WO2000036770A1

WO2000036770A1 - Method and apparatus for measuring the range performance of a wireless communication device

Info

Publication number: WO2000036770A1
Application number: PCT/US1999/027911
Authority: WO
Inventors: James R. Bress; Richard C. 890 Tasmania Drive COMPTON; Edward Bress
Original assignee: Ast Technology Labs Inc.
Priority date: 1998-12-14
Filing date: 1999-12-01
Publication date: 2000-06-22
Also published as: AU1921500A

Abstract

In a system and method for evaluating the electrical range performance of a transmitter (12) and receiver (14) which communicate via a wireless link, the transmitter (12) and receiver (14) are placed in separate enclosures (enclosures A (20) and B (22) respectively). A probe (29) in enclosure (A) (20) detects signals from the transmitter (12). These signals are converted to electrical signals, which propagate via coaxial cable (32) (or some other transmission medium) to enclosure (B) (22) where the receiver (14) is located. Before reaching the second enclosure, the signal passes through a signal modifier (34), which varies the signal amplitude and noise floor in a precise manner using noise generators (85) and attenuators. The signal reaching enclosure (B) (22) is then transmitted by a second probe (30) to the receiver. The performance of the transmitter (12)/receiver (14) combination is evaluated by monitoring the receiver (14) as the signal strength and noise levels are varied. The test system is bi-directional such that the transmitter (12) or receiver (14) may be located in either enclosure (A) (20) or enclosure (B) (22).

Description

Method and Apparatus for Measuring the Range Performance

of a Wireless Communication Device

Background of the Invention

This invention relates, in general, to the testing of wireless electronic

equipment, wherein a transmitter sends a signal in the form of electromagnetic waves

or acoustic waves of any predetermined form to one or more receivers, to determine

the range performance of the transmitter and receiver.

If a transmitter and a receiver are too far apart, it becomes difficult to maintain

an acceptable communication level between them. The threshold distance for which

communication remains acceptable is called the range, and this is usually measured in

a large outdoor test facility, where the physical distance between the transmitter and

the receiver can be gradually increased until the receiver performance drops below

some predefined, or threshold, level. Because either the transmitter or receiver must

be moved in such a procedure, the accompanying diagnostic measurement equipment

must also be transported. This equipment is often cumbersome or impractical to

move, so less precise diagnostics such as an operator's ear must often be used.

Furthermore, outdoor tests can also be highly inaccurate because multiple signal

reflections, obstacles, and a continuously changing outdoor signaling environment due

to such things as atmospheric variations can lead to irreproducible results. In addition, outdoor test ranges are subject to disruption due to inclement weather.

The impairments associated with performing precise range measurements

outdoors may be somewhat circumvented through the use of an indoor anechoic

chamber, but not without introducing another set of problems. In indoor testing, the

separation between the transmitter and receiver is difficult to increase beyond some

limited value. Instead, the transmitter and receiver must be physically modified to

simulate the effects of increasing spatial separation. When testing a receiver, an

attenuator is inserted in the transmit chain so that the signal can be continuously

decreased to emulate the effects of increasing spatial separation. Conversely, when

testing a transmitter, the receiver is modified by inserting a variable attenuator at the

input. These techniques, though widely used, rely on the fabrication of elaborate and

expensive simulators, which may not faithfully reproduce the equipment under test. In

addition, techniques that require modification of the device to be tested may not

provide an accurate indication of the device's actual range performance when the

device is used in its un-modified state.

The complexity, impracticality, and cost of these testing procedures often

precludes their use except in very specialized cases, but even if these problems can be

overcome, outdoor range tests are subject to external interference which cannot be

controlled, and do not yield repeatable results. During manufacturing of wireless

communication equipment, the high cost and impracticality of using the test range

and/or specialized test equipment for simulation makes it unreasonable to perform these tests on each unit, or on a statistically significant number of production units.

Accordingly, there is a need for a system which can be used in a laboratory or

on a production floor to simulate the effects of an outdoor range. This system should

also be able to provide a controlled environment where tests are repeatable for any

level of production sampling, are not subject to uncontrollable external interference,

do not rely on human subjective measurements, and should require no modifications

of the devices to be tested.

