CA2565032A1 - A reconfigurable mimo antenna system for ieee 802.11n systems - Google Patents

Description

A RECONFIGURABLE MIMO ANTENNA SYSTEM FOR
IEEE 802.11N SYSTEMS

FIELD OF THE INVENTION

The present invention relates to wireless local area networks and in particular to a novel reconfigurable MIMO antenna system for use in wireless local area networks supporting the evolving IEEE 802.11n standard.
BACKGROUND TO THE INVENTION

Wi-Fi, or WLAN, is the name sometimes given to the 802.11 series of wireless telecommunications standard developed by the IEEE. The various standards were intended for wireless communications with portable devices, when in proximity to an access point (AP) of a local area network.

The 802.11 standards leave connection criteria and roaming totally open to the client or subscriber station (SS). An AP periodically broadcasts its Service Set Identifier (SSID) and other system configuration information via packets or beacons. Based on the received information, the client may decide whether to connect to an AP.

Traditional standards within the IEEE 802.11 family include 802.11a, 802.11b and 802.11g, each of which differ in detail. 802.11b, the first widely accepted wireless networking standard, was released in 1999. It uses the 2.4 GHz band as its operating frequency in North America and boasts a typical data rate of 6.5 Mbit/s, up to a maximum of 11 Mbit/s. It uses a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) method (technically Complementary Code Keying (CCK)), usually in a point to multipoint configuration, wherein an access point communicates via an omni-directional antenna with one or more clients that are located in a coverage area around the access point.
802.11a, also released in 1999, has an operating frequency in the 5 GHz band, a typical data rate of 11 Mbit/s up to a maximum of 25 Mbit/s. It uses a 52-subcarrier Orthogonal Frequency-Division Multiplexing (OFDM) across 12 non-overlapping channels, 8 of which are dedicated to indoor use and 4 to point to point. Of the 52 OFDM subcarriers, 48 are for data and 4 are pilot subcarriers with a carrier separation of 0.3125 MHz (20 MHz/64), each of which can be BPSK, QPSK, 16-QAM
or 64-QAM encoded.

802.11g was released in June 2003. Like 802.11b, it has an operating frequency in the 2.4 GHz band, but boasts a typical data rate of 25 Mbit/s up to a maximum of 54 Mbit/s. OFDM is used for data rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbit/s, CCK for 5.5 and 11 Mbit/s and DBPSK/DQPSK DSSS for 1 and 2 Mbit/s data rates.

All three standards are intended to operate across an indoor range of about 100 feet.

By contrast, the 802.11n standard, while still evolving, is expected to operate in either the 2.4 GHz or 5 GHz bands, with a typical data rate of 200 Mbit/s up to

-2-a maximum of 540 Mbit/s, and at an indoor range of up to 160 feet. As such, it should be up to 50 times faster than 802.11b and well over 10 times faster than 802.11a and 802.11g. The release date of the standard is estimated to be April 2008.

802.11n introduces multiple input multiple output (MIMO) processing into 802.11. In MIMO
processing, a plurality of transmitter and receiver antennas are used to allow for increased data throughput through spatial multiplexing and increased range by exploiting spatial or other diversity characteristics, by coding schemes and otherwise.

Accordingly, each SS and each AP may contain a multiplicity of antennas (the standard authorizes up to 4).

The 802.11n-oriented antenna systems for use with APs in the prior art have contemplated using a plurality of fixed pattern sectorized microstrip antennas, including omni-directional antennas, because of their relatively well-understood design and operation and simple implementation. Nevertheless, the engineering trade-off in so doing is that such implementations are constrained by the limitations of such rudimentary antennas.
SUMMARY OF THE INVENTION

Accordingly, it is desirable to provide an 802.11n-oriented wireless MIMO antenna system which

-3-provides improved signal to noise and interference ratio (SNIR) performance.

