GB2107556A - Communication system - Google Patents

Abstract

A wireless communication system is described for enabling simultaneous operation of a plurality of portable communication devices each having its fixed base station with which it communicates. The operating frequencies of the system are such that communication can occur within a specified area without harmful interference caused by receiver desensing and intermodulation products. The input-output of each device comprises a resistive attenuator pad.

Description

SPECIFICATION Communication system Prior two-way radio communication systems have been concerned with producing as large a dynamic range as practical to provide communication over a large area. The present invention relates to a method whereby a plurality of portable units can operate within a few feet of each other, all within a predetermined dimensioned enclosed area, and communicate simultaneously each with its own base unit on different frequencies in the same band without harmful interference. When interference occurs, an individual has difficulty in communicating clearly with someone on the other end of a radio link because other radio signals somehow jam or interfere with the conversation in progress. If this interference precludes normal conversation from occuring, it is called harmful interference.Occasional static, strange sounds or voices being heard by the users that does not affect a conversation in progress in not considered harmful interference. If the communication link is for data transmission, interference would be considered harmful if transmission errors occur that could not be corrected by digital techniques.

Prior art in this specific area is truly limited. As disclosed in U.S. Patent No. 4,152,648 by Delogne, a radio-communication system for confined spaces merely discloses a radiating transmission line. U.S. Pat. No. 4,165,487 describes a low power communication system that provides for only one-way transmission and uses only one frequency. Indeed, there are situations where many discreet communication channels need to be operating simultaneously in a limited area without interfering with each other. Prior communication systems operate portable transceivers at output power levels of + 30 dBm or more and receive sentistivities at a minimum of - 115 dBm. This results in a dynamic range in excess of 145 dB. This wide dyanmic range is very useful where maximum area coverage is desired.However, a wide dynamic range in a confined area, where portable units are relatively close to base antennas and to each other, will cause harmful interference to other units operating in the system. If the variations in signal strengths between the portable units and base units are minimized as the portable units move about the confined area, then a system could be designed such that any spurious signals generated would be near or below the threshold level of sensitivity of the receivers in the system.

If several two-way radios operate in close physical proximity on different frequencies in the same band, receiver desensitization and spurious signals such as intermodulation (IM) products result in interference. A very severe interference problem results when third order intermodulation products are generated when the mixing of radio signals occur. The offending signals on F, and F2 are generated according to the well known equations: 2A - B = F, (1) A+B-C=F2 (2) Where: A=Txfreq 1 B = Tx freq 2 C=Txfreq 3 F1, F2 = Intermodulation (IM) product frequencies Where frequency F, or F2 coincide with another users frequency, interference occurs. Fifth and seventh order IM products are also generated but usually their energy levels are so low that they of not cause harmful interference.

Intermodulation products become significant when two strong radio frequency (RF) fields exist at the same point and mixing takes place. One of the places this mixing can occur is in the transmitter output stages of each unit transmitting. This is called high level mixing. In the case of several portable radios operating within a few feet of each other and two or more are transmitting simultaneously, RF energy radiated from the antenna of each unit is received by the other transmitting antennas and is coupled back to each transmitter's output amplifying stage.

This results in a mixing of the signals whereby intermodulation products are formed and reradiated by the transmitting antenna.

Mixing can also occur in the front-end stages of a radio receiver. When two strong signals are present at the input of a receiver, mixing can take place and IM products are internally generated. This is called receiver IM.

Careful selection, then, of the portable transmitter frequencies is imperative. No IM product should form under any combination of portable transmit frequencies which coincide with other portable receiver frequencies. Even if variations in the signal strength from portable to base are minimized, weak signals generated spuriously in the portable's transmitter output stages will be received by other portables operating close by in the receive mode. Further, front-end mixing will invariable take place due to the close proximity of other portables transmitting. IM products in the portable transmit frequency range are unavoidable; however, they can be minimized so that these spurious signals do not interfere with bona fida signals received by any base unit receiver.

Another very severe interference problem is receiver desensitization. Receivers are desensitized when a strong RF signal in the same band as the receiver appears at the receiver input. If the signal is on the frequency the receiver is tuned to, then no problems exist. However, if the signal is not on the receiver frequency, the receiver will be desensitized and be hampered from receiving weaker signals coming in that are on the receiver frequency. Without protection from strong signals, harmful interference will result when a plurality of portable radio units operate in close proximity using prior art techniques.

