WO2007091203A1

WO2007091203A1 - Wlan transmit power control

Info

Publication number: WO2007091203A1
Application number: PCT/IB2007/050366
Authority: WO
Inventors: Steve Shearer; Andrew Prentice
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-02-06
Filing date: 2007-02-03
Publication date: 2007-08-16

Abstract

A system comprises a WLAN transmitter connected to periodically receive null packets and to transmit them into a dummy load. A power measurement scheduler controls a DPDT switch connected to the WLAN transmitter's input and output. An envelope detector is associated with the dummy load and provides measurements to a power corrector. The WLAN transmitter output power is thus updated and its settings are held until the next update. Between updates, a data queue provides the normal data traffic to the WLAN transmitter input through one pole of the DPDT switch, and the WLAN transmitter output is connected to the antenna through the other pole of the DPDT switch.

Description

WLAN TRANSMIT POWER CONTROL

The present invention relates to wireless local area network (WLAN) circuitry, and more particularly to inexpensive methods and equipment to measure and control WLAN transmitter output power.

Wireless local area network (WLAN) transmitters need to limit the radio output power they apply to their antennas to comply with federal regulations and to keep the interference they cause to a minimum. Temperature changes and aging can significantly affect the radio output power. But determining how much power is being output is not so easy to do in mass-produced consumer products where manufacturing costs need to be tightly controlled.

In February 2004, the Federal Communications Commission (FCC) issued a revision to the regulations for the unlicensed national information infrastructure (UNII) bands and 5-GHz channel usage. Such revision added eleven channels, for a total of twenty-three. But, in order to use the eleven new channels, radios must incorporate two new features. These are part of the IEEE 802.1 Ih standard, e.g., transmitter power control (TPC) and dynamic frequency selection (DFS). The 5-GHz band included the UNII- 1 , UNII-2, and UNII-3 bands, which had four channels each. The channels were spaced 20-MHz apart with an RF spectrum bandwidth of 20 MHz, for non- overlapping channels. There were differing restrictions for each related to transmit power, antenna gain, antenna styles, and usage. The UNII-I band was designated for indoor use, and initially required permanently attached antennas. The UNII-2 band was designated for indoor/outdoor use, and permitted external antennas. The UNII-3 band was for outdoor bridge products that could be used for indoor/outdoor WLAN's, and it also permitted external antennas. Portions of the 5-GHz band can be used by radar systems. DFS dynamically instructs a transmitter to listen and switch to a channel clear of radar signals. Prior to transmitting, the DFS listens for a radar signal that could be on that channel. If a radar signal is detected, the channel will be vacated and flagged as unavailable for use. The transmitter will continuously monitor the environment for the presence of radar, both prior to and during operation. This allows WLAN's to avoid interference with incumbent radar users in instances where they are collocated. Such features can simplify enterprise installations, because the devices themselves can automatically optimize their channel reuse patterns.

TPC technology allows the clients and access points to exchange information about their mutual signal levels. Each device dynamically adjusts its transmit power to uses only enough energy to maintain the communication at a given data rate. Adjacent cell interference is thus minimized, allowing for more densely deployed high-performance WLAN's. As a secondary benefit, client devices enjoy longer battery life because less power is used by the radio.

Antennas will deliver the maximum transmitter power to the environment when the impedance match is optimum. Antenna matches are measured in terms of their standing wave ratio (SWR). The closer the SWR is to unity, the better. If an antenna is tuned right, the SWR will be at minimum. But nearby objects can load the antenna and cause reflections back into the transmitter. Measuring the output power becomes problematic because the antenna tuning is not stable and the power measurements are not reliable. Directional couplers can be used to tap out a sample of the transmitter output power while rejecting reflected power returning back from the antenna. But directional couplers are relatively expensive and too costly to be used in mass produced WLAN devices.

What is needed is a reliable and cost-effective circuit and method for measuring and controlling the real output power of a WLAN transmitter, regardless of antenna loading and mistuning. Briefly, a system embodiment of the present invention comprises a WLAN transmitter connected to periodically receive null packets and to transmit them into a dummy load. A power measurement scheduler controls a DPDT switch connected to the WLAN transmitter's input and output. An envelope detector is associated with the dummy load and provides measurements to a power corrector. The WLAN transmitter output power is thus updated and its settings are held until the next update. Between updates, a data queue provides the normal data traffic to the WLAN transmitter input through one pole of the DPDT switch, and the WLAN transmitter output is connected to the antenna through the other pole of the DPDT switch.

An advantage of the present invention is a WLAN transmitter is provided that measures power output while connected into a non-reflective load, and that will not interfere with other users because the null packets it transmits will be discarded according to the medium access rules.

A further advantage of the present invention is a WLAN transmitter power control is provided that can be implemented inexpensively. A still further advantage of the present invention is that a method is provided that can be used in all radio transmitters with antennas subject to random loading and detuning.

The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.

Fig. 1 is a schematic diagram of a wireless local area network (WLAN) system embodiment of the present invention; and

Fig. 2 is a flowchart diagram of a wireless transmitter power control method of the present invention.