Summary of the Invention

An object of this invention is to provide an accurate and simple apparatus and

procedure for testing the range performance of wireless communication equipment.

Another object of the invention is to provide a system for testing the range

performance of wireless communication equipment in confined spaces without the

need for expensive test instruments or device modification, without the need for an

outdoor range, and without the need for human subjective measurements.

In accordance with a preferred form of the present invention, a transmitter and a

receiver to be tested for range are placed in corresponding separate enclosures. Both

enclosures are shielded to prevent signals from leaking in or out of the enclosure so

that they effectively isolate the units under test. The enclosures can be quite small and

can be fabricated relatively easily. Probes such as antennas are located in each of the

enclosures and couple signals in the respective enclosures to a transmission medium

(e.g. a coaxial cable) to provide a signal communication link from the transmitter enclosure to the receiver enclosure. The enclosure probes and the transmission

medium are able to simulate a continuous range of separations between the transmitter

and receiver by variably attenuating the signal in the transmission medium.

Although the use of separate shielded enclosures for transmitters and receivers

to be tested is preferred, in some circumstances it is possible to provide adequate

testing with only one small shielded enclosure. In this case, the second enclosure

might be a room in which the first enclosure is located, and the large enclosure might

be unshielded or partly shielded. Further, for some purposes there need not be a

second enclosure; a single shielded enclosure containing a probe connected through a

variably attenuated transmission medium to a second probe might be sufficient.

Although the invention will be described hereinafter as incorporating two shielded

enclosures, it will be understood that both may not be required in all cases.

The testing procedure of the present invention can be used for example, to test

the range of electronic equipment such as cordless telephones. In such a test, a

telephone base unit, or base transceiver, is placed in a first enclosure and a handset, or

mobile transceiver unit, is placed in a second enclosure. To test these units, a

telephone line interface is connected to the base unit and an electrical signal

representing an audible tone (e.g. 1000 Hz) is supplied to the base unit through the

interface. The base unit transmits the audible signal via a modulated RF carrier

supplied to a transmit antenna, and the RF signal is detected by a first probe in the first

enclosure. The RF carrier received by the first probe travels through the transmission medium to a second probe which is located in a second enclosure, where the RF

carrier signal is transmitted into the second shielded enclosure by the probe. The

handset unit antenna receives the RF carrier signal from the probe and the handset

receiver extracts the audible tone, which is then played on the handset speaker. The

quality of the tone may be precisely monitored at the handset speaker by connecting it

to a transducer (e.g. a microphone) and then connecting the transducer output to a

signal analyzer. The range of the cordless telephone can be measured by attenuating

the RF carrier signal in the transmission medium until the quality of the audible tone

received at the handset drops below some prescribed level. Often this signal quality

level is clearly defined by accepted industry standards. The actual range is then

proportional to the level of attenuation required to reach the prescribed signal quality

limit. The apparatus can be used to make relative measurements of range, and

absolute values for range can also be determined by calculating the constant of

proportionality with a simple one-time calibration.

It is important to note that with this invention no modification of the handset

unit or of the base unit is required for this test. Furthermore, to perform this test the

only information needed is the approximate RF carrier frequency of operation of the

transceivers under test to ensure that the probes, transmission media and attenuator

will function adequately, and the enclosure dimensions and shielding are appropriate

for the frequencies in use. No information is required regarding the communication

protocol or modulation technique. The test apparatus is also bi-directional in the sense that it can simulate a range test when the handset is receiving or transmitting.

Likewise, the measurement system is applicable to two-way point-to-point systems

like cordless telephones, cellular telephones, and radios, and works equally well for

two-way point-to-multipoint systems. This apparatus can also be used to test both

two-way communication systems and one way systems such as remote controls,

garage door openers, security devices, and televisions or radios.

The method of communication is not limited to radio waves, for with some

modifications to the probes, the apparatus can be used with infrared (IR) and optical

links and with acoustic communication devices. In the case of such devices a

transducer to convert the IR, optical or acoustical signal into an electrical signal

suitable for transmission along the enclosure-to-enclosure transmission medium is

needed. The injection of interfering signals into the transmission medium can also

easily be used to test the effects of multi-channel interference that may occur in

normal every-day use of the equipment under test.