In accordance with one aspect of the present invention, a method for substantially eliminating interference in an operational wireless communication system, the system having at least three portable signal receiving and transmitting means, at least three base signal receiving and transmitting means, and associated radiation transmitting and receiving means, upon the simultaneous operation of all the signal receiving and transmitting means and while movement of the portable signal receiving and transmitting means occurs comprises locating the communication system in a predetermined enclosed area; connecting the signal outputs of the base signal transmitting means to a base radiation transmitting means; connecting the signal inputs of the base signal receiving means to a base radiation receiving means; positioning the base radiation transmitting means and the base radiation receiving means so that the difference between the minimum propagation loss and the maximum propagation loss between the base radiation transmitting means and the portable radiation receiving means is minimized and the difference between the minimum propagation loss and the maximum propagation loss between the base radiation receiving means and the portable radiation transmitting means is minimized; reducing high level mixing in the portable signal transmitting means and receiver IM in the portable signal receiving means; selecting system operating frequencies such that all third order intermodjlation prdducts generated from the mixing of signals from any portable signal transmitting means are different from any operating frequencies of the portable signal receiving means; and selecting transmitting power outputs such that an acceptable system operating margin is achieved and at the same time RF fields that would result in harmful desensitization of the portable signal receiving means are avoided.

In accordance with a second aspect of the present invention, an operational wireless system for use in such a method comprises at least three portable signal receiving and transmitting means; three base signal receiving and transmitting means; means for connecting signal outputs of the base transmitting means to a base radiation transmitting means; means for connecting signal inputs of the base receiving means to a base radiation means; first means for reducing high level mixing in each portable signal transmitting means; second means for reducing the internal generation of receiver intermodulation products in the portable signal receiving means; and third means providing transmitting power outputs resulting in acceptable system operating margins and simultaneously avoiding RF fields of a magnitude that results in harmful desensitization of the portable signal receiving means.

This method and system solves the difficulties encountered when a plurality of portable radio transmitter-receivers (transceivers) are operated in a confined space. The portable radio tranceivers (portables) are small devices that can be hand-carried or mounted on a users belt. Any person carrying a device can be anywhere within the predetermined enclosed area and communicate with his or her associated base signal transmitting and receiving means (or base unit) without harmful interference from other portables or base units.

In one example RF energy from each of the base unit transmitters is mixed together through a commercially available linear transmitter combiner. The combined signal output from this combiner is fed through a transmission line in the operating area. The antenna system could be one of several commercially available antennas or radiating transmission lines or a combination of these.

A similar antenna system may be utilized for the base signal receiving means. Signals received by the receiving antenna system travel through a transmission line through a preselector to a receiver multicoupler and then to the receiving means.

Preferably, the first and second means comprise protective pads. Each protective pad may be added in series with antennas in the portable means. In a preferred example, each protective pad is a non-reactive attenuator. Its primary purpose is to reduce high-level mixing in the transmitter output stage of the portable radio. Its secondary purpose is to reduce the effective receiver sensitivity and improve receiver IM rejection. Each channel may be further protected by the use of sub-audible tones to prevent any signal receiving means or receiver from "hearing" any signals on that channel that does not have the specific tone the receiver is designed to respond to.

Unique equations which determine the operating frequencies may be used as is described later. Classical system gain verses loss equations are used to determine transmitter power output.

This method and system provides an effective, virtually interference-free communication system enabling a plurality of portable units a two-way communication link with their associated base units in a confined area.

An example of a method and system in accordance with the present invention is illustrated in the accompanying drawing, in which: Figure 1 is a diagrammatic view of the system; and, Figure 2 illustrates diagrammatically a portable unit.

I. Determination of the System Operating Frequencies To Be Used.

The system operating frequencies are the frequencies that all transmitters and receivers operate on in the system disclosed.

