WO2011145271A1

WO2011145271A1 - Band-pass filter, and wireless access device

Info

Publication number: WO2011145271A1
Application number: PCT/JP2011/002277
Authority: WO
Inventors: 健寿神内
Original assignee: 日本電気株式会社
Priority date: 2010-05-20
Filing date: 2011-04-19
Publication date: 2011-11-24

Abstract

A band pass filter that handles milli-wave zone signals is formed as a plurality of types, for example two types, of band pass filter having mutually differing wavelength properties by means of sandwiching in a state parallel to the E plane of a waveguide (2) a thin metal plate (1) wherein a plurality of square holes (4a, 4b) having differing sizes and spacing corresponding to a wavelength band that is allowed to pass through by means of the waveguide (2), which is bisected by the center of an H plane, are bored in a plurality of rows, for example two rows. Furthermore, as a switching mechanism that switches which waveguide to cause to operate as a band pass filter, screw holes (A, A, B, B) that can thread to screws that form a reflective rod (3) for suppressing input/output of a signal to another waveguide are bored at both ends of each of the plurality of rows, for example two rows, of waveguides for connecting to the signal input/output terminals of an external circuit. As a result, it is possible to provide a variable-frequency band pass filter that can switch between a plurality of types of band pass filter.

Description

Band pass filter and radio access device

The present invention relates to a bandpass filter and a wireless access device.

In the outdoor device of the millimeter wave band wireless access device, the frequency band used for each customer is different, and even in the same frequency band, the high frequency input / output unit has a high frequency band for removing unnecessary waves. A pass filter (BPF: Band-Pass Filter) is used, but a band-pass filter has a wide band (for example, a frequency band of 1 GHz or more in the 23 GHz band), high attenuation, and low loss. Therefore, it is impossible to cover with one type of band-pass filter, and it will be divided into multiple band-pass filters according to the frequency band. Has become enormous.

It has been proposed that a waveguide changeover switch is used to switch a plurality of, for example, two types of bandpass filters (BPF). Japanese Patent Application Laid-Open No. 63-028102 discloses a “waveguide switch”. Patent Document 2, Japanese Patent Application Laid-Open No. 63-248201, “Waveguide Switch”, Japanese Patent Application Laid-Open No. 59-047802, “Waveguide Switch”, Japanese Patent Application Laid-Open No. 2009-212855, “Waveguide”. All of the waveguide changeover switches proposed in “Tube Switch” and the like are very large and difficult to incorporate in outdoor equipment.

For this reason, it has become an important issue to reduce the number of types of wireless access devices, both in terms of production and use of wireless access devices for transmitting and receiving millimeter-wave band signals.

JP-A-63-028102 (2nd page) JP 63-248201 A (page 2) JP 59-047802 A (page 3-4) JP 2009-212855 A (page 4-5)

The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a bandpass filter and a radio access device that can switch a plurality of types of bandpass filters with a simple switching mechanism. It is said.

In order to solve the above-described problems, the band-pass filter and the wireless access device according to the present invention mainly adopt the following characteristic configuration.

(1) A band-pass filter according to the present invention is a band-pass filter including a waveguide that passes only a signal in a frequency band selected from millimeter-wave band signals, and is divided into two equal parts at the center of the H plane. A thin metal plate in which a plurality of rectangular holes having a size and interval determined in accordance with a frequency band to be passed are formed in a row by the divided waveguide is parallel to the E plane of the waveguide. It is characterized by having a structure sandwiched between.
(2) A radio access apparatus according to the present invention is a radio access apparatus that transmits and receives millimeter-wave band radio signals. At least the bandpass filter according to (1) is replaced with a bandpass filter for transmission and a bandpass for reception. It is implemented as a filter.

According to the bandpass filter and the radio access apparatus of the present invention, a plurality of types of bandpass filters (BPFs) can be used as a bandpass filter indispensable for removing unnecessary frequencies in the millimeter waveband radio access apparatus by an extremely simple switching mechanism. ), It is possible to configure a variable frequency bandpass filter that switches to a bandpass filter having a desired frequency characteristic, so that the number of types of wireless access devices can be greatly reduced. .