Additionally, it is desirable to provide an 802.11n-oriented wireless MIMO antenna system that implements and coordinates a plurality of directional beam antennas that mimic and improve upon the performance of fixed pattern sectorized antennas.

Furthermore, it is desirable to provide an 802.11n-oriented wireless MIMO antenna system having a simple, easily manufactured and configurable architecture.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which:
Figure 1 is a block diagram of the inventive antenna assembly in accordance with a first embodiment of the present invention;

Figure 2 is a perspective view of a MIMO sub-assembly in accordance with the embodiment of Figure 1;
Figure 3 is a plan view of the layout of a side of a double-sided PCB monopole Yagi-Uda antenna used in the MIMO sub-assembly of Figure 2;

-4-Figure 4 is a block diagram of a measurement setup to derive simulated and measured results for the MIMO sub-assembly of Figure 2;

Figure 5 is a plot of simulated and measured input reflection for the double-sided PCB monopole Yagi-Uda antenna in accordance with the embodiment of Figure 1;

Figure 6 is a plot of a simulated three-dimensional antenna pattern at 2.4 GHz derived using ANSOFT Corporation HFSS in accordance with the measurement setup of Figure 4;

Figure 7 is a plot of the azimuth pattern measurement for the measurement setup of Figure 4;
Figure 8 is a plot of the azimuth pattern measurement for the measurement setup of Figure 4;
Figure 9 is a block diagram of components in the embodiment of Figure 1; and Figure 10 is a flow chart showing processing steps in a beam selection algorithm used in the embodiment of Figure 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention accomplishes these aims by providing a novel MIMO antenna system that may interface with a WiFi access point system using multiple antenna technology oriented toward the evolving 802.11n standard.

-5-The novel antenna system comprises an antenna assembly as shown generally at 100 in Figure 1. The assembly 100 comprises a plurality (preferably 3) of circular metallic disks 110, preferably deposed upon an antenna PCB substrate 111, each of which holds a plurality (preferably 6) of directional beam antennas 210, which in the preferred embodiment each comprise a double-sided printed circuit monopole Yagi-Uda antenna vertically oriented around the disk 110 circumference and connected by an SMA conductor 220 soldered therewith, as shown in Figures 2 and 3. Preferably, each circular metallic disk 110 is deposited upon the antenna PCB
substrate 111 and has a 24 cm diameter. The centre conductor 220 of the Yagi-Uda antenna 210 is soldered to its driver 213, while the reflector 212 and directors 214, 215 are soldered to ground.

The disks 110 are suspended below a planar board 120 having holes 121 cut therein co-axial with the centre of each disk 110. Preferably, the planar board 120 is comprised of Perspex (TM) material. The holes 121 permit electrical connection by way of cables 901 to pass from the antenna components mounted on one side thereof to the control circuitry 900 mounted on the other side.

Because of the large number of identical Yagi-Uda antennas 210 that make up the inventive antenna system (in the preferred embodiment 18), manufacturability and reproducibility concerns are addressed by printing the monopoles 210 on a monopole

-6-dielectric substrate 211 that is vertically mounted in spoke-like fashion around each horizontal disk 110.
The inventive use of double-sided PCB
manufacturing technology permits more accurate monopole heights that correspond to more accurate beam shapes and design flexibility, while maintaining stability, good gain, wide bandwidth and low return loss characteristics.
Simulations have shown that performance is also generally improved over discrete wire monopole Yagi-Uda antennas.

Each Yagi-Uda antenna 210 has one reflector 212, one driver 213 and two director elements 214, 215 and is made by etching identical strips on both sides of the PCB dielectric substrate 211. The double-sided pattern of such an antenna provides both symmetrical pattern and a higher radiation efficiency over prior attempts to mass produce Yagi-Uda antennas using single sided PCB techniques.