To prevent any IM signals generated by the mixing formula shown in equation (1) from coinciding with a portable's receive frequency, the transmit frequencies must be carefully selected. The following equations disclose the means to determine the system operating frequencies, bandwidth and the transmit-receive split in which the system can operate:: 2F2-F3=F4 (3) F2 - S = F, (4) S + F3 = F4 (5) F2 + B = F4 (6) F, = lowest portable receiver frequency F2 = lowest portable transmitter frequency F3 = highest portable receiver frequency F4 = highest portable transmitter frequency S = the separation (or split) in frequency between a given portable's receive and tranmit frequencies and is the same for all portables B = the bandwidth needed for the portable transmit frequencies (or receive frequences since they only differ by a constant S) The total bandwidth of the system is the difference between the lowest portable receive frequency, F1, and the highest portable transmit frequency, F4.The total bandwidth, TB, is determined as follows: TB=(F3-F1)+G +(F4-F2) (7) Where: G = the guard band between F3 and F2 and G=F2-F3 (8) Thus the equation (5) is more simply expressed as: TB = F4 - F1 (9) Since the highest and lowest frequencies within which the system operates is governed by regulation or other parameters, F, and F4 are already determinable. Using equations (3) through (6) above, F2, F3, B and S are determined. In some instances, S is predetermined, in which case F2, F3 and B can be calculated.

If the operating frequencies are determined in this fashion, third order IM signals that result according to the mixing formula shown in equation (2) will not cause interference. For the frequencies determined by equations (3) through (6), the following relationship is always true: F2 + (F2 + d)F4 > F3 (10) Where: F2, F3 and F4 are defined above, and d = the channel spacing which is the difference in frequency between two adjacent operating frequencies In essence, the worst operating situation occurs when the highest portable transmitter frequency is subtracted from the sum of the lowest portable transmit frequencies. For the frequencies determined by equations (3) through (6), the result of any such mixing will always result in a frequency that is above the highest portable receive frequency, F3.

Once the limits of the portable's transmit and receive frequencies are calculated, the number of channels that can be used in that spectrum is given by: N=B/d (11) Where: N = the number of channels available B = the bandwidth as defined earlier d = the channel spacing as defined earlier Then the portable receive frequencies are: Fm = (m1)d + F, (12) Where: Fm = a portable receive frequency as m goes from 1 to N.

The portable transmit frequencies are then given by (Fm + S).

The base unit frequencies are reciprocal of the portable's frequencies. That is, if a portable unit transmits on a frequency X and receives on a frequency Y then the base unit receives on frequency X and transmits of frequency Y.

II. Description of the Base Unit System and Propagation Loss Proceeding now with a detailed description of a method to implement the present invention, Fig. 1 is referenced. The base unit transmitters 1 are designated TX1, TX2. . . TXN. TX, operates on frequency F1, TX2 operates on F, + d and TXn operates on F, + (n - 1)d. In the preferred embodiment these transmitters are housed in a cabinet separate from the cabinets housing the receivers 1 2. In an alternative embodiment, these transmitters 1 may be housed in the same enclosure as receivers 1 2. Commercial transmitters are available with a wide range of output powers. The selection of the power output of these transmitters is discussed later in Section IV of this description.Audio feeding the transmitters enters on a line 30 which is separate from a line 31 which activates the transmitter. The signals from each transmitter travel along their respective transmission cables 2 and are combined together by means of a transmitter combiner 3. The transmitter combiner 3 is comprised of circuiators, hi-Q cavities and critical length coaxial cables acting to linearly mix the signals from all the transmitters. The output of the combiner 3 travels along a coaxial transmission line 4 to the transmitter antenna system. In the preferred embodiment, the main coaxial transmission cable feeds a passive power divider 5 which distributes the RF energy to the multiple base antennas 7 through antenna transmission cables 6. The base antennas are commercially available devices.In the preferred embodiment, a directional antenna is utilized, although other types can be used in alternative embodiments. The power divider 5 is composed of critical length coaxial cables connected with "T" connectors.

In an alternative embodiment, the antenna system may be comprised of a single antenna or other radiating means, such as radiating transmission lines (not shown).

The receiver antenna system, comprised of divider 17, cables 1 8 and antennas 1 9 of the base units is identical to and independent of the transmitter antenna system in the preferred embodiment. Signals radiating from the portable unit transmitters are picked up by the receiving antennaa and fed through a transmission line 20 to a pre-selector 1 6. The pre-selector 1 6 allows only signals between F4 and F2 to reach the receiver multicoupler 14. The multicoupler 1 4 is comprised of either passive power dividers or active tuned circuits. The multicoupler 14 distributes all the RF energy present on its input cable 1 5 to each and every base unit receiver via coaxial cables 1 3. Each base unit receiver selectively tunes in the frequency it is designed to operate on and feeds the demodulated audio signal through its respective cables 32. Not shown in the figure for simplicity and understood to be present are conventional power supplies for all transmitters 1, receivers 12 and multicoupler 14.