Fig. 1 represents a wireless local area network (WLAN) system embodiment of the present invention, and is referred to herein by the general reference numeral 100. WLAN system 100 comprises a WLAN transmitter 102 with an input switch 104 normally connected to select a data queue 106, and an output switch 108 normally connected to an antenna 110. A power management scheduler 112 periodically toggles switches 104 and 108 in parallel to input null packets from a generator 114, and provide the WLAN transmitter 102 power output into a dummy load 116. The switches 104 and 108 are in a double-pole, double-throw (DPDT) configuration.

The circuits and methods of the present invention would be useful in any radio system where the power output levels must be controlled. In particular, they are advantageously used in a WLAN environment. There are three different variations of WLAN defined by IEEE standards, e.g., (1) 802.11a, (2) 802.11b, and 802.11g. IEEE-802.11a operates in the 5-GHz ISM band, while 802.1 Ib and 802.1 Ig operate in the 2.4-GHz ISM band. A variety of data rates and modulation techniques are used to encode data rates varying from lMb/s to 54Mb/s. All of these systems use time division duplexing, and the data is transmitted in variable-length frames. Each IEEE standard also specifies several test modes with fixed times and duty cycle rates. An envelope detector 118 connected to the dummy load 116 provides power measurements to a power corrector 120. These measurements are compared to some set-point and controls are sent to the WLAN transmitter so that its power output will scale up or down appropriately. Such power settings are held until the power corrector receives new measurements from the envelope detector 118, e.g., as signaled by a flag from the power management scheduler 112.

A 802.1 Ib WLAN transmitter can use either binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK). The modulation in the power envelope has an effect on the peak to average power ratio. Both 802.11a and 802.1 Ig WLAN transmitters can use orthogonal frequency division multiplexing (OFDM) that uses forty-eight separate data sub- carriers, and four pilot carriers. A correspondingly lower data rate is used on each carrier. The advantage of this system is that it reduces errors introduced by multi-path propagation at high data rates. Systems based on OFDM can offer higher data rates or longer range at lower data rates compared with conventional single carrier systems. A variety of modulation schemes are used to convey the data ranging from the lowest data rate with BPSK to 54Mb/s with 64-state quadrature amplitude modulation (64QAM).

The peak power has a significant effect on the design of the power amplifier in the transmitter. It is important to test accurately the peak power to ensure that the transmitter is not over-compressing and distorting the transmitter signal. This could lead to higher error rates and lower overall performance of the WLAN signal. It is also important to calibrate the power loop that dynamically controls the power output of the WLAN transmitter. Many transmitters have to co-exist with each other and accurate control of the power is required to do this. A power meter offers the most accurate means of measuring power, which ensures an accurately calibrated WLAN system supporting the highest number of users. The particular modulation technique in use will significantly effect the radio output signal power envelope.

Fig. 2 represents a radio transmit power control method embodiment of the present invention, and is referred to herein by general reference numeral 200. The method 200 begins with a step 202 for scheduling a series of automatic periodic measurements of the output power of a radio transmitter. A step 204 then switches the output of the radio transmitter into a dummy load during an automatic periodic measurement period which was scheduled in the previous step. A step 206 selects null packets to be switched into the data input of the radio transmitter during the automatic periodic measurement period. Such null packets are preferably as defined in IEEE Standard 802.11. A step 208 provides for measuring the transmitter power delivered to the dummy load during the automatic periodic measurement period. A step 210 sets a new control level on the output power of the radio transmitter that depends on measurements obtained in the previous step. A step 212 then returns the input of the radio transmitter to a data source, and the output of the radio transmitter to an antenna after and between successive automatic periodic measurement periods.

Although particular embodiments of the present invention have been described and illustrated, such is not intended to limit the invention. Modifications and changes will no doubt become apparent to those skilled in the art, and it is intended that the invention only be limited by the scope of the appended claims.

Claims

CLAIMS:

1. A method of radio transmitter power control, comprising: scheduling a series of automatic periodic measurements of the output power of a radio transmitter; switching the output of said radio transmitter into a dummy load during an automatic periodic measurement period scheduled in the previous step; inputting null packets into the data input of said radio transmitter during said automatic periodic measurement period; measuring the transmitter power delivered to the dummy load during said automatic periodic measurement period; setting a new control level on the output power of the radio transmitter depending on a measurement obtained in the previous step; and returning the input of said radio transmitter to a data source and the output of said radio transmitter to an antenna after and between successive said automatic periodic measurement periods.

2. The method of Claim 1, wherein: the scheduling, switching, inputting, measuring, setting, and returning occur within a wireless local area network (WLAN) device.

3. The method of Claim 1, wherein: the inputting null packets conforms to those defined in IEEE Standard 802.11.

4. A transmitter system, comprising: a radio transmitter with an adjustable power output level; an output switch providing for the selectable connection of the output of the radio transmitter to either an antenna or a dummy load; an input switch providing for the selectable connection of the input of the radio transmitter to either a data source or a null source; a power corrector connected to set the operating power output of the radio transmitter according to a periodic measurement taken from said dummy load.

5. The transmitter system of Claim 4, further comprising: a power measurement scheduler connected to the input and output switches and providing for periods in which the power corrector can obtain said periodic measurement.

6. The transmitter system of Claim 4, further comprising: an envelope detector associated with said dummy load for providing said periodic measurement to the power corrector.

7. The transmitter system of Claim 4, wherein: the radio transmitter is included in a WLAN device, and the input switch is able to select a source of null data packets according to IEEE Standard 802.1.