Brief Description of the Drawings

The foregoing and other objects, features and advantages of the present

invention may best be understood from the following detailed description of a

preferred embodiment thereof, take with the accompanying drawings, in which:

Figure 1 is a block diagram illustrating a preferred form of the test apparatus of

the invention;.

Figure 2 is a schematic illustration of one of the enclosures used in the apparatus of Figure 1 ;

Figure 3 is a diagrammatic illustration of a signal modifier having a variable

attenuator, used in the apparatus of Figure 1;

Figure 4 is a diagrammatic illustration of an alternate attenuator for use in the

signal modifier of Figure 3 ;

Figure 5 is a schematic illustration of a test procedure utilizing the apparatus of

Figure l; and

Figure 6 is a block diagram illustrating a multipoint test system in accordance

with the invention.

Detailed Description of Preferred Embodiment

Referring now in detail to the drawings, a block diagram of a test apparatus, or

tester, constructed in accordance with the present invention is generally illustrated at

10 in Figure 1. For purposes of illustration, the tester 10 is configured to measure the

range performance of wireless devices under test ("DUT"); for example, test devices A

and B indicated by blocks 12 and 14, respectively. The wireless devices 12 and 14 can

take many forms, but for purposes of the following description, the devices will be

considered to be a cordless telephone base transceiver (or unit) 12 and its

corresponding handset transceiver (or unit) 14. Normally in such devices,

electromagnetic radio frequency signals are communicated bi-directionally between

the base unit 12 and the handset 14 through the atmosphere by means of their

corresponding antennas 16 and 18. In order to evaluate the performance of the transmitter and receiver in each of the base transceiver unit 12 and the handset

transceiver unit 14, these units are placed in corresponding enclosures 20 and 22,

respectively. The base transceiver 12 may be connected by way of line 23 to an

external telephone line interface 24, while the handset 14 may be connected by way of

a coupler 26 which may be a cable, to an external transducer 27 such as a microphone

and/or loudspeaker. Alternatively, coupler 26 may be an acoustic coupler, or tube,

coupled to the handset microphone, and a separate acoustic coupler, or tube, coupled

to the handset speaker to transmit acoustic signals into and out of the chamber 22.

The tube would then be interfaced to the microphone or speaker transducer 27 outside

the enclosure. Use of an acoustic tube eliminates the need to run wires inside the

enclosure for acoustic coupling. The base unit and the handset unit may be battery

operated or may be connected to exterior power supplies, as needed. The signals on

lines or couplers 23 and 26 carried to and from the enclosures 20 and 22 are connected

to suitable test equipment such as a controller/monitor 28 which can generate audio

signals for delivery (via an appropriate interface) into either enclosure and can analyze

the received signal from the other enclosure.

A probe 29, which preferably is in the form of an R.F. antenna, is located in

enclosure 20 for communicating with antenna 16 of the base unit 12. Similarly, a

probe 30 is located in enclosure 22 for communication with antenna 18. The probes

29 and 30 are connected to each other through a transmission medium 32 such as a

coaxial cable which passes through the walls of the respective enclosures 20 and 22. A signal modifier 34 is included in the transmission medium so that the probes 28 and

30 are interconnected to each other through the signal modifier.

The probes 29 and 30 and the transmission medium 32 with the signal modifier

34 act as a broadband two-way coupling device for the base unit and the handset unit.

The probes serve as antennas which can simultaneously receive signals in their

respective enclosures and transmit signals fed from the transmission medium. It will

be understood that the antennas and the transmission medium must be compatible with

the frequency of operation of the wireless devices 12 and 14 under test.

Enclosure 20 of Figure 1 is illustrated in greater detail in Figure 2, it being

understood that in the preferred form of the invention enclosure 22 is similar. As

illustrated, the enclosure 20 is constructed of a material which does not pass

electromagnetic signals. For example, sheet metal or a metallic screen may be

suitable. The enclosure includes top and bottom walls 40 and 42, left and right side

walls 44 and 46, a rear wall 48, and a door 50 forming a front wall for the enclosure.