The combined signals from the transmitter combiner 3 radiating from the antennas 7 travel a distance in space in the confined area and reache all portable radio units P1, P2.. N All portable unit receivers are subject to all signals but will only receive the frequency they are tuned to operate on in the range from F, to F3. Similarly, signals radiating from portable units P1, P2. . . PN will reach their respective base unit receivers through the base unit antenna system 17, 18, 19 as described above.

In an alternative embodiment, a single antenna system and a duplexer is employed. In this embodiment, both the output of the transmitter combiner 3 appearing on a transmission line 4 and the input of the receiver multicoupler 14 appearing on a transmission line 15 are connected to a single antenna system by means of a duplexer (not shown).

The antennas are placed in the confined area and as explained below, and closest a portable can be to any base unit antenna is called distance R,. The maximum distance any portable unit can be removed from any base antenna is represented by distance R2. The dynamic operating range of the system is determined by the relation: D=L2-L1 (13) Where: D = the dynamic operating range, in dB L2 = the propagation loss a radio signal experiences along distance R2j, in dB L2 = the propagation loss a radio signal experiences along distance R1, in dB and the free space propagation loss, L, is given by the classical relation: L=36.6+2ologf+20logr 36.6 + 20 low f + 20 low r (14) Where:: L= loss in dB f = frequency in MHz r = distance in miles The base antennas 7, 19 must be placed in the area to minimize the dynamic operating range, D. In the preferred embodiment, the base antennas 7, 19 are unidirectional antennas placed high above the service area with the maximum lobe pointing down on this area. Thus no portable Pn can get very near to the base antennas 7, 19.

With the base antennas placed such that the distances R, and R2 are minimized, L, is comparable to the value of L2 making the difference between them small. Thus a compressed dynamic range is realized.

In the preferred embodiment of the system, the propagation loss is simply the free space loss since all portable units will have line-of-sight to the base antennas. In alternative embodiments, the radio signals may experience other propagation losses as they travel through walls or other structures. If this is the case, these propagation losses need to be empirically determined and then added to the worst case loss L2 in order to determine the dynamic operating range of the system. In such instances, the base antennas are placed such that the difference between L, and the new L2 is minimized.

III. Description of the Portable Radio Turning now to Fig. 2, the portable radio 33 in the preferred embodiment is described. All RF signals are radiated from and received by an antenna 43 which is a short whip mounted on top of the radio unit. The RF signals travel through a protective pad 44. In the preferred embodiment, the pad 44 is a non-reactive attenuator made up of resistors and is designed to introduce a signal loss to RF energy passing through it. The insertion loss of this pad will be determined by the distances R, and R2 and other system losses. Typical values are between 10 to 20 dB.The pad 44 accomplishes the following: 1-Reduces the transmitted energy reaching the antenna; 2 Reduces the effective receiver sensitivity; 3-Reduces the receiver IM rejection; Reduces the level of IM products resulting from high level mixing in the transmitter output stage.

The protective pad 44 has input and output impedances that match the respective impedances of the receiver, transmitter and antenna. In an alternative embodiment, an RF circulation device, also referred to as an isolator, is put on the output of the transmitter in the portable in addition to any non-reactive attenuator. This circulation device allows RF signals to flow in one direction without experiencing significant loss, while any RF signals travelling in the other direction experience a large loss. Also, alternatively, separate attenuators may be used for the receiver and transmitter.

A transmit-receive switching means 45 couples the receiver and transmitter to the antenna 43 through the pad 44. The remaining aspects of the portable radio operation are conventional. A microphone 40 picks up the user's voice and modulates a transmitter 49. A sub-audible tone encoder 41 also modulates the transmitter continuously with a sub-audible tone. A receiver 46 demodulates the RF siganl received and when a sub-audible tone is decoded by a tone decoder 47, a speaker 49 is enabled, allowing the user to hear any message on the channel. The speaker 48 could be replaced with an earphone or earplug (not shown). A battery 42 provides the power for all active elements in the portable.

In an alternative embodiment, the receiver and transmitter in the portables may be enclosed in separate cases with or without a common power source.

