceased and transmission of speech or when data has resumed.

Alternatively, it may be sufficient to observe whether the RSSI of the alternate slot has increased or decreased from frame to frame and whether the same increase or decrease is applied to the transmit power level without first correcting the RSSI value as described previously (approximately the opposite). This accumulation of incremental changes has the tendency to slowly drift the absolute value of the correction and the absolute power value can be corrected each time a known reference power for a SACCH burst is received, thereby limiting the drift to thirteen frames or 120 ms. Thus, the drift is negligible.

Another method that may be employed when intermittent transmission is used is to allow the mobile unit to hop to a different carrier frequency in one of 12 alternate frames in order to receive the SACCH signal in one of 12 alternate frames. If the SACCH signal is transmitted at a constant power, the mobile terminal can decide the path and from that decide whether the path has changed.

The frequency hopping scheme is shown in figure 9. As shown in fig. 9, the mobile terminal receives the SACCH signal within its own assigned receive slot within frame 13. If no transmission is received within frame 14, the mobile terminal switches to an alternate channel (channel 12 in this case) to receive the SACCH signal within the alternate channel. If no signal is received in its own assigned slot, the mobile terminal switches to the third channel of frame 15. The mobile terminal is thereby able to hop from channel to channel in order to receive the SACCH signal on each access channel. If SACCH signals on all channels are transmitted at a constant power setting, the mobile terminal can decide whether the path loss is changing by comparing the SACCH received within each frame with the SACCH signal received within the previous frame (even if SACCH signals are received on different channels). To take advantage of this approach, the mobile terminal preferably derives information from the broadcast control channel that provides the relative timing staggering of SACCH transmissions on different carrier frequencies.

The invention can advantageously be modified to decide on the transmission power control for mobile stations within a terrestrial wireless system like the trunked radio system known as such. Trunked land mobile radios differ from conventional land mobile radios in that the user does not receive a permanent frequency assignment. Instead, many users or groups of users share a smaller number of frequencies. Generally, each shared frequency is actually a pair of frequencies, one for transmission in the base-to-mobile (downlink) direction and the other for transmission in the mobile-to-base (uplink) direction; this configuration is referred to as "dual frequency simplex".

One of the shared downlink frequencies is used to broadcast control messages to the mobile radio and one of the shared uplink frequencies is used to receive control messages from the mobile radio. When a user wishes to talk, he presses a push-button switch (push-talk button) on the radio station and the radio station automatically sends a channel request message on the uplink control channel and then temporarily goes to the receive state. The base station receives the message informing the mobile radio station that it will be used for the next free channel used during the "end" period and immediately responds by sending an access-acknowledge message on the downlink control channel. The mobile radio then resumes transmission using the notified channel number until the user releases the push-to-talk switch.

A problem that arises within the prior art land mobile radio systems is that the mobile station may be located at a greater or lesser distance from the base station and accordingly the mobile station may be more or less receptive. A stronger received mobile station signal may interfere with the reception of a weaker signal, particularly when the weaker signal is using an adjacent frequency channel. A conventional solution to this problem is to either design expensive radio stations or other stations with higher adjacent channel rejection to not use adjacent channels within the same base station. Neither solution has benefits in the current state of the art, the best solution being to implement power control of the mobile station transmitter so that the base station receives signals near the desired strength or at a strength that is not too strong.

In accordance with the present invention, a method is provided for determining an appropriate transmit power for use by a mobile radio station.

In trunked radio systems, all mobile stations listen to the downlink control channel in idle mode. If the members of the group become active transmitters, the entire group receives an access confirmation message assigning a free channel to the transmitter. The remainder of the group (i.e., the group of non-transmitting radio stations) then tunes to the downlink of the assigned channel pair to receive the transmitted radio message relayed by the base station.

Once transmission begins, the transmitting radio remembers the signal strength (rssi (o)) that it was previously receiving the control channel before turning to transmission; the access request message is then transmitted by the mobile radio at a known power level p (o).

The base station measures the signal strength of the received access request and decides whether the signal strength is too strong or too weak. The base station then sends an access confirmation message to the mobile station including the number of free channels to be used and an indication of the DELTA that the access request signal was too strong or too weak.

The mobile radio then performs the following operations:

CONSTANT(o)＝P(o)-RSSI(o)+DELTA

in order to determine the initial power control constant constand (o). It then transmits on the designated channel with the power given by:

P＝CONSTANT(o)-RSSI

here RSSI is any RSSI (o) or signal strength of the received access confirmation message or any subsequent measurement of signal strength received from the base station. In TDMA terrestrial mobile radio systems, mobile radios transmit in bursts occupying less than 100% of the available time and can momentarily switch between transmission pauses to receive information from a base station. The received information may include timing information to keep the transmission bursts synchronized, frequency error information that can be used to keep the exact transmission frequency, and signal strength information that can be used to update the power level P to be used for the next burst in accordance with the present invention. Periodically cycling back and forth to receive also provides a useful facility for PRIORITY INTERRUPTs (PRIORITY INTERRUPTs), where normally transmitting radio stations can be commanded to switch to receive in order to allow the user to receive more urgent messages. Even if a non-TDMA radio station can be readily switched momentarily to receive within a few milliseconds to decide if a more urgent communication is required, such interruption will cause a negligible loss of voice quality, while also providing the opportunity for signal strength measurements whereby the transmit power can be updated once transmission is resumed.