It is a block block diagram which shows an example of the block configuration of the radio | wireless access apparatus which concerns on this invention. It is a block diagram which shows an example of the structure of the band pass filter (BPF) mounted in the outdoor apparatus of the radio | wireless access apparatus which concerns on this invention. It is a block diagram which shows an example of the structure of the band pass filter (BPF) mounted in the outdoor apparatus of the radio | wireless access apparatus which concerns on this invention. It is a characteristic view which shows the simulation result of the frequency characteristic of the band pass filter shown to FIG. 2A and 2B. It is structural drawing which shows the other example of the structure of the band pass filter (BPF) mounted in the outdoor apparatus of the radio | wireless access apparatus which concerns on this invention. It is a block block diagram which shows an example of the block configuration of the conventional radio | wireless access apparatus.

Hereinafter, preferred embodiments of a bandpass filter and a wireless access device according to the present invention will be described with reference to the accompanying drawings.

(Features of the present invention)
Prior to the description of the embodiments of the present invention, an outline of the features of the present invention will be described first. The present invention makes it possible to switch a plurality of types of bandpass filters (BPF) to bandpass filters having desired frequency characteristics by a very simple switching mechanism. Furthermore, it is possible to simultaneously realize a plurality of band pass filters (BPF) with one thin metal plate (fin).

More specifically, the present invention divides the waveguide into two equal parts at the center of the H plane (H-plane: magnetic field plane), and depending on the frequency band to be passed by the two divided waveguides. A band-pass filter (BPF) having a structure in which thin metal plates (fins) each having a plurality of rectangular holes having a predetermined size and interval are formed in a row and sandwiched in parallel with an E plane (E-plane: electric field plane). It is characterized by forming.

Further, in such a bandpass filter (BPF), a plurality of rectangular holes having different sizes and intervals for each row are formed in one metal plate as a plurality of waveguides. A plurality of band-pass filters (BPF) having different frequencies are formed, both ends of each waveguide are joined by T-branch, and any one of the plurality of band-pass filters (BPF) is joined to the joined portion. Switching mechanism for switching to a band pass filter (BPF) (for example, a screw for forming a reflection rod is screwed into screw holes formed at both ends of each waveguide, and a signal in an unnecessary frequency band is obtained. Is provided with a very simple switching mechanism that suppresses input and output to the waveguide through which the signal passes. It is characterized by constituting a variable band-pass filter (BPF).

Furthermore, the present invention is characterized in that the frequency variable bandpass filter as described above is mounted as a bandpass filter for transmission / reception in a radio access device that transmits / receives a millimeter-wave band signal.

Thus, by changing the band pass filter (BPF) having different center frequencies of N types (N: an integer of 2 or more) according to the frequency band to be used as a frequency variable band pass filter (BPF), wireless access The number of types of devices can be greatly reduced.

(Configuration example of embodiment of the present invention)
Next, a configuration example of the wireless access device according to the present invention will be described with reference to FIG. FIG. 1 is a block configuration diagram showing an example of a block configuration of a radio access apparatus according to the present invention.

A wireless access device that handles millimeter-wave band signals is generally composed of an outdoor device and an indoor device. Between the outdoor device and the indoor device, an IF (Intermediate Frequency: intermediate frequency) band signal and an outdoor device are used. Are connected by a coaxial cable in which a control signal for controlling the power and a power source are superimposed. As shown in FIG. 1, the outdoor device has an IF signal, a control signal, and a power source that are transmitted and received with the indoor device, and a multiplexer (Multiplexer) 11 that separates and combines the power, and a transmission IF signal. A transmission circuit unit 12 that converts and amplifies an RF (Radio Frequency) signal, a reception circuit unit 13 that amplifies the received RF signal and converts it to a reception IF signal, a transmission bandpass filter 14 that removes unnecessary waves, and reception It is composed of a band pass filter 15.

Here, as shown in FIG. 1, the transmission bandpass filter 14 is configured as a so-called variable frequency bandpass filter, and includes two transmission bandpass filters 14a (center frequency f1) and transmission bandpass having different center frequencies. It comprises a filter 14b (center frequency f2) and two change-

over switches

14c and 14d provided at both input and output ends in order to switch the two

transmission bandpass filters

14a and 14b according to the used frequency band. The pass filter 15 is also configured as a so-called frequency variable bandpass filter, and includes two reception bandpass filters 15a (center frequency f1 ') and reception bandpass filter 15b (center frequency f2') having different center frequencies. Frequency of receiving band-

pass filters

15a and 15b Two changeover switches 15c provided in the input and output ends in order to switch in accordance with the frequency band, and a 15d.