The door is hinged at 52 to provide ready access to the interior of the enclosure and

preferably is equipped with a radio frequency gasket 54 (such as Chomerics Part No.

81-02-11825-2000) to prevent electromagnetic signals within the enclosure from

leaking out or to prevent external signals from leaking into the enclosure. The interior

surfaces of the enclosure may be covered with an absorbent material 55 (such as

Ecosorb SF-0.9) capable of absorbing RF signals radiating from the device under test.

The absorbent material 55 is used to reduce R.F. signal reflections to provide a consistent signaling environment for the probe within the enclosure. In some

applications, as where the device under test will be coupled to acoustic test equipment,

the enclosure 20 may incorporate soundproofing material 56 on the interior of the

enclosure which could be integrated with, or applied separately from the R.F.

absorbent material 55.

The enclosure may be any desired shape, and preferably will be generally

rectangular. However, it may be cylindrical, or may incorporate a cylindrical

enclosure, for use in testing devices having omni-azimuthal antennas.

In one form of the invention, the enclosure may be divided by a horizontal floor

57 which divides the enclosure into upper and lower chambers 58 and 59, respectively.

The device under test 12 may be located in the upper chamber, while support

equipment such as an uninterruptable power supply (UPS) 60 and a signal interface

box 61 may be located in the lower chamber. It is also possible to have an enclosure

with just one chamber and all support equipment outside the enclosure.

Signal coupler 23 passing from outside the enclosure through the enclosure

wall to box 62 may be connected through an R.F. isolator 62 (such as Spectrum

Controls Part# SCI-2250-001) which allows power and audio signals to pass but

attenuates R.F. signals that could be coupled to metallic wires. If the interface cable

23 is non-metallic (e.g. optical or acoustic) then R.F. isolator 62 is not required.

The UPS device 60 may be connected to an external power supply 63 by way

of line 64 and an RF isolator 65 to provide power to the device under test by way of line 66 and a suitable docking station 68, if the device under test is not battery

powered. The docking station 68 may be rotatable to serve as a rotating stage, and

provides suitable connectors for supplying power to the device under test 12. The

rotating stage may be used to mount devices to be tested which have nonomni

antennas to permit measurement of the dependence of the transmitted signal on the

angle of the device with respect to the probe.

R.F. isolator 65 may be like R.F. isolator 62 or, alternatively, it may be a relay

that is opened when the enclosure door is shut thereby providing electrical isolation

from external wires. The power supply (UPS) 60 would in that case, charge while the

door is open and provide battery power to the device under test when the door is

closed and tests are performed. Another configuration is to omit wall 57 and have

only one chamber in enclosure 20, with all interface equipment outside the enclosure.

In this case, any metallic cables used for interface signals passing through the

enclosure wall would be connected through an R.F. isolator. If the signals to be

coupled are acoustic (as is the case with a telephone handset) a non-metallic acoustic

tube (e.g. plastic) may be used to carry the acoustic signal from inside the enclosure to

outside the enclosure. In this case, interface transducers such as a microphone and

speaker would be coupled to the tube outside the enclosure, thereby eliminating the

need to pass metallic wires through the enclosure.

The signal interface box 62 is connected to the device under test through the

docking station 68 by way of cable 72, and permits audio, optical, or electrical signals, for example, to be supplied to or be received from the device under test 12 and

transferred to the exterior of the enclosure by way of input/output line 23. The line 23

may be a fiber optic cable, a telephone line, or other suitable medium which is either

electrically isolated (e.g. an optical interface) or R.F. isolated (e.g. using R.F. isolator

62) for passing through the enclosure wall. The RF isolators 62 and 65 allow the test

enclosure 20 to be electrically isolated, to inhibit signals outside the enclosure from

producing interference inside the enclosure and disturbing the test environment. This

isolation also prevents signals inside the enclosure from leaking out and disturbing

other test environments. The docking station 68 positions the device under test above

the floor 57 of the enclosure for further isolation.

