Patent Application
20060183432
1. Field of the Invention
The present invention relates to a wireless network and in particular to a calibration of a transmitter using a range of transmit powers.
2. Related Art
Transmit power calibration is typically performed by a manufacturer with some margin (also called backoff) to account for board-to-board variation and to cover a range of less than optimal environmental conditions (e.g. temperature). Therefore, during live operation, a given wireless device may support a higher transmit power than the calibration specifies.
Note that transmit power calibration is essentially a tradeoff between range and throughput per modulation rate supported. That is, as the transmit power used for a given modulation format is increased the range is extended at the expense of the maximum throughput supported. During live operation, a given device may reduce its transmit power if range extension is not required to increase the maximum throughput provided.
In a common wireless network, a receiver can determine a signal quality of an incoming signal from a transmitter and then transmit that signal quality back to the transmitter. The transmitter can then adjust the power based on that signal quality. Notably, if the signal quality is “acceptable”, then no adjustment is made. Unfortunately, this feedback technique can easily fail to determine an optimal transmitter power.
Therefore, a need arises for a technique that can accurately determine an optimal transmitter power.
In accordance with one aspect of the invention, a transmitter can send a plurality of frames with a range of transmitter powers to another device in a wireless network. The quality metrics computed from these frames can be advantageously used to determine an optimal transmit power.
In one method, using the plurality of transmit powers, a receiver can compute quality metrics and send these quality metrics to the transmitter as feedback. Each quality metric can include at least an error vector magnitude (EVM). Note that in some embodiments, each quality metric can further include a received signal strength indicator (RSSI). The transmitter can calibrate its optimal transmit power using the feedback.
In another method, using its own receiver to monitor a plurality of signals, a transceiver (which includes both a transmitter and a receiver) can compute quality metrics. Each quality metric can include at least an EVM (and in some embodiments, an RSSI). Using the quality metrics, the transceiver can calibrate its optimal transmit power.
In one embodiment, the optimal transmit power can be defined as a maximum power that meets a minimum quality specification for a given supported modulation format. In another embodiment, the optimal transmit power can be defined as a transmit power that allows for a greatest path loss while maintaining a given packet error rate (PER). In yet another embodiment, the optimal transmit power can be defined as a transmit power that maximizes a throughput supported in the wireless network.
These calibration steps can be performed during association of the transmitter and the receiver and/or periodically during a connection between the transmitter and the receiver. These techniques can be advantageously computer implemented in wireless devices, e.g. transmitters and transceivers, using instructions.
In accordance with one aspect of the invention, a range of transmit powers can advantageously facilitate the optimal calibration of transmitter power.
To perform technique 100, a wireless network can include a transmit device (transmitter) capable of modifying its transmit power and a receive device (receiver) capable of reporting a quality metric back to the transmitter. This quality metric can include, for example, the error vector magnitude (EVM). In one embodiment, the receive device can also be capable of reporting a signal strength, e.g. the received signal strength indicator (RSSI), back to the transmitter.
Notably, in step 101, the transmitter can transmit a plurality of frames to the receiver using a plurality of transmit powers. For example, the transmitter could use a range of transmit powers from 10 dBm to 30 dBm. This range of transmit powers can advantageously improve the quality of the feedback provided by the receiver.
Specifically, for each frame and associated transmit power, the receiver can compute a quality metric in step 102. In one embodiment, this quality metric can include the error vector magnitude (EVM). In another embodiment, the receiver can also compute the received signal strength, e.g. the received signal strength indicator (RSSI). (Note that a combination of EVM and RSSI can be used to maximize link budget, which can reduce the margin, and throughput.) In step 103, the receiver can report its computation results to the transmitter, thereby allowing the transmitter to calibrate its transmit power based on that feedback in step 104. Note that calibration steps 101-104 can be performed during association and/or periodically throughout the wireless connection between the transmitter and the receiver.
In this calibration, the transmitter can determine its optimal transmit power. The optimal transmit power can be defined as the maximum power that meets the minimum quality specification for a given supported modulation format. Thus, based on the quality metrics, the transmitter can determine the maximum transmit power for its given hardware and environmental conditions, per modulation format supported.
Alternatively, the optimal transmit power can be defined as the transmit power that allows for the greatest path loss while maintaining a given packet error rate (PER). One way to determine the greatest path loss per given PER is by selecting the output power that minimizes the total contribution of the transmitter noise (such as due to non-linearities) as well as the receiver noise. This optimal power will be different depending on the path loss because the path loss impacts the relative impact of the receiver noise. In yet another embodiment, the optimal transmit power can be defined as the power that maximizes the throughput supported on the wireless link.
Note that the transmitter can also reduce its transmit power once it knows that the receiver is receiving a signal that has excess signal such that the signal to noise ratio (SNR) of the receiver is not limited by antenna-referred noise, but rather the internal dynamic range of the transmitter or receiver (or at least the contribution of the internal noises increases relative to that of the external antenna-referred noise). This level can be set heuristically, through manufacturing calibration, or through live calibration.
Specifically, in step 201, the transceiver can transmit a plurality of signals using a plurality of transmit powers. For each signal and associated transmit power, the transceiver can monitor those signals using its own receiver and compute quality metrics based only on those signals (using certain generalized assumptions regarding those quality metrics because another device is not providing feedback) in step 202. In step 203, the transceiver can calibrate its transmit power based on those computed quality metrics. Thus, in technique 200, also called a “loopback” technique, a transceiver can, without feedback from another device, choose its optimal transmit power for given hardware and environmental conditions per modulation format supported.
Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying figures, it is to be understood that the invention is not limited to those precise embodiments. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. As such, many modifications and variations will be apparent.
For example, the above-described techniques can be advantageously computer implemented in wireless devices, e.g. transmitters and transceivers, using instructions embodied on a computer readable medium. Accordingly, it is intended that the scope of the invention be defined by the following Claims and their equivalents.
This application claims priority of U.S. Provisional Patent Application 60/643,460, entitled “Calibration Using Receiver-Based EVM Detector” filed Jan. 12, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60643460 | Jan 2005 | US |