In order to more clearly show the difference between the outdoor device of the wireless access device according to the present invention shown in FIG. 1 and the outdoor device of the conventional wireless access device, FIG. 5 shows a block configuration of the conventional wireless access device. Yes. FIG. 5 is a block diagram illustrating an example of a block configuration of a conventional wireless access device. Unlike the outdoor device of the wireless access device according to the present invention illustrated in FIG. 1, a transmission bandpass filter and a reception bandpass filter are Each has one configuration. That is, as shown in FIG. 5, the outdoor device of the conventional wireless access device is configured by a multiplexer 21, a transmission circuit unit 22, a reception circuit unit 23, a transmission bandpass filter 24 and a reception bandpass filter 25.

Next, the structures of the transmission bandpass filter 14 and the reception bandpass filter 15 mounted on the outdoor device of the wireless access device according to the present invention shown in FIG. 1 will be described with reference to FIGS. 2A and 2B. 2A and 2B are structural diagrams showing an example of the structure of a bandpass filter (BPF) mounted on an outdoor device of a wireless access device according to the present invention, and FIG. 2A shows switching for two types of bandpass filters. FIG. 2B is an exploded perspective view showing a state in which a bandpass filter (BPF) having a mechanism is disassembled and viewed obliquely from above, and FIG. 2B is a bandpass filter (BPF) having a switching mechanism for two types of bandpass filters. It is sectional drawing which shows the cross-section of the center part. Note that both the transmission bandpass filter 14 and the reception bandpass filter 15 shown in FIG. 1 that need to be configured as a frequency variable bandpass filter are configured by waveguides having the same structure as shown in FIGS. 2A and 2B. It ’s fine.

As shown in FIG. 2A, a bandpass filter (frequency variable bandpass filter) having a switching mechanism for two types of bandpass filters has a thin metal plate 1 at the center of the H plane (H-plane: magnetic field plane). It is configured by being sandwiched in parallel with the E plane (E-plane: electric field plane) by the waveguide 2 divided into equal parts. As shown in FIG. 2B, the metal plate 1 has two rows of

rectangular holes

4a and 4b having intervals and sizes determined according to the frequency characteristics of the band-pass filter (that is, the frequency band to be passed). A plurality of holes are perforated.

That is, as shown in FIG. 2A, in order to form two types of band-pass filters (center frequencies f1, f2) having different frequency characteristics as a frequency variable band-pass filter on a single metal plate 1, the metal plate 1 The

rectangular holes

4a and 4b having different intervals and sizes are formed in two rows, and the portion of the metal plate 1 in which the

rectangular holes

4a and 4b are formed is divided into two equal parts. By sandwiching between the divided waveguides 2, two types of band-pass filters are formed. Furthermore, in order to connect the two types of formed bandpass filters to the input / output ends of signals input / output to / from an external circuit, the two types of formed bandpass filters are joined by a T-branch that excites with opposite phase equal amplitude, The joint portion is provided with a switching mechanism for switching which of the two types of band pass filters is operated as the frequency variable band pass filter.

That is, as shown in FIGS. 2A and 2B, a switching mechanism for switching between two types of bandpass filters (transmission-side changeover switches 14c and 14d or reception-side changeover switches 15c and 15d shown in FIG. 1) Screw holes A and A for screwing screws and screw holes so that the reflecting rods 3 can be disposed at both ends of each of the two rows of waveguides 2 forming a kind of band-pass filter. B and B are drilled. In the present embodiment, the reflection rod 3 for constituting the switching mechanism is realized by a screw, and the waveguide 2 which is to suppress the operation among the two rows of waveguides 2 (two types of bandpass filters). One of the waveguides 2 (bandpass filter) is operated by screwing a screw forming the reflecting rod 3 into the screw hole A, A or screw hole B, B on the (bandpass filter) side. An example is shown.

(Operation example of the embodiment of the present invention)
Next, an example of the operation of the bandpass filter switching mechanism in the wireless access device shown in FIGS. 1, 2A, and 2B will be described.