Preferably, the monopole dielectric substrate 211 is thin, of approximately 0.787 mm, to avoid dielectric losses, and manufactured from FR4 material having a permittivity of 4.2 and a tangent loss of 0.02.
In a preferred embodiment, each monopole dielectric substrate has a length of 8 cm and a width of 3 cm. To achieve operation in the 2.43 GHz, band as shown in Figure 3, the reflector 212 is preferably 27.5 mm long, the driver 213 is 24.6 mm long, the first director 214 is 21.7 mm long and the second director 215 is 22.4 mm long. The reflector 212 and driver 213 are

-7-separated by 25.5 mm, while the driver 213 and first 214 and second directors 215 are each separated by 21.2 mm.
Each strip is 2 mm wide and the distal ends of each strip are connected through the substrate by vias 218. An SMA
conductor 219 is connected to the driver 213 and adapted to conductively engage the SMA conductor 220 on the disk 110.

The positioning of the directional beam antenna 210 about the disk 110 is an important factor for determining the performance of the inventive MIMO antenna sub-assembly shown generally in Figure 1. Preferably, the near edge of each antenna substrate is positioned 25 mm from the centre of the disk 110. This can be facilitated by a cylindrical horizontal spacer hub 140 mounted coaxially with the centre of the disk 110 as shown in Figure 1. The horizontal spacer hub 140 is not shown on Figure 2 for clarity purposes only.

Simulations have shown that the double sided PCB monopole Yagi-Uda antenna 210 has high efficiency and a beam pattern that is symmetrical due to the double sided etching. The input reflection is well below -10dB
through a bandwidth of 150 MHz without any extra matching circuits.

The monopole implementation has the advantage of being of shorter length compared to a dipole Yagi-Uda antenna and an easier feed with good matching. In the preferred embodiment, the Yagi-Uda monopole antenna 210 has a height of almost a quarter wavelength, that is half

-8-of the height of conventional printed circuit Yagi-Uda dipole antennas and thus boasts a low profile.

Each of these directional beam antennas 210 generate a beam with an azimuth beamwidth so that together, the antennas 210 co-located on a disk 110 generate a 360 coverage pattern.

A sectorized sleeve monopole antenna 231, which is preferably an omni-directional antenna, is co-located coaxially with each disk 110 on a small ground plane 230 above the directional beam antennas 210 as shown on Figure 1.

Any desired vertical spacing between the ground plane 230 and the directional beam antennas 210 may be provided by a second cylindrical vertical spacer hub 141 positioned coaxially with and abutting against the horizontal spacer hub 140. Preferably, the vertical spacer hub 141 is of larger diameter than the horizontal spacer hub 140, as shown in Figure 1, to provide a more stable base for the ground plane 230. However, those having ordinary skill in this art will readily appreciate that a single hub of appropriate dimension could replace both the horizontal 140 and vertical 141 spacer hubs.

Each disk 110, together with the vertical directional beam antennas 210 and the omni-directional antenna 231 comprise a single MIMO antenna sub-assembly 200.

Each MIMO antenna sub-assembly 200 is separated from its neighbour, in order that it intercepts an

-9-independent data stream from a wireless subscriber. The throughput is thus additively enhanced when compared to single antenna systems such as that covered by the 802.11a, 802.11b and 802.11g standards.

Preferably, the MIMO antenna sub-assemblies 200 are positioned in a linear array to minimize mutual shadowing. Simulations suggest that the antenna assembly 100 is preferably mounted on a ceiling 130 similar to a fluorescent tube light fixture.as shown in Figure 1. In order to protect the sensitive antenna elements, the antenna assembly 100 is preferably enclosed by a radome 150.

Figure 4 shows the measurement setup used to generate certain simulation and measurement results relating to the MIMO sub-assembly 200.

Figure 5 shows a three-dimensional simulated pattern at 2.4 GHz derived using ANSOFT Corporation HFSS.
The pattern shown is for the inventive MIMO sub-assembly 200 at an elevation angle of 75 and an omni-directional monopole antenna operating at a frequency of 2.5 GHz.

