Advanced Antenna Diversity Mechanism

This disclosure relates to a process for selecting an antenna in a wireless communication system having at least two antennas. In particular, the present invention is designed to select the best antenna based on a plurality of quality indicators.

In wireless communication systems, incoming signals often experience time dispersion and fading which is referred to as multipath effect. Multipath effect occurs when a signal and its replica arrive at a receiver out of phase to any extent. Generally, multipath effect occurs because artificial and natural obstructions cause transmitted signals to follow several propagation paths from a transmitter to a receiver. In wireless technology, the phase differences of the signals arriving at the receiver produce background noise, and in severe cases, cause the receiver to squelch. Conventional wireless technology utilizes antenna diversity to minimize this problem. A wireless network commonly includes more than one antenna positioned such that the signals received by the antennas are not correlated.

In DECT (Digital Enhanced Cordless Telecommunications) and PWT (Personal Wireless Telecommunications) systems, for example, two antennas are available in the base station and/or portable. FIG. 1 illustrates a block diagram of a conventional wireless base station having a diversity of antennas.

When more than one antenna is used in a wireless system, a protocol is employed to select the best antenna. Normally, antenna selection is based on a singular quality indicator adapted from the field of cryptography. The kinds of quality indicators may be divided into two categories namely: (1) those which are designed to authenticate signal transmissions and select an antenna as the signal is received and (2) those which are designed to authenticate signal transmissions and select an antenna after the signal is received. Known devices of both types have drawbacks and disadvantages.

In a TDMA-based system, for example, the antenna selection is controlled by software or logic circuitry. In this system, a cyclical redundancy check (CRC) like parameter 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.

The antenna selection process is illustrated in FIG. 2. A CRC error rate that produces good speech quality is used as a threshold for selecting an appropriate antenna. If the present antenna provides a CRC error that is below the threshold value, no switching occurs. However, when the CRC error rate rises above the threshold value, another antenna is selected.

While CRC provides antenna selection by authenticating 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 authenticates 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 antennas 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. An example of a preamble diversity switching process is shown in FIG. 3.

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 antennas are similar and therefore, an antenna selection will not necessarily improve receiving quality. Therefore, a preamble diversity switching process alone may not be the best parameter for selecting an antenna. An additional indicator may be desirable to select an optimal antenna.

It is therefore an object of the invention to implement an appropriate decision strategy in a DECT, PWT, and TDMA telecommunication systems that are based on real and predicted quality criteria.

Another object of the invention is to provide an improved antenna selection process for a TDMA telecommunication system that may pre-select an optimal antenna before an error is detected.

It is a further object of the invention to implement simple antenna-selection algorithms that identify the antenna that will receive the highest quality signals.

Still another object of the invention is to provide an improved antenna selection process for TDMA telecommunication systems that tracks ancillary quality factors such as error vectors and gradient error vectors.

Yet another object of the invention is to seamlessly integrate multiple quality indicators contemporaneously into a sophisticated antenna switching strategy for TDMA telecommunication systems.

A system for selecting an antenna from a diversity of antennas in a DECT, PWT, or TDMA-based telecommunication system is disclosed. The system combines multiple criteria to improve antenna diversity selections. The principal selection process is based on preamble diversity comparisons that operate in real-time. A preamble diversity switching circuit measures receive signal strength indicators (RSSI) from a plurality of antennas and selects the antenna that provides the highest RSSI value when the difference between the RSSI values exceeds a predetermined threshold. When the value of the RSSI difference falls below the predetermined threshold, a cyclical redundancy check process (CRC) and a vector detection process (SVD) are employed jointly. The CRC process decodes a CRC value from a signal's content and compares it to a predetermined threshold. If a CRC error is detected a first weighted function is calculated. In a first SVD process, the difference between a reference signal vector and a received signal vector is computed. If an SVD error is detected a second weighted function is calculated. When the sum of the first and second weighted factors is greater than a predetermined threshold antenna selection occurs. When the threshold is not met, diversity selections remain unchanged and control of the antenna selection process returns to the preamble diversity switching process. It should be noted, the SVD process may also be based on the gradient error vector, .gradient.f(E.sub.k,n1), which represents the magnitude of the received signal's rate of change.

FIG. 1 is a block diagram of a conventional wireless base station having a diversity of antennas.

FIG. 2 is a block diagram of a Cyclical Redundancy Check antenna switching process used in the prior-art station of FIG. 1.

FIG. 3 is a block diagram of a Preamble Diversity Switching Process also used in the prior-art station of FIG. 1.

FIG. 4 is a flow chart of a first embodiment of the antenna switching process.

FIG. 5 is a flow chart of a second embodiment of the antenna switching process.

FIG. 6 is a flow chart of a third embodiment of the antenna switching process.

FIG. 7 is a flow chart of a fourth embodiment of the antenna switching process.

FIG. 8 is a schematic diagram of the of the antenna switching circuits used in the antenna switching processes of FIGS. 4-7.

