Patent Application
20080240280
1. Field of the Invention
This invention relates to wireless voice and data communications, and more particularly to systems and methods to provide antenna diversity in wireless transmission communication systems.
2. Description of the Prior Art
The Institute of Electrical and Electronic Engineers (IEEE) has established a wireless standard, IEEE 802.16e. The IEEE 802.16e standard (IEEE 802.16e) outlines Media Access Control (MAC) and Physical Layer (PHY) specifications for wireless networks. The specification of the IEEE 802.16e addresses transmission of data in wireless networks. In particular, the IEEE 802.16e standard addresses communication in wireless asynchronous transfer mode (ATM) systems, covering frequencies of operation between 2.5 gigahertz (GHz) and 6 GHz. As is known in the art, IEEE 802.16e uses a modulation method called orthogonal frequency-division multiplexing access (OFDMA), which allows communication to occur at extremely high data speeds by transmitting data over multiple frequency channels over a wide frequency range.
The IEEE 802.16e specification takes into account successful and unsuccessful transmission of packets, for example data packets, and includes mechanisms designed to thwart problems with packet transmission, such as requiring in-order transmission of packets, and retransmission by a transmitting entity of packets that were not received properly by a receiving entity.
The antennae used for the transmission or receipt of these packets play a crucial role. An antenna is a device that transmits or receives electromagnetic wave signals. The signals may be, for example, received by another antenna located at a proximate or a distant location. The antennae may be mounted within, for example, a transmission device in a wireless communication network. Some examples of transmission devices include wireless base station or access point devices, and mobile station devices. One example wireless communication network system is disclosed in the Mobile WiMAX Technical Overview and Performance Evaluation document prepared on behalf of the WiMAX Forum and published on Feb. 21, 2006, which is hereby incorporated by reference.
The mechanism of selecting an antenna from a plurality of antennae to attain a superior channel is called switch diversity. Typically, switch diversity selects a new antenna when directed and continues operations by using the new antenna. Unfortunately, since mobile station devices usually have a single radio chain, it extremely difficult for a mobile station device to determine which antenna offers the best channel without actually using the antenna. Thus, it is likely that the mobile station device will perform worse after the switch to the new antenna. When this occurs, the mobile station device often iterates through untried antennae searching for an antenna that would work. Such antenna search iterations can result in a lengthy interval of service outage for the mobile station device.
Normally, antenna selection is based on the value of a quality metric, related either to the antenna used and/or the communication channel (e.g., an antenna gain figure, a cyclical redundancy check (CRC) parameter, a receive signal strength indicator (RSSI), a carrier to interference+noise ratio (CINR), a signal-to-noise figure, a bit error rate, a symbol error rate, or an equivalent quality metric). The types of quality metrics may also be divided into two major categories: (1) those which are designed to monitor signal transmissions and select an antenna as the signal is received and (2) those which are designed to monitor signal transmissions and select an antenna after the signal is received.
In a time division multiplexed access (TDMA) wireless system, for example, the antenna selection is controlled by software or logic circuitry. In this system, a CRC parameter or an equivalent is generally used to select an antenna after the signal is received. CRC is based on polynomial division in which each bit of a packet of data represents one coefficient of a polynomial. The polynomial is then divided by a pre-programmed polynomial to yield a quotient polynomial and in some cases a remainder polynomial. When the division yields a remainder polynomial, the system assumes that a transmission error occurred and selects another antenna. If, however, the division does not yield a remainder polynomial, the system assumes no transmission errors occurred and therefore does not select another antenna.
One example of a current antenna selection process is illustrated in
While CRC provides antenna selection by monitoring transmitted data, it has disadvantages. Its primary shortcoming is that antenna selections are not made in real time. The present antenna selected is based on a previous CRC comparison, which does not change until the antenna receives a poor quality signal. The time delay that exists between receiving an incoming signal and selecting another antenna makes the selection process susceptible to errors due to interference. A CRC selection may be accurate if a transmitter or receiver is stationary or moves at a slow rate of speed, because the communication environment is subject only to slight variations in time. However, when a transmitter or receiver moves at a high rate of speed, this time delayed process may be ineffective because it may not react to a changing environment and thus, it may be susceptible to interference.