IV. Determination of Transmitter Power Outputs The RF power output of all transmitters is the minimum necessary to achieve an acceptable operating margin. Generally, a system operating margin of 20 dB or more is acceptable in the industry. The following classical relationships are used in calculating this operating margin: (15) Operating Received Receiver Margin, dB = Power, dB - Sensitivity, dBm (16) Received System System Power = Gain, dBm - Loss, dB Where: (17) System Gain, dBm = The transmitter power output in dBm (18) System Loss, dB = The sum of all losses which include:: -free space loss -loss due to protective pads -15 dB system use factor -loss through the transmitter combiner or receiver multicoupler -transmission cable losses -loss due to power dividers For a given installation, all the above losses are determinable except the loss due to the protective pad 44. The amount of attenuation the pad provides is maximized in order to minimize spurious IM signals being generated while allowing practical transmitter power (20 to 30 dBm) to be utilized. The use of the protective pad in the portable unit lowers the effective radiated power of the portable.Thus the attenuation value of the pad and the transmitter power output are chosen such that the resulting weak RF fields will not desense the receiver of any nearby portable. Similarly, when energy from the base unit transmitters experience their loss through the combiner, transmission cables, power dividers and propagation through the shortest distance R1, the resulting RF field in the service area is so weak that it will not desence any portable receiver operating in that area. Since the transmitter combiner 3 introduces more loss to a signal than the receiver multicoupler 14, the base unit transmitters have more power output than the portable unit transmitters in order to have a comparable system operating margin in both directions.

After the system losses listed in (18) are determined, the optimum power and the value of the insertion loss of the protective pad can easily be determined through the use of relationships (1 5) through (18) above.

Claims (9)

1. A method for substantially eliminating interference in an operational wireless communication system, the system having at least three portable signal receiving and transmitting means, at least three base signal receiving and transmitting means, and associated radiation transmitting and receiving means, upon the simultaneous operation of all the signal receiving and transmitting means and while movement of the portable signal receiving and transmitting means occurs the method comprising locating the communication system in a predetermined enclosed area; connecting the signal outputs of the base signal transmitting means to a base radiation transmitting means; connecting the signal inputs of the base signal receiving means to a base radiation receiving means; positioning the base radiation transmitting means and the base radiation receiving means so that the difference between the minimum propagation loss and the maximum propagation loss between the base radiation transmitting means and the portable radiation receiving means is minimized and the difference between the minimum propagation loss and the maximum propagation loss between the base radiation receiving means and the portable radiation transmitting means is minimized; reducing high level mixing in the portable signal transmitting means and receiver IM in the portable signal receiving means; selecting system operating frequencies such that all third order intermodulation products generated from the mixing of signals from any portable signal transmitting means are different from any operating frequencies of the portable signal receiving means; and selecting transmitting power outputs such that an acceptable system operating margin is achieved and at the same time RF fields that would result in harmful desensitization of the portable signal receiving means are avoided.

2. A method according to claim 1, wherein the system operaing frequencies are selected by using the following formulae: 2F2 - F3 = F4 F2 - S = F, S + F3 = F4 F2 + B= F4 where F, = lowest portable signal receiving means frequency, F2 = lowest portable signal transmitting means frequency, F3 = highest portable signal receiving means frequency, F4 = highest portable signal transmitting means frequency, S = the separation in frequency between a given pair of portable signal receiving and transmitting means, and B = the bandwidth needed for the portable signal transmission means frequencies.

3. A method according to claim 1 or claim 2, wherein the operating margin is 20dB or more.

4. A method according to claim 1 substantially as described with reference to the accompanying drawings.

5. An operational wireless communication system for use in a method according to any of the preceding claims, the system comprising at least three portable signal receiving and transmitting means; three base signal receiving and transmitting means; means for connecting signal outputs of the base transmitting means to a base radiation transmitting means; means for connecting signal inputs of the base receiving means to a base radiation receiving means; first means for reducing high level mixing in each portable signal transmitting means; second means for reducing the internal generation of receiver intermodulation products in the portable signal receiving means; and third means providing transmitting power outputs resulting in acceptable system operating margins and simultaneously avoiding RF fields of a magnitude that results in harmful desensitization of the portable signal receiving means.

6. A system according to claim 5, wherein the first and second means comprise protective pads.

7. A system according to claim 6, wherein each protective pad is a non-reactive attenuator.

8. A system according to any of claims 5 to 7, wherein each portable signal transmitting means is located with a respective portable signal receiving means to form a portable unit.

9. A system according to claim 8, wherein the first and second means are formed by the same means.

1 0. A system according to claim 5, substantially as described with reference to the accompanying drawings.