Figure 6 shows a measured antenna beam pattern for the MIMO sub-assembly 200.

Figure 7 shows an overlay of simulation results of an omni-directional monopole antenna operating at a frequency of 2.5 GHz and the MIMO sub-assembly at an elevation angle of 75 .

-10-Figure 8 shows an additional overlay of a simulation result of the MIMO sub-assembly at an elevation angle of 90 .

Figure 9 is a block diagram of processing components, shown generally at 900 in Figure 9. The processing components 900 are connected to the omni-directional antenna 231, the plurality of directional beam antennas 210 and an 802.11n compliant processor 970 and comprise a beam selector switch 905, a plurality of pre-selector filters 910, 915, a plurality of RF transmit / receive converters 920, 925, a plurality of power amplifiers for transmission 921, 926, a line switch 930, a processor assembly 940, a beam transmit / receive switch 950, an omni transmit / receive switch 955, a receive omni / beam switch 960 and a transmit omni / beam switch 965.

The beam selector switch 905 is connected to each of the directional beam antennas 210 corresponding to a single MIMO antenna sub-assembly 200 via cables 901 and to the pre-selector filter 910. It provides electrical RF connection between the selected antenna from the directional beam antennas 210, each of which corresponds to a beam, and the pre-selector filter 910.
Typically, the switching time for the beam selector switch 905 is on the order of 150 ns.

There is a pre-selector filter 910 associated with the selected directional beam antennas 210 and a pre-selector filter 915 associated with the omni-directional antenna 231. The pre-selector filter 910 is

-11-connected to the output of the beam selection switch 905 and the RF transmit / receive converter 920. The pre-selector filter 915 is connected between the omni-directional antenna 231 via cables 901 and the RF
transmit / receive converter 925. The pre-selector filters 910, 915 condition the signal received/transmitted along its associated antenna.

The RF transmit / receive converter 920 is connected to the pre-selector filter 910, to the line switch 930 and receives control signals 947 from the processor assembly 940. The RF transmit / receive converter 925 is connected to the pre-selector filter 915, to the omni transmit / receive switch 955 and receives control signals 946 from the processor assembly 940. The RF transmit / receive converters 920, 925 convert signals received from its associated pre-selector filter 910, 915 from RF down to baseband I & Q signals and I & Q signals received for transmission to its associated pre-selector filter 910, 915 from baseband up to RF. Preferably, the RF transmit / receive converters 920, 925 are MAX 2829 IC chips with an internal low noise amplifier and paired with an associated power amplifier PA model number MAX 2247 and has a noise figure [NF] of 3.5 dB. Those having ordinary skill in this art will readily appreciate that if the RF transmit / receive converters 920, 925 do not have internal low noise amplifiers, a discrete amplifier may be connected therewith as appropriate.

-12-The line switch 930 is connected to the RF
transmit / receive converter 920, the processor assembly 940 and the beam transmit / receive switch 950 and receives control signals 943 from the processor assembly 940. It tests the various beams and passes baseband I &
Q signals received from the various Yagi-Uda antennas 210 through the pre-selector filter 910 and the RF transmit /
receive converter 920 to the processor assembly 940, in order to identify the best beam to be used for a SS, and an operational mode in which baseband I & Q signals directed to and emanating from the identified best beam along the RF transmit / receive converter 920 are passed from and to the beam transmit / receive switch 950.

The processing assembly 940 is connected to the line switch 930. It receives control signals 971, 972 from the 802.11n compliant processor 970 and issues control signals 947, 946, 943, 944, 945 to the RF
transmit / receive converters 920, 925, to the line switch 930, to the beam transmit / receive switch 950, to the omni transmit / receive switch 955 respectively.
Preferably the processing assembly 940 comprises an analog to digital conversion subsystem 941 and a processing subsystem 942. In a preferred embodiment, the processing assembly is implemented in an field-programmable gate array (FPGA). An exemplary logical block diagram of the FPGA is shown in Figure 10.