Another technique for antenna diversity switching monitors signal transmissions and selects an antenna as the signals are received. Preamble diversity switching is an example of a system that provides real-time measurements and real-time antenna selection. Preamble diversity switching sequentially measures the receive signal strength of a diversity of antennae at the beginning of each extended preamble. The receive signal levels of each antenna, which are the receive signal strength indicators (RSSI), are stored and compared. The antenna with the higher RSSI value is selected. When the RSSI value associated with another antenna is higher, that antenna is then selected.
The preamble diversity switching process provides the benefit of selecting an antenna as signals are received. The system is less affected by rapid environmental change. However, problems arise when differences between RSSI values are insignificant. When insignificant differences exist, the system may experience some uncertainty when selecting an antenna. This is simply because minor differences in RSSI values indicate that the signal qualities received by the antennae are similar and therefore, an antenna selection will not necessarily improve receiving quality. Therefore, a conventional preamble diversity switching process may not be the best method for selecting an antenna.
It is not unusual for an antenna to receive a signal across a fading channel. Multiple antennae are typically used in communication systems to provide another option to turn to, in the event of poor signal reception due to a fading channel, so that a good channel with no fading can be found. Some examples of causes of a fading channel include phase shift in the signal and multi-path interference errors. The RF energy that is transmitted between antennae can experience destructive and constructive interference due to multiple paths taken by the energy with multiple delays on the way to a receive antenna. The interference can cause a receive antenna to receive a packet in error or to miss a packet entirely.
Ideally, antenna diversity techniques are used when a particular channel is fading due to multi-path effects so that changing from one antenna to another antenna provides another communication channel that in all probability is not fading. Trying and testing multiple antennae using antenna diversity typically takes place during a preamble, header, or training portion of the packet. The preamble is examined rather than the data so that no data are lost while the different antennae are being tested.
There are several reasons why this approach is undesirable for the IEEE 802.16e standard, and for any other high data rate radio system. First, the packet preamble in IEEE 802.16e is quite short, because a short preamble is desirable in any high data rate communications system in order to keep the efficiency of the communications system high. If the preamble is a long period in time, then the efficiency is low. While having a short preamble is good for efficiency, the short preamble reduces the time available to test the antennae. Switching between antennae takes a certain time based on the physical constraints of driving electrical switches. In addition, there is a minimum time needed to measure the signal from a given antenna to effectively determine the quality of the signal. When the measurement time (i.e., the duration of the preamble) is very short, a very poor estimate of the quality may be obtained.
In contrast to IEEE 802.16e systems, most wireless systems are narrowband signal systems. Narrowband signals are generally thought of in terms of having signal bandwidths of hundreds of kilohertz (kHz), for example, 500 kHz or 1 megahertz (MHz), or less, depending on the transmitting and receiving channel response. Wideband and broadband signals are generally thought of in terms of having signal bandwidths above 1 MHz depending on the transmitting and receiving channel response. In IEEE 802.16e systems, the signals have operating carrier frequencies in the neighborhood of 2-6 GHz.
At higher frequencies the signal more directional and more easily interrupted by relative movements of the transmitter and/or receiver. Furthermore, at higher frequencies the amount of data transmitted in a unit of time increases, creating a need to avoid or minimize interruptions caused by antenna failure. Therefore, an antenna switching algorithm should be optimized as much as possible to deal with the greater vulnerabilities and consequences of higher frequency and faster data transmission environments.
In view of the foregoing, what is needed is an improved method and system to more closely optimize the selection of an antenna from a plurality of antennae when an antenna and/or channel is degrading during use. Wideband wireless antenna applications and narrowband wireless antenna applications could both benefit from such methods and systems.
The present invention can be implemented in numerous ways, such as by a method, a circuit, or a system. Two aspects of the invention are described below.