For example, in the transmission bandpass filter 14 that operates as a frequency variable bandpass filter in the wireless access device of FIG. 1, the transmission bandpass filter having the frequency characteristic of the transmission center frequency f1 out of the two types of

transmission bandpass filters

14a and 14b. When the switch 14a is operated, as the changeover switches 14c and 14d, the screws of the reflecting bar 3 are screwed into the screw holes B and B at both ends of the waveguide 2, and the reflecting bar 3 operates as a band pass filter 14b. When the input / output of signals to / from the waveguide 2 is suppressed and the transmission bandpass filter 14b having the frequency characteristic of the transmission center frequency f2 is operated, the screws of the reflector 3 are used as the changeover switches 14c and 14d. Is screwed into the screw holes A and A at both ends of the waveguide 2, and the band pass filter 14 is reflected by the reflector 3. To suppress the input and output of signals to the waveguide 2 that operates as a. This switching mechanism is the same for the reception bandpass filter 15 that operates as a frequency variable bandpass filter.

As described above, according to the frequency band to be applied, the screws forming the reflecting rod 3 are screw holes A and A or screw holes B and B formed in both ends of the two rows of waveguides 2. It is used as a frequency variable bandpass filter that can easily switch between two types of bandpass filters (a bandpass filter having a center frequency f1 and a bandpass filter having a center frequency f2) simply by performing an operation of screwing into the band. be able to.

FIG. 3 is a characteristic diagram showing a simulation result of the frequency characteristics of the bandpass filter shown in FIGS. 2A and 2B. The screw of the reflector 3 is screwed into the screw holes B and B at both ends of the waveguide 2. The pass characteristic and the return loss characteristic when operated as a bandpass filter having the frequency characteristic of the center frequency f1, and the screw of the reflector 3 are screwed into the screw holes A and A at both ends of the waveguide 2 to be centered. The pass characteristic and the return loss characteristic when operated as a bandpass filter having the frequency characteristic of the frequency f2 are respectively illustrated. Note that FIG. 3 shows a simulation result in the case where the center frequency f1 is 21.5 GHz and the center frequency f2 is 22.1 GHz.

When using the millimeter wave band of 23 GHz as a radio frequency band, the radio access device generally needs to have a frequency band of about 1,200 MHz as a cover band. It is impossible to cover such a wide frequency band with one type of band-pass filter. As shown in the f1 pass characteristic curve or the f2 pass characteristic curve in FIG. One type of bandpass filter alone is limited in covering a frequency band of about 700 MHz.

Therefore, in the conventional radio access device, as described above, at least two types of radio access devices, that is, a radio access device including a bandpass filter having a frequency characteristic such as an f1 pass characteristic curve, and an f2 pass characteristic It has been necessary to separately prepare a radio access apparatus including a bandpass filter having a frequency characteristic like a curve. Therefore, in the conventional radio access device, it is necessary to arrange radio access devices that cover the respective frequency bands as radio sections facing each other. Therefore, a total of four types of radio access devices in which transmission and reception frequencies are switched are provided. It was necessary to prepare, and the number of devices was increased.

However, in the present embodiment, as shown in the simulation result of FIG. 3, by appropriately positioning the position where the reflector 3 is mounted, two kinds of metal plates are used as shown in FIGS. 2A and 2B. Even if the band pass filter is configured, a good pass characteristic and return loss characteristic can be obtained as characteristics of each band pass filter, and a band pass filter having a frequency characteristic with a center frequency of f1 (= 21.5 GHz) Each bandpass filter having a frequency characteristic with a center frequency f2 (= 22.1 GHz) can cover a frequency band of about 700 MHz with good characteristics.

A bandpass filter having a switching mechanism for two types of bandpass filters as described above is provided as a frequency variable bandpass filter on the transmitting side and the receiving side of the wireless access device, and screws 2 forming the reflector 3 are provided. It is extremely simple just to screw into one of the two screw holes A and A and screw holes B and B formed at both ends of the waveguides 2 in the row. By a simple operation, it can be operated as a transmission / reception circuit having the frequency characteristic of the center frequency f1, or can be operated as a transmission / reception circuit having the frequency characteristic of the center frequency f2 (f2 ≠ f1).