The analog to digital conversion subsystem 941 converts baseband I & Q signals received from the line switch 930 while in best beam selection mode to digital

-13-form and forwards the digital information to the processing sub-system 942, where the information is evaluated and a best beam selected for the SS in question.

The processing subsystem 942 also receives signals from the 802.11n compliant processor 970 in the form of commands to move from an omni-directional mode to a beam mode and vice versa 971, and to move from a transmit to a receive mode and vice versa 972. The processing subsystem 942 thereafter generates control signals to the various switches and RF transmit / receive converters to give effect to the commands received from the 802.11n compliant processor 970.

The beam transmit / receive switch 950 is connected to the line switch 930, to the receive omni /
beam switch 960, to the transmit omni / beam switch 965 and receives control signals 944 from the processing assembly 940. The beam transmit / receive switch 950 moves, in response to control signals 944 from the processing assembly 940, between a transmit mode in which it passes information from the transmit omni / beam switch 965 to the line switch 930 and a receive mode in which it passes information from the line switch 930 to the receive omni / beam switch 960.

The receive omni / beam switch 960 is connected to the beam transmit / receive switch 950, the omni transmit / receive switch 955 and the 802.11n compliant processor 970. The receive omni / beam switch 960 moves in response to control signals from the processing

-14-assembly 940 between an omni mode in which signals originally received at the omnidirectional antenna 231 are passed by the omni transmit / receive switch 955 to the 802.11n compliant processor 970 and a beam mode in which signals originally received at a designated best beam corresponding to one of the directional beam antennas 210 are passed on by the beam transmit / receive switch 950 to the 802.11n compliant processor 970.

The transmit omni / beam switch 965 is connected to the 802.11n compliant processor 970, the beam transmit / receive switch 950 and the omni transmit / receive switch 955. The transmit omni / beam switch 970 moves in response to control signals from the processing assembly between an omni mode in which signals generated by the 802.11n compliant processor 970 are passed to the omni transmit / receive switch 955 for transmission by the omnidirectional antenna 231 and a beam mode in which signals generated by the 802.11n compliant processor 970 are passed to the beam transmit / receive switch 950 for transmission to the designated best beam corresponding to one of the directional beam antennas 210.

The 802.11n compliant processor 970 is connected to the receive omni / beam switch 960, the transmit omni / beam switch 965 and generates commands to the processing assembly 940 including to switch between a transmit mode and a receive mode 972. It is anticipated that third party manufacturers will design and implement such processors to provide Wi-Fi functionality in accordance with the evolving 802.11n standard. In most

-15-of these cases, such processors will be expecting that each of the MIMO antennas will be a sectorized antenna such as the omnidirectional antenna 231, rather than a plurality of directional beam antennas 210. In such a scenario, some additional logic to permit the 802.11n compliant processor 970 to generate commands to the processing assembly 940 to switch between an omni mode and a beam mode 971 may be appropriate.

Where, as in the preferred embodiment, the antenna assembly 100 comprises a plurality of MIMO
antenna sub-assemblies 200, each of the MIMO antenna sub-assemblies will interface with a common 802.11n compliant processor 970, where the combining of the signals from each of the MIMO antennas takes place, to increase throughput and SINR.

In operation, when a SS is detected, preferably by receipt of an RTS signal along the omnidirectional antenna 231, the received antenna signal is filtered by the pre-selector filter 915, attenuated and/or amplified as required for noise minimization purposes and down-converted to a series of baseband I & Q signals by receiver circuitry on the RF transmit / receive converter 925 and fed to the the omni transmit / receive switch 955, which is initially in receive mode.

The signal is fed out of the omni transmit /
receive switch 955 and into the receive omni / beam switch 960, which is initially in omni mode.