Claims (15)

1. An open loop power control method for controlling the transmit power of a mobile terminal in a TDMA system, wherein the mobile terminal communicates with a base station using time slots within a repeating TDMA frame period, the method comprising:

a) receiving at said mobile terminal a signal transmitted from said base station in a time slot of said TDMA frame period other than said mobile terminal's assigned own time slot;

b) measuring the signal strength of the received signal at the mobile terminal;

c) calculating a transmit power setting for the mobile terminal at said mobile terminal based on said measurements of said received signal strength; and

d) transmitting a signal from said mobile terminal to said base station in a second time slot within said TDMA frame period in accordance with said calculated transmit power setting.

2. The method according to claim 1, further comprising receiving at said mobile terminal a signal from said base station in a third time slot of said TDMA frame and decoding said signal to recover information.

3. A closed loop power control method for controlling the transmit power level of a mobile terminal in a mobile radio communication system, said method comprising:

a) receiving, at a base station, a signal transmitted from the mobile terminal;

b) measuring the signal strength of the received signal at the base station;

c) calculating a power offset value based on a difference between a measured signal strength of said received signal and a predetermined standard power;

d) adjusting a transmit power setting for the mobile terminal based on the power offset value and a predetermined delay factor, wherein the adjustment is a predetermined fraction of the power offset value;

e) transmitting the adjusted transmit power setting from the base station to the mobile terminal; and

f) adjusting the transmit power of the mobile terminal in response to the transmit power setting received from the base station.

4. A closed loop power control method as claimed in claim 3, wherein said transmit power setting is calculated by dividing the power offset value by said delay factor and adding the result to the previous transmit power setting.

5. A closed loop power control method as claimed in claim 3, further comprising the steps of comparing said power offset value to a predetermined power difference threshold, and determining a new transmit power setting for said mobile terminal if said power offset value exceeds said power difference threshold.

6. The closed loop power control method of claim 3 wherein the signal strength of the received signal is measured between a plurality of frames and the average of the received signal strength between said plurality of frames is used to calculate the power offset value.

7. The closed loop power control method of claim 6 wherein the average of the received signal strength is calculated over a traffic information reception period defined by the duration of signaling information transmission.

8. A closed loop power control method as claimed in claim 3, wherein the power setting for the mobile terminal is a power control constant and the transmit power level for the mobile terminal is determined by subtracting the received signal strength from said power control constant.

9. A closed loop power control method as claimed in claim 3, further comprising the steps of receiving a transmitted signal at said mobile terminal, measuring the signal strength of the received signal, calculating an adjusted transmit power setting at said mobile terminal based on said measured received signal and said transmit power setting received from said base station, and transmitting a signal from said mobile terminal in accordance with said adjusted transmit power setting.

10. The closed loop power control method of claim 9 wherein signals received at said mobile terminal are transmitted by said base station within assigned receive time slots to said mobile terminal.

11. The closed loop power control method of claim 9 wherein signals received at said mobile terminal are transmitted by said base station within time slots other than its assigned self-receive time slot.

12. An open loop power control method for controlling the transmit power level of a mobile terminal in a TDMA communication system, said method comprising:

g) receiving a signal transmitted from a base station to said mobile terminal within an assigned receive timeslot of a repeating TDMA frame period to said mobile terminal;

h) switching to an alternate time slot and receiving a signal transmitted from said base station within said alternate time slot when no signal is received within the assigned receive time slot of the mobile terminal;

i) measuring the signal strength of the received signal within the assigned time slot of the mobile terminal and within the alternate time slot;

j) adjusting the transmit power level of the mobile terminal based on the measured signal strength of the received signal when the signal is received within its assigned receive time slot; and

k) the transmit power level of the mobile terminal is adjusted based on the measured strength of the received signal in the alternate time slot when no signal is received in the mobile unit's assigned own receive time slot.

13. The open loop power control method of claim 12 further comprising the steps of receiving a first signal within said alternate time slot, receiving said second signal within said alternate time slot, determining a change in signal strength between said first and second received signals within said alternate time slot, and adjusting the transmit power of said mobile terminal based on said change in signal strength between said first and second signals.

14. An open loop power control method according to claim 13 wherein said first signal is compared with a signal of known standard power to obtain a correction value based on the difference between the signal strengths of the two signals and the correction value is added to said first and second signals before determining the change in signal strength between the first and second signals.

15. The open loop power control method of claim 14 wherein a signal of known standard power is transmitted on a slow associated control channel.