In the drawings, depicted elements are not necessarily drawn to scale and alike and similar elements may be designated by the same reference numerals through several views.

The signal vector detection (SVD) process used in a first embodiment focuses on phase modeling and phase prediction of broadcast signals for TDMA communication systems. In this process, a signal vector detection error or error vector E.sub.k,n1 is calculated by subtracting an estimated signal vector S.sub.k,n1 from a reference signal vector S.sub.k,n1 (E.sub.k,n1 =S.sub.k,n1 -S.sub.k,n1). A function, f(E.sub.k,n1), is then calculated. In this embodiment, a sum (SUM) of the absolute value (ABS) of E.sub.k,n1, is calculated (f(E.sub.k,n1)=SUM(ABS E.sub.k,n1)). However, other mathematical functions may be used. Since E.sub.k,n1 is measured during the receiving process and is compared to a predetermined threshold, an antenna selection decision will not occur in real time as in a batch process. However, the signal detection process may also be based on a second criteria, a gradient error vector, .gradient.f(E.sub.k,n1).

The term "gradient" as used in this specification is used to describe a vector that represents the magnitude of a signal's rate of change. With respect to TDMA utilization, an antenna selection based on the gradient error vector can initiate antenna selections before signal quality declines.

In accordance with the first embodiment, an antenna selection system is illustrated in FIG. 4. This embodiment seamlessly integrates a preamble diversity switching process (PDS), a first authentication process, with a cyclical redundancy check (CRC) process, a second authentication process, and the signal vector detection (SVD) process, a third authentication process, to form a unified antenna switching strategy that tracks signal quality.

As shown, radio frequency broadcasts are sequentially processed into receive signal strength indicators RSSI.sub.0 and RSSI.sub.1 and are stored, where RSSI.sub.0 is measured from a first antenna (Antenna.sub.0) and RSSI.sub.1 is measured from a second antenna (Antenna.sub.1) (steps 102 and 104). RSS.sub.0 and RSSI.sub.1 are then compared to generate a RSSI difference signal (.increment.RSSI) (step 106). When .increment.RSSI is greater than or equal to a first predetermined quality threshold value T.sub.1, the antenna having the highest RSSI value is selected and an N-bit counter is reset (steps 108 and 112). However, when .increment.RSSI is less than T.sub.1, the N-bit counter is incremented and then compared to a predetermined slot count N.sub.1 (steps 108 and 110). If the value of the N-bit counter is less than N.sub.1, protocol requires that the preamble diversity switching process begin a new cycle (step 110). If, however, the value of the N-bit counter is equal to or greater than N.sub.1, protocol initiates joint CRC and SVD control (steps 110).

As described previously, a CRC like parameter authenticates data after the data is received as in a batch process. In this process, the transmitted signal is encoded with a circular redundant code derived from the signal's content. After the signal is received and decoded into a CRC error rate a weighted CRC error rate is calculated by scaling the difference between the CRC error rate and a second predetermined quality threshold value, T.sub.2 (.sigma.(CRC-T.sub.2)) (steps 114 and 116). In this embodiment a predetermined CRC error rate is used as the threshold value to ensure high quality data and audio communications. A CRC error rate of 1%, for example, may be used as the threshold T.sub.2 for audio communications, because a CRC error rate below 1% generally produces good voice quality.

In the SVD process, f(E.sub.k,n1) associated with the current antenna is read and stored (step 118). A weighted SVD error rate is then calculated by scaling the difference between f(E.sub.k,n1) and a third predetermined quality threshold value, T.sub.3 (.beta.(f(E.sub.k,n1)-T.sub.3)) (step 120). When the sum of the weighted CRC and SVD error rates (.sigma.(CRC-T.sub.2)+.beta.(f(E.sub.k,n1)-T.sub.3) where .sigma.+.beta.=1) is greater than a fifth predetermined quality threshold value T.sub.5, another antenna is selected and control is returned to the PDS process (steps 124, 126, and 128). However, when the sum of the weighted CRC and SVD error rates is equal to or is less than T.sub.5, no diversity selections occur and control is returned to the PDS process (steps 124 and 126). In this embodiment, the fifth predetermined threshold is equal to zero. However, those skilled in the art understand that T.sub.5 will vary according to the environment of the embodiments use.

The second embodiment illustrated in FIG. 5 is similar to that depicted in FIG. 4. However, in this embodiment the SVD process is based on the gradient error vector, .gradient.f(E.sub.k,n1), rather than the error vector, f(E.sub.k,n1). In this configuration, f(E.sub.k-1,n1) from multiple previous time frames are measured and stored. The f(E.sub.k,n1) associated with the current antenna is then read and compared to f(E.sub.k-1,n1) to generate the .gradient.f(E.sub.k,n1) value (.gradient.f(E.sub.k,n1)=f(E.sub.k,n1)-f(E.sub.k-1,n1)) (step 202). A weighted SVD error rate is calculated by scaling the difference between .gradient.f(E.sub.k,n1) and a third predetermined quality threshold value, T.sub.3 (.beta.((.gradient.f(E.sub.k,n1)-T.sub.3)) (step 204). Like the first embodiment, when the sum of the weighted CRC and SVD error rates (.sigma.(CRC-T.sub.2)+.beta.(.gradient.f(E.sub.k,n1)-T.sub.3) where .sigma.+.beta.=1) is greater than the fifth predetermined quality threshold value T.sub.5, another antenna is selected and control is returned to the PDS process (steps 206, 126, and 128).