The signal modifier 34 (Fig 1) is illustrated in greater detail in Figure 3, to

which reference is now made. As schematically illustrated, the transmission medium,

or coaxial line, 32 is connected through a calibrated variable attenuator 80 (such as a

series pair of HP model 33320H and 33322H attenuators) which is adjustable by the

controller/monitor 28 to vary the insertion loss between probes 29 and 30. A pair of

directional couplers 82 and 84 (such as HP model 778D) connected to line 32 may be

provided in the signal modifier 34 to allow the signal level in the coaxial line 32 to be

monitored in controller/monitor 28 and also to allow interfering channel noise,

generated in the controller/monitor or by a separate signal generator 85, to be added in

both directions of the communication link, if desired. The signal couplers 82 and 84

may be connected to suitable test equipment in or through controller/monitor 28, such as the signal generator 85, signal monitors, and the like, by way of lines 86 and 88.

The signal modifier 34 is a reciprocal-port network which enables signals passing

from left to right (as viewed in Figure 3) to be attenuated by the same amount as

signals traveling in the opposite direction, i.e. from right to left. A variable delay 89

may also be added in series with the attenuator to simulate the temporal effects

associated with the spatial separation of the base station and handset.

In some instances, for example in the testing of digital cellular phones, it is

desirable to attenuate the transmit and receive signal by unequal amounts. This can be

accommodated by the modified attenuator arrangement 90 illustrated in Figure 4, in

which oppositely traveling transmit and receive signals 91 and 92 are separated with

the use of back-to-back circulators 93 and 94. These signals are then independently

attenuated in attenuators 95 and 96, which may be a series pair of HP model 33320H

and 33322H attenuators, and corresponding amplifiers 97, 97' and 98, 98'.

In operation, RF signals transmitted by device 12 in enclosure 20 are sampled

by the probe antenna 29 within the enclosure. The sampled signals then propagate by

way of the transmission medium, including in this example the coaxial cable 32 and

the signal modifier 34, to the probe 30 in enclosure 22. The RF signals supplied to

this probe radiate into enclosure 22 and are received by device 14 through its antenna

18. The signal strength propagated through the transmission medium is varied in a

controlled manner by way of the calibrated variable attenuator 80 located in the signal

modifier 34. By increasing the insertion loss of the transmission medium; i.e. by increasing its attenuation, the effects of moving the base unit 12 and the handset 14

further apart are simulated. The attenuator may also include amplifiers to enable the

device to also simulate a signal gain through the transmission medium. The useful

range of the transmitted signals from the devices under test 12 and 14 can be

accurately measured by increasing the attenuation of the transmission medium until

the receiver ceases to function adequately. In the case of a cordless telephone, this

point is reached when the distortion and/or noise level of the signal from the handset

speaker, as measured through coupler 26 and transducer 27 in monitor 28, for

example, exceeds a known industry standard. A microphone interfaced to a signal

analyzer can be used to precisely monitor the quality of this audio signal.

The reverse signal path from the handset unit to the base unit may also be

tested, where an acoustic signal such as a tone is acoustically coupled to the handset

microphone by way of coupler 26, for example, and signal quality measurements are

made on the telephone line interface of the base unit, at line 23, for example. The

range is again proportional to the attenuator setting. The greater the attenuation

needed to shut down the communications link, or reach a prescribed minimum

acceptable quality of transmission, the longer the range of the transmitter and receiver

system.

The variable attenuator 80 is controlled by a conventional controller 28, which

may be automated to programmatically vary the attenuator to provide a known, rapidly

variable, and repeatable pattern of testing for the devices under test. The monitor 28 which is coupled to the transmission medium 32 at couplers 82 and 84 serves to detect

the signal strength. In addition, the controller 28 is coupled to the medium 32 to

permit injection of selected noise or interference signals from a suitable signal

generator in the controller. Audio test signals on couplers 23 and 26 may be generated

and analyzed by the controller/monitor 28 to create a completely automated system

which can programmatically vary the simulated distance and simultaneously measure

the received audio signal quality.

The probes preferably are RF antennas, as described above, but may be dual

mode antennas to enable them to receive a variety of signals from the devices under

test.