Therefore, by applying a switching mechanism using the reflecting rod 3, the bandpass filter (frequency variable bandpass filter) having the structure shown in FIGS. 2A and 2B having the two types of bandpass filters can be obtained as shown in FIG. As shown in the overall pass characteristic curve of the pass characteristic curve + f2 pass characteristic curve), the frequency band of 1,300 MHz from 21.15 GHz to 22.45 GHz is obtained as the total frequency characteristic obtained by summing the characteristics of both band pass filters. It can be seen that it can be a cover band and can sufficiently cover a wide frequency band of 1,200 MHz required in the radio frequency band of 23 GHz band.

That is, the reflection rod 3 is formed by providing the band-pass filter (frequency variable band-pass filter) having the switching mechanism of the two types of band-pass filters as described above on the transmission side and the reception side of the wireless access device. The frequency characteristic of the center frequency f1 can be obtained by an extremely simple operation of simply screwing the screws to be screwed into the screw holes A and A or the screw holes B and B formed at both ends of the two rows of waveguides 2. It can be operated as a transmission / reception circuit having the same frequency, or can be operated as a transmission / reception circuit having a frequency characteristic of the center frequency f2 (f2 ≠ f1). As a result, a wireless access device equipped with such a bandpass filter (frequency variable bandpass filter) is placed opposite to the wireless section by changing the transmission / reception frequency. Compared to the case of four, it is possible to halve the amount of one facing to two.

(Configuration example of another embodiment of the present invention)
In the band-pass filter (frequency variable band-pass filter) shown in FIG. 2A and FIG. 2B described above, a single metal plate 1 is sandwiched between two rows of waveguides 2 so as to switch between two types of Pand-pass filters. Although the case of branching has been illustrated, a configuration in which three types of band-pass filters are switched as three branches sandwiching one metal plate by three rows of waveguides is also possible.

FIG. 4 is a structural diagram showing another example of the structure of a band-pass filter (BPF) mounted on the outdoor device of the wireless access device according to the present invention. One metal plate is constituted by three rows of waveguides. An example is shown in which three types of bandpass filters having different center frequencies are configured by being sandwiched.

As shown in FIG. 4, a bandpass filter (frequency variable bandpass filter) provided with a switching mechanism for three types of bandpass filters has a thin metal plate 5 formed on an H surface (H -Plane: magnetic field plane) is sandwiched in parallel with the E plane (E-plane: electric field plane) by the waveguide 6 divided into two equal parts. The metal plate 5 has a plurality of

rectangular holes

7a, 7b, 7c each having an interval and a size determined according to the frequency characteristics of the bandpass filter (that is, the frequency band to be passed) in three rows. It has been drilled.

That is, as the frequency characteristics of the frequency variable bandpass filter, in order to form three types of bandpass filters (center frequencies f1, f2, and f3) having different frequency characteristics on one metal plate 5, The

rectangular holes

7a, 7b and 7c having different intervals and sizes are formed in three rows, and the portion of the metal plate 5 in which the

rectangular holes

7a, 7b and 7c are formed is divided into 2 etc. Three types of band-pass filters are formed by sandwiching them between the divided waveguides 6. Further, in order to connect the formed three types of band-pass filters to the input / output ends of signals to be input / output to / from an external circuit, the formed three types of band-pass filters are joined by a T-branch that excites with opposite phase equal amplitudes. The joint portion is provided with a switching mechanism for switching which of the three types of bandpass filters is operated as a frequency variable bandpass filter.

As the switching mechanism, as shown in FIG. 4, the reflecting rods can be disposed at both ends of each of the three rows of waveguides 6 forming three types of bandpass filters. Screw holes A and A, screw holes B and B, and screw holes C and C for screwing the screws are formed.

When operating as a band-pass filter having the frequency characteristic of the center frequency f1 among the three types of band-pass filters, the screws of the reflector are screw holes B and B and screw holes C and C at both ends of the waveguide 6. And the input / output of a signal to the waveguide 6 that operates as a band-pass filter having the center frequencies f2 and f3 is suppressed by the reflector 3. Further, when operating as a band-pass filter having a frequency characteristic of the center frequency f2, the reflection rod screw is screwed into the screw holes C and C and the screw holes A and A at both ends of the waveguide 6 for reflection. The rod 3 suppresses input / output of signals to / from the waveguide 6 operating as a bandpass filter having center frequencies f3 and f1. Further, when operating as a bandpass filter having a frequency characteristic of the center frequency f3, the reflection rod screw is screwed into the screw holes A and A and the screw holes B and B at both ends of the waveguide 6 for reflection. The rod 3 suppresses input / output of signals to / from the waveguide 6 that operates as a bandpass filter having the center frequencies f1 and f2.