-16-The signal is fed out of the receive omni /
beam switch 960 and into the 802.11n compliant processor 970, where it is processed in conventional fashion.
Additionally, the 802.11n compliant processor 970 identifies an opportunity to identify a best beam for the communications between the AP with which the antenna assembly 100 is associated and the SS in question. It initiates this task by notifying the processor assembly 940, along control signal 971, to move from an omni mode to a beam mode.

In consequence thereof, if necessary, the processor assembly 940 issues control signal 947 to the RF transmit / receive converter 920, activating it and sending it into receive mode, control signal 943 to the line switch 930, sending it into best beam calculation mode, control signal 944 to the beam transmit / receive switch 950, sending it into receive mode.

As a result, antenna signals received at the various directional beam antennas 210 will be successively switched by beam selector switch 905 into connection with the pre-selector filter 910 where it is filtered and then attenuated and/or amplified as required for noise minimization purposes and down-converted to a series of baseband I & Q signals by receiver circuitry on the RF transmit / receive converter 920 and fed to the line switch 930, which is in best beam selection mode.
The signal received at each of the directional beam antennas 210 is received as baseband I & Q signals by the analog to digital conversion subsystem 941, converted

-17-into digital format and fed to the processing subsystem 942 where it is processed. From a comparison of the digitized antenna signals, the processing subsystem 942 identifies the best beam for the SS in question.

Thereafter, processing assembly 940 issues control signal 943 to the line switch 930 to send it into operational mode and the 802.11n compliant processor 970 will thereafter use the identified best beam for all communications with the SS, until superseded by a subsequent best beam or the best beam is declared non-functioning.

For signals received from the SS while in the beam operational mode, antenna signals received at the directional beam antenna 210 identified with the best beam will be switched by beam selector switch 905, which is activated by a control signal from the processing assembly 940, into connection with the pre-selector filter 910 where it is filtered and then attenuated and/or amplified as required for noise minimization purposes and down-converted to a series of baseband I & Q
signals by receiver circuitry on the RF transmit /
receive converter 920 and fed to the line switch 930, which is in operational mode. As a result, the signal received at the directional beam antenna 210 is passed as baseband I & Q signals through the beam transmit /
receive switch 950, which, if necessary, will have been sent into receive mode by the processing assembly 940 along control signal 944, through the received omni /
beam switch 960, which, if necessary, will have been sent

-18-into beam mode by the processing assembly 940 along control signal 944 and to the 802.11n compliant processor 970 for processing in conventional fashion.

Those having ordinary skill in this art will readily recognize that the processing involved in determining the best beam will require some time and that in the interim, there may be incoming signals from the SS. In such an eventuality, the signals are received by the omnidirectional antenna 231 and processed in the same manner as the initial RTS signal. This permits the antenna assembly 100 the luxury of not having to identify the best beam within any minimum time period.

Additionally, there may be occasions when the directional beam antennas 210 are not available or the best beam estimate may be considered no longer current, such as because of movement of the SS. In such situations, the antenna assembly 100 may revert to using the omnidirectional antenna 231.

During the course of processing, the 802.11n compliant processor 970 may identify some signal that needs to be communicated to the SS. If there is no best beam identified for communications with the SS, the 802.11n compliant processor 970 signals the processing assembly 940 along control signal 972 to move to a transmit mode.

This in turn prompts the processing assembly 942, if necessary, to issue control signal 946 to the RF
transmit / receive converter 925 sending it into transmit

-19-mode, control signal 945 to the omni transmit / receive switch 955 sending it into transmit mode, and control signal to the transmit/omni beam switch 965 sending it into omni mode.

As a result, signals from the 802.11n compliant processor 970 are sent to the transmit omni / beam switch 965, to the omni transmit / receive switch 955 and to the RF transmit / receive converter 925, where they are converted from baseband I & Q signals into RF signals, conditioned as necessary, sent to the pre-selector filter 915 to be filtered and transmitted to the SS along the omnidirectional antenna 231.