The proportionality of the range to the attenuator setting can be verified with

the aid of the Friis formula for received power P_r. Thus, the received power P_r is a

function of the transmitted power P_t and the transmit and receive antenna gains G _t G _r ,

respectively, and is given by :

P = P_t G_t G_r λ² / 4 π r² (Eq. 1)

where λ is the wavelength and r is the spacing between the transmitter and receiver.

The Friis formula assumes that the receiver is in the far field of the transmitter and

vice versa. The far field assumption requires r » D²/λ ,where D is the largest

characteristic dimension of the transmit or receive antenna.

If the probe antenna in enclosure A is in the far field of the device under test,

then the received power P_a is given by: P = P, G, G_a λ² / 4 π r_a ² = P, G, F_a (Eq. 2)

where F =G_a λ²/4 π r_a ²

Pi is the transmitted power, while G, and G_a are the gains of the probe and the

transmitter respectively. The probe and transmitter are spaced a distance r_b , as

illustrated in Figure 5.

Similarly for the receiver in enclosure B, under the far field assumption, the

received power P₂ is given by:

P₂= P_b G_b G₂ λ²/4 π r_b ² = P_b G₂ F_b (Eq. 3)

where F_b=G_bλ²/4 π r_b ²

P_b is the transmitted power of probe b, while G_b and G₂ are the gains of the probe and

the receiver respectively. The probe and receiver are spaced a distance r_b. Now P_b= A

P_a where A is the attenuation introduced by the signal modifier and so from (3):

P₂/G₂ = F_b A P_a (Eq. 4)

If the transmitter and receiver were in free space separated by an effective

distance r then from (1):

P₂/ G₂ = P, G,G₂ λ² / 4 π r² -P_aG₂λ²/F _a 4 π r² (Eq. 5)

Equating these last two equations (4) and (5) yields:

r² = λ² F_aF_b / 4 πA (Eq. 6)

so that the square of the effective spatial separation of the receiver and transmitter is

inversely proportional to the attenuation of the signal modifier. If absolute distance is

required, the constant of proportionality F_a can be calculated from (2): F. = P^ G, (Eq. 7)

where P_a can be measured by inserting a directional coupler in the coaxial line and P,

G, is the transmit EIRP which can be readily measured using an antenna with known

gain G_r.

P_. G, = P_r 4 π r₀ ²/G_r λ² (Eq. 8)

where P_ris the received power from the transmitter with the known antenna spaced r₀

away. In a similar manner, F_b can be calculated.

The probe setup is bi-directional so range can be measured in both directions.

The derivation above assumed that the enclosures were sufficiently large that

the probes were in the far field of the device under test. However, in practice, the

procedure is equally applicable to smaller enclosures where the far-field assumption

may not be applicable. By combining equations (2) and (3) above, the following is

obtained:

P₂ = P_b G₂ F_b = A P_a G₂ F_b= A G₂ F_b F_a G, P, (Eq. 9)

In general, for any size enclosure, P₂ must be proportional to P, because this is a linear

system, so

P₂= A L P, (Eq. 10)

where L is the fixed loss constant of proportionality equal to the apparatus loss when

the attenuator in the signal modifier is set at unity (zero attenuation). The fixed loss

may be measured by replacing device A under test by a radiator with known transmit

power and replacing the device B under test with a receiving antenna connected to a power meter. The radiator and receiving antenna must have gains similar to their

respective test devices. The effective spatial separation is then given by:

r² = λ²G,G₂/4πAL (Eq. 11)

As illustrated in Fig. 6, point-to-multipoint systems where a base station 100

communicates with multiple subscribers can also be tested using multiple (N)

transceivers each in a separate enclosure 102, 104, ...N, that are each connected via a

corresponding transmission medium 110, 112....114 to the base station enclosure 100,

through corresponding variable attenuators 120, 122,...124, using anN-way combiner

130. Each variable attenuator is connected to a controller/monitor in the manner

illustrated in Fig. 3 to vary the communications links between the base station and the

subscriber devices under test.