By providing a band-pass filter (frequency variable band-pass filter) having a switching mechanism for switching between three types of band-pass filters as described above on the transmitting side and the receiving side of the wireless access device, a screw that forms a reflecting rod is provided. Two pairs of screw holes among the three pairs of screw holes A and A, screw holes B and B, and screw holes C and C formed at both ends of each of the three rows of waveguides 6 It is possible to operate as a transmission / reception circuit having the frequency characteristic of the center frequency f1 or to operate as a transmission / reception circuit having the frequency characteristic of the center frequency f2 (f2 ≠ f1) by an extremely simple operation only by screwing to the center. It can also be operated as a transmission / reception circuit having a frequency characteristic of a center frequency f3 (f3 ≠ f2 ≠ f1).

Furthermore, the band-pass filter according to the present invention is not only for switching between two or three types of band-pass filters, but more generally, a switching mechanism for switching N-type (N: integer greater than or equal to 2) band-pass filters. It can also be configured as a bandpass filter (frequency variable bandpass filter) provided with

The configuration of the preferred embodiment of the present invention has been described above. However, it should be noted that such examples are merely illustrative of the invention and do not limit the invention in any way. Those skilled in the art will readily understand that various modifications and changes can be made according to a specific application without departing from the gist of the present invention.

This application claims priority based on Japanese Patent Application No. 2010-116401 filed on May 20, 2010, the entire disclosure of which is incorporated herein.

The present invention can be used for an outdoor device of a millimeter wave band wireless access device.

1 Metal plate 2 Waveguide 3 Reflector (screw)
4a Square-shaped hole 4b Square-shaped hole 5 Metal plate 6 Waveguide 7a Square-shaped hole 7b Square-shaped hole 7c Square-shaped hole 11 Multiplexer (Multiplexer)
12 transmission circuit unit 13 reception circuit unit 14 transmission bandpass filter 14a transmission bandpass filter (center frequency f1)
14b Transmission bandpass filter (center frequency f2)

14c selector switch

14d selector switch 15 reception band pass filter 15a reception band pass filter (center frequency f1 ')
15b Reception band pass filter (center frequency f2 ')

15c changeover switch

15d changeover switch 21 multiplexer 22 transmission circuit unit 23 reception circuit unit 24 transmission bandpass filter 25 reception bandpass filter

Claims

A bandpass filter comprising a waveguide that passes only a signal in a frequency band selected from millimeter waveband signals, and the frequency that is passed by the waveguide divided into two equal parts at the center of the H plane. A band having a structure in which a thin metal plate in which a plurality of rectangular holes having a size and interval determined according to a band are formed in a row is sandwiched in a state parallel to the E surface of the waveguide. Path filter.
A plurality of band-pass filters having different frequency characteristics as a plurality of rows of waveguides by drilling a plurality of rows of the rectangular holes having different sizes and intervals according to the frequency band to be passed through the metal plate And switching which waveguide is operated as a band-pass filter at each end of each of the plurality of columns of waveguides for connection to the input / output ends of signals input / output to / from an external circuit The bandpass filter according to claim 1, wherein the bandpass filter is operated as a variable frequency bandpass filter by including a switching mechanism.
As the switching mechanism, it is possible to attach a reflecting rod for suppressing input of a signal to the waveguide not to be operated and output of the signal from the waveguide at both ends of each of the plurality of rows of waveguides. The band-pass filter according to claim 2, further comprising a mounting mechanism.
As the mounting mechanism, the reflecting rod is constituted by a screw, and screw holes that can be screwed with the screws of the reflecting rod are formed at both ends of each of the plurality of rows of waveguides. The band-pass filter according to claim 3.
The band-pass filter according to any one of claims 2 to 4, wherein a connecting portion between the input / output terminal of the signal and each of the plurality of rows of waveguides is configured as a T-branch.
A radio access apparatus for transmitting and receiving millimeter-wave band radio signals, wherein the bandpass filter according to any one of claims 1 to 5 is mounted as a transmission bandpass filter and a reception bandpass filter. Wireless access device.