On the other hand, if there is a best beam identified for the SS, the 802.11n compliant processor 970 signals the processing assembly 940 along control signal 972 to move to a transmit mode and, if necessary, along control signal 971 to move to a beam mode.

This in turn prompts the processing assembly 942, if necessary, to issue control signal 947 to the RF
transmit / receive converter 920 sending it into transmit mode, control signal 943 to the line switch 930 to send it into operational mode, control signal 944 to the beam transmit / receive switch 955 sending it into transmit mode.

As a result, signals from the 802.11n compliant processor 970 are sent to the transmit omni / beam switch 965, to the beam transmit / receive switch 950, to the line switch 930 and to the RF transmit / receive

-20-converter 920, where they are converted from baseband I &
Q signals into RF signals, conditioned as necessary, sent to the pre-selector filter 910 to be filtered, through the beam selector switch 905, which is configured to connect it to the directional beam antenna 210 associated with the designated best beam, for transmission to the SS.

In addition to the time interval during which the processing assembly 940 is attempting to determine which of the directional beam antennas 210 is to be designated as providing the best beam, there are two other occasions when the antenna system 100 may enter the omni mode, namely in the case of hidden nodes, or where there is a desire on the part of the user to transmit omni.

Turning now to Figure 10, there is shown a flow chart showing the processing steps involved in determining the best beam for an SS.

Initially, one of the plurality (in the preferred embodiment, 6) of directional beam antennas 210 is selected 1010 using the beam selector switch 905. The received antenna signal is processed by an edge-detection module 1020 in order to identify the start of the packet.
Once the start of the packet has been identified, the process of calculating covariance matrices 1030 is commenced. The performance of each beam is identified by measuring the coefficients of the channel defined between the multiple antennas of the SS

-21-and the multiple antennas of the antenna system 100 of the AP. The coefficients define the effective impedance of the channel, in terms of the phase shift and attenuation of the signal encountered by the channel.

From the computed covariance matrices, the maximum and minimum eigenvalues are determined by eigen decomposition 1040.

With this information, the condition number for the selected beam may be determined 1050. The condition number is calculated as the ratio of the largest eigenvalue over the smallest eigenvalue.

This process is repeated for each of the potential best beams 1060, by returning 1062 to the select beam step 1010 if not all of the beams have been selected and proceeding to the next step 1061 only when all of the beams have been selected and condition numbers identified for each beam.

Once all of the condition numbers have been identified, the beam corresponding to the best condition number is stored in a look-up table entry corresponding to the SS 1070. In addition to the best beam number (and optionally the condition number), the MAC address for the SS is also stored so that the best beam associated with that SS can be later retrieved.

Those having ordinary skill in this art will readily recognize that while the processing just described contemplates the choice of the best beam from among the plurality of directional beam antennas 210 in a

-22-single multi-beam assembly 200, it is certainly feasible to consider selecting the best beam for each of the MIMO
antenna sub-assemblies 200 from among all of the directional beam antennas 210 in order to maximize the performance of the antenna system 100 overall, and not on a MIMO antenna sub-assembly 200 basis only. Thus, in the exemplary embodiment of a linear array of three MIMO
antenna sub-assemblies 200 each comprising six directional beam antennas 210, three best beams could be designated, each associated with one of the MIMO antenna sub-assembly 200, but based on the overall performance provided across all eighteen directional beam antennas 210 in the antenna assembly 100.

After all the beams are selected, the best beam number and MAC address are stored in the look-up table.
At this point, the processing determines if the next general interval has been reached 1080. The general interval is a time interval chosen to represent a convenient point at which the best beam information can be used to switch from omni mode to beam mode. If the general interval has not been reached 1082, the beam selection process is repeated. If, however, the general interval has been reached 1081, the best beam information stored in the look-up table is handed over.