Although the invention has been described in terms of a preferred embodiment,

it will be understood that variations and modifications can be made without departing

from the true spirit of the invention. For example, enclosure 22 can be a room (or a

larger enclosure) in which enclosure 20 is located, or enclosure 22 can represent an

outdoors location where a device under test 14 is located. In such a case, the antenna

18 of the device 14 may be near a probe 30 so that interference is minimized, with the

variable attenuator in signal modifier 34 permitting controlled simulation of changes

in the distance between test unit 12 and test unit 14, in the manner described above.

Thus, the scope of the invention is defined by the following claims.

Claims

What is claimed is:

1. A system for simulating an outdoor distance test range, comprising:

at least a first shielded enclosure;

a first probe located in said enclosure and a second probe outside and spaced

from said enclosure;

a transmission medium connecting the probes; and

a reciprocal attenuator in the transmission medium.

2. The system of claim 1 further including a second shielded enclosure, said second

probe being located within said second enclosure.

3. The system of claim 1 wherein said attenuator includes a pair of back-to-back

circulators and attenuators..

4. The system of claim 1 wherein a variable delay is inserted in series with the

attenuator.

5. The system of claim 1 wherein said enclosure is lined with an absorber material.

6. The system of claim 1 further including directional couplers connected to said attenuator for injecting interfering signals in said transmission medium.

7. The system of claim 6, further including a noise generator connected to said

couplers to simulate the effects of external noise interference.

8. The system of claim 1 wherein said enclosure includes an operable access door.

9. The system of claim 8, further including a battery charger connectable to a battery

for a device under test in said enclosure which charges when the door of the enclosure

is open.

10. The system of claim 1 wherein said attenuator is a bi-directional amplifier to

simulate signal gain or loss through said transmission medium.

11. The system of claim 2, wherein:

said first shielded enclosure and its probe comprise a base station and said

second shielded enclosure and said second probe comprises an individual subscriber

station;

and further including:

N additional shielded enclosures and corresponding probes comprising

individual subscriber stations; N+l couplers connecting each of said probes to the transmission media; and

N attenuators inserted in the transmission medium, one for each subscriber

station.

12. The system of claim 2, wherein each enclosure contains dual mode probes to

accommodate devices under test capable of receiving or transmitting in different

modes.

13. The system of claims 2, wherein each said enclosure is a cylindrical chamber for

receiving devices to be tested having omni-azimuthal antennas.

14. The system of claims 2, wherein at least one of said enclosures incorporates a sub

floor to house support electronics.

15. The system of claims 2, wherein at least one of said enclosures includes a rotating

stage for receiving devices to be tested having non omni antennas, whereby the

angular range dependence of a device under test can be monitored.

16. The system of claims 2, further including means for coupling an information

signal to be carried by a device under test into at least one of said enclosures to

determine the quality of a wireless link.

17. The system of claim 16, further including a non-metallic tube for coupling audio

signals from a device under test through a corresponding enclosure wall.

18. The system of claim 2, further including an automated controller connected to

said attenuator to programmatically vary said attenuator and to measure the quality of

the carried signal.

19. The system of claim 2, further including a monitor connected to said transmission

medium.

20. The system of claim 19, wherein said attenuator is variable.

21. The system of claim 20, wherein said first and second probes are connected to

said transmission medium to receive signals transmitted by devices under test located

in one of said enclosures and to transfer said received signals through said

transmission medium to the probe in the other of said enclosures.

22. The system of claim 21, wherein said first and second probes are connected to

transmit signals received from said transmission medium.

23. The system of claim 22, wherein said probes receive and transmit RF signals.

24. The system of claim 22, wherein said probes receive and transmit optical signals.

25. The system of claim 22, wherein said probes receive and transmit audio signals.

26. The system of claim 22, wherein said first enclosure includes a first RF

transceiver and said second enclosure includes a second RF transceiver, said probes in

said enclosures receiving and transmitting RF signals, and said attenuator being

variable to vary the impedance of said transmission line to RF signals.

27. The system of Claim 22, wherein said first enclosure includes a first R.F.

transceiver and said second enclosure includes a second R.F. transceiver, said probes

in said enclosures receiving and transmitting R.F. signals, and said attenuator being

variable to vary the impedance of said transmission line to R.F. signals.