A first aspect of the invention is directed to a method to switch among a plurality of antennae based on at least one quality metric. The method includes monitoring over a first period of time a first quality metric relating to the use of a first antenna, and determining a time variation indicator of the first quality metric; storing the time variation indicator of the first quality metric in a first memory location; and switching to a second antenna; monitoring over a second period of time a second quality metric relating to the use of the second antenna, if the second quality metric relating to the use of the second antenna fails the pre-defined quality metric value, and the time variation indicator of the first quality metric is small, switching back to the first antenna and repeating substantially all preceding operations to switch to a third antenna when necessary; if the second quality metric relating to the use of the second antenna fails the pre-defined quality metric value, and the time variation indicator of the first quality metric is large, switching to a third antenna and repeating substantially all preceding operations with the third antenna substituted for the first antenna.
A second aspect of the invention is directed to a method to switch among a plurality of antennae based on at least one quality metric. The method includes monitoring over a first period of time a first quality metric relating to the use of a first antenna, and determining a time derivative slope of a first quality metric; if the first quality metric does not fail a pre-defined quality metric value, remaining with the first antenna; if the first quality metric fails the pre-defined quality metric value; storing the time derivative slope of the first quality metric in a first memory location; switching to a second antenna; monitoring over a second period of time a second quality metric relating to the use of the second antenna; if the second quality metric does not fail the pre-defined quality metric value, remaining with the second antenna; if the second quality metric relating to the use of the second antenna fails the pre-defined quality metric value, and the time derivative slope of the first quality metric has an absolute value less than a pre-defined time derivative slope threshold, switching back to the first antenna and repeating substantially all preceding operations to switch to a third antenna when necessary; if the second quality metric relating to the use of the second antenna fails the pre-defined quality metric value, and the time derivative slope of the first quality metric has an absolute value not less than the pre-defined time derivative slope threshold, switching to a third antenna and repeating substantially all preceding operations with the third antenna substituted for the first antenna.
These and other aspects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The invention provides a method and a system to more closely optimize the selection of antennae in wireless communication systems. Various embodiments of the invention can be applied to communication applications, biological applications, medical applications, electronic applications, and any other applications where a better antenna or channel selection algorithm can be beneficially used in communications. In this specification, drawings, and claims, any instance of the term radio-frequency is defined as any electromagnetic signal frequency in the frequency range of 50,000 to 100,000,000,000 cycles per second (Hertz).
Other terms used below and in the figures are defined as follows. Thressw—This is the switch threshold. This is the point at which degradation of antenna or channel condition (e.g., determined by at least one metric of quality) indicates that a switch is necessary. The antenna or channel is still usable until its quality falls below the failure threshold. This can be computed from simulated performance of the device. Thresfailure—This is the failure threshold. This is the point at which degradation of the antenna or channel condition indicates that the current antenna or channel is unusable. This can be computed from a simulated performance of the device. εhysteresis—This is a factor that adds additional margin to ensure an antenna or channel switch is performed at the proper condition. τhysteresis—This is the interval needed to determine whether an antenna or channel switch is needed. The antenna or channel condition must be below the switch threshold for this amount of time before an antenna or channel switch is attempted. These parameters are illustrated below in
Switch diversity, by definition, is the process of selecting a single antenna with an acceptable quality metric (e.g., an antenna quality, channel quality, or equivalent) at a given time. Quality can be quantified by a set of one or more quality metrics. Various embodiments of the invention can:
One way to describe the trend of channel degradation can use a linear interpolation of successive samples of at least one quality metric.
In one embodiment, failing a pre-defined value for least one first quality metric could mean the quality metric decreases with decreasing quality and fails by falling below a pre-defined quality metric value. In another embodiment, the quality metric may increase with decreasing quality and fail by going above a pre-defined value. In one embodiment of the invention, a plurality of quality metrics may be used and some weighted average of the plurality of quality metrics is used to determine whether an antenna and/or a channel in use is failing to satisfy some pre-defined value.
Several embodiments of the invention are possible. The phrase “in one embodiment” used in the specification can refer to a new embodiment, a different embodiment disclosed elsewhere in the application, or the same embodiment disclosed earlier in the application. The exemplary embodiments described herein are for purposes of illustration and are not intended to be limiting. Therefore, those skilled in the art will recognize that other embodiments could be practiced without departing from the scope and spirit of the claims set forth below.