After the best beam information has been handed over, the processing checks to see if a predetermined time interval (in the preferred embodiment 10 ms) has expired. The time interval represents the repeat frequency of checking for the best beam. If the time

- 23 -interval has not expired, the processing loops back around 1102. If the time interval has expired 1101, the beam selection process is repeated.

The present invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combination thereof. Apparatus of the invention can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and methods actions can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The invention can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language.
Suitable processors include, by way of example, both general and specific microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks;
magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program

-24-instructions and data include all forms of volatile and non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks;
CD-ROM disks; and buffer circuits such as latches and/or flip flops. Any of the foregoing can be supplemented by, or incorporated in ASICs (application-specific integrated circuits), FPGAs (field-programmable gate arrays) or DSPs (digital signal processors).

Examples of such types of computers are the processing sub-assembly 942 and the 802.11n compliant processor 970 contained in antenna assembly 100, suitable for implementing or performing the apparatus or methods of the invention. The system may comprise a processor, a random access memory, a hard drive controller, and an input/output controller coupled by a processor bus.

It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiments disclosed herein, consistent with the present invention, without departing from the spirit and scope of the present invention.

Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein.

Accordingly, the specification and the embodiments are to be considered exemplary only, with a

-25-true scope and spirit of the invention being disclosed by the following claims.

-26-

Claims (17)

THE EMBODIMENTS OF THE PRESENT INVENTION FOR WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE:

1. An access point having a plurality of diverse access point antennas for a multiple-input multiple-output (MIMO) wireless communications system for communication with a subscriber station having a plurality of diverse subscriber antennas, each of the access point antennas having coverage areas and comprising:

a sectorized antenna adapted to generate an antenna pattern substantially corresponding to the coverage area of its associated access point antenna;

a plurality of directional beam antennas each adapted to generate an associated beam pattern, the beam patterns corresponding to each of the plurality of directional beam antennas combining to provide a beam coverage area substantially corresponding to the coverage area of their associated access point antenna;

wherein performance of the plurality of access point antennas while communicating with the subscriber station may be maximized in a beam mode by concentrating available access point antenna energy on a selected directional beam in each of the plurality of access point antennas.

2. An access point according to claim 1, wherein the system complies with a MIMO Wi-Fi communication standard.

3. An access point according to claim 2, wherein the standard is an IEEE 802.11n standard.

4. An access point according to claim 1, wherein the plurality of access point antennas are oriented in a linear array.

5. An access point according to claim 4, wherein the array of access point antennas are adapted to be ceiling-mounted.

6. An access point according to claim 1, wherein the plurality of access point antennas are three in number.

7. An access point according to claim 1, wherein the plurality of directional beam antennas are mounted on a circular metallic disk.

8. An access point according to claim 7, wherein the circular metallic disk is normal to and coaxial with a central axis of the access point antenna.

9. An access point according to claim 7, wherein the circular metallic disk is deposited on a side of a printed circuit board.

10. An access point according to claim 8, wherein the plurality of directional beam antennas are positioned radiating outward from the central axis.

11. An access point according to claim 1, wherein each of the plurality of directional beam antennas comprise a double-sided printed circuit board.

12. An access point according to claim 1, wherein the plurality of directional beam antennas in an access point antenna is six in number.

13. An access point according to claim 7, wherein the sectorized antenna is mounted on a ground plane parallel to the circular metallic disk.

14. An access point according to claim 13, wherein the directional beam antennas lie between the circular metallic disk and the ground plane.

15. An access point according to claim 13, wherein the sectorized antenna is normal to the ground plane.

16. An access point according to claim 13, wherein the sectorized antenna lies on the side of the ground plane facing away from the circular metallic disk.

17. An access point according to claim 1, wherein, if the beam mode is not available, communication with the subscriber station may be maintained by diverting available access point antenna energy to the sectorized antenna in each of the plurality of access point antennas.