FIG. 6 shows a third embodiment wherein the authentication process includes the PDS, f(E.sub.k,n1), and .gradient.f(E.sub.k,n1) processes. Like the prior configurations, the authentication process begins with the PDS process that does not delegate antenna diversity control until N.sub.1 consecutive instances in which .increment.RSSI is less than T.sub.1 (steps 108 and 110). When .increment.RSSI is less than T.sub.1, f(E.sub.k,n1), and .gradient.f(E.sub.k,n1) process criteria are employed. Like the prior two embodiments, the SVD criteria are identical to the prior embodiments and therefore are not repeated. When control is delegated to the SVD criteria, the decision threshold is calculated by summing the scaled SVD error rates (.sigma.(f(E.sub.k,n1)-T.sub.2)+.beta.(.gradient.f(E.sub.k,n1)-T.sub.3) where .sigma.+.beta.=1). When the scaled sum of the SVD error rates is greater than T.sub.5, another antenna is selected and control is returned to the PDS process (steps 126 and 128).

The embodiment depicted in FIG. 7 is similar to that depicted in FIGS. 6, but further comprises the steps of contemporaneously evaluating the CRC process. Thus, FIG. 7 constitutes a further improvement of the embodiment shown in FIG. 6 by evaluating additional reliability criteria. In accordance with a fourth embodiment, illustrated as FIG. 7, the processing steps of reading and calculating the respective CRC and f(E.sub.k,n1), and .gradient.f(E.sub.k,n1) are identical to the prior embodiments and therefore are not repeated. In this embodiment, when control authority is delegated to the synchronous time-delayed indicators, the decision threshold is the scaled sum of the CRC, f(E.sub.k,n1), and .gradient.f(E.sub.k,n1) processes (.sigma.(CRC-T.sub.2)+.beta.(f(E.sub.k,n1)-T.sub.3)+.gamma.(.gradient.f(E. sub.k,n1)-T.sub.4) where .alpha.+.beta.+.gamma.=1) (step 402). When the sum of the weighted CRC and SVD error rates is greater than the fifth predetermined quality threshold value, T.sub.5, another antenna is selected and control is returned to the PDS process (steps 402, 126, and 128). However, when the weighted sum of the CRC and SVD error rates is equal to or is less than T.sub.5, no diversity selections occur and control is returned to the PDS process (step 126).

The concepts and processes previously illustrated may be implemented through software or logic circuitry. The aforementioned embodiments were employed using logic circuitry. As depicted in FIG. 8, when the broadcast signals are received, the signals are processed using a PDS circuit 502. The PDS circuit 502 produces the .increment.RSSI signal which is fed to a comparator circuit 504 by a first output. When the .increment.RSSI is greater than or equal to the first predetermined quality threshold value T.sub.1, the comparator 504 resets an N-bit counter circuit 506 and enables a latch circuit 508. The latch circuit 508 latches a second output from the PDS circuit 502 to a multiplexer circuit 510 enabled by the N-bit counter circuit 506 control to initiate antenna selections. Once the .increment.RSSI falls below T.sub.1, the counter circuit 506 is incremented in response to a third output from the PDS control circuit 502. In this embodiment, after more than N.sub.1 consecutive low .increment.RSSI cycles, control of the antenna selection is transferred to a plurality of synchronous time-delayed criteria circuitry 512 by the N-bit counter circuits 506 control of the multiplexer circuit 510. The time delayed circuitry 512 that processes CRC, f(E.sub.k,n1) and .gradient.f(E.sub.k,n1) are described in the U.S. patent applications that were incorporated by reference with additional signal conditioning implemented through conventional adders, multipliers, and comparator circuitry. The time delayed circuitry 512 controls antenna selection until the .increment.RSSI equals or exceeds T.sub.1. When .increment.RSSI equals or exceeds T.sub.1, the PDS circuit 502 reacquires control of the selection process and the comparator circuit 504 resets the counter circuit 510.

Those skilled in the art will implement the steps necessary to provide the device and methods disclosed herein and understand the process parameters including T.sub.1, T.sub.2, T.sub.3, T.sub.4, T.sub.5, .alpha., .beta., .gamma. and N.sub.1 vary according to the environment of the embodiment's use as rural and urban areas have different receiving characteristics. The disclosed embodiments and concepts of the invention enjoy utility in any type of TDMA telecommunication system where data authentication occurs.

Only the embodiments of the invention and examples of its versatility are described within this disclosure. Variations and modifications of the embodiments disclosed herein may be made based on the description herein disclosed, without departing from the scope and spirit of the invention set forth in the following claims.