Voice guard digital voter for multiple site PST RF trunking system

What is claimed is:

1. In an RF trunking system of the type including plural spatially disparate receiving sites S1-SN each including digital RF receiving means for receiving digitally encoded RF signals transmitted by mobile/portable RF transceivers and further including means for decoding said received signals to provide corresponding digital messages to a digital voter apparatus, said voter apparatus comprising:

plural digital receiving means, each operatively connected to a corresponding one of said means for decoding, each of said digital receiving means for receiving said digital messages provided by the receiving site corresponding thereto;

bit error rate calculating means coupled to said plural digital receiving means for calculating, in parallel, plural bit error rate values associated with each of said received messages; and

scheduling means, coupled to each of said plural digital receiving means and also coupled to said bit error rate calculating means, for controlling said plural digital receiving means to select and output a message having the lowest calculated bit error rate value associated therewith and for controlling said plural digital receiving means to discard all other received messages, said scheduling means including:

a bus, and

means coupled to said bus for scheduling application of said messages onto said bus at times responsive to said calculated bit error rates and for aborting scheduled application of messages to said bus in response to detection of signals on said bus.

2. An RF trunking system of the type including first and second geographically located receiving sites S1 and S2 each including digital RF receiving means for receiving digitally encoded RF signals transmitted by mobile/portable RF transceivers and further including means for decoding said received signals to select corresponding digital messages said system further comprising:

first digital RF receiving means operatively connected to said means for decoding included in said first receiving site S1 for receiving and temporarily storing a first digital message provided by said first receiving site S1;

second digital RF receiving means operatively connected to said means for decoding included in said second receiving site S2 for receiving and temporarily storing a second digital message provided by said second receiving site S2;

digital selector means for selecting only one of said stored first and second digital messages and for synchronizing said first and second digital RF receiving means to a time T.sub.0 ; and

a bus for connecting said first and second digital RF receiving means and said digital selector means;

said first digital receiving means including first transmitting means for calculating a first time delay Delta T.sub.0 associated with the reliability of said first digital message, and for transmitting said first digital message over said bus to said digital selector means beginning at a time T.sub.0 +Delta T.sub.1 ; and

said second digital receiving means including:

second transmitting means for calculating a time delay Delta T.sub.0 corresponding to the reliability of said second message and for transmitting said second message over said bus to said selector means beginning at time T.sub.0 +Delta T.sub.2,

detecting means connected to said bus means for detecting the occurrence of first and/or second messages on said bus, and

means connected to said detecting means and to said second transmitting means for selectively inhibiting said second transmitting means from transmitting in response to detection of messages on said bus by said detecting means between time T.sub.0 and time T.sub.0 +Delta T.sub.1.

3. A system as in claim 2 wherein said first transmitting means includes means for transmitting said first message over a time period longer than time period Delta T.sub.2 -Delta T.sub.1.

4. In an RF trunking system of the type including a receiving site S1 including digital RF receiving means for receiving digitally encoded RF signals transmitted by mobile/portable RF transceivers and means for decoding said received signals to generate corresponding digital messages, a method comprising:

(a) preassigning a slot number associated with said receiving site S1;

(b) receiving and temporarily storing a digital message provided by a first decoding means;

(c) calculating a bit error rate value corresponding to said stored digital message;

(d) determining a window delay time in response to said calculated bit error rate value;

(e) timing said window delay time beginning at a times T.sub.0 ;

(f) timing a further transmission slot delay time in response to said preassigned slot number;

(g) concurrently with said timing steps (e) and (f), monitoring a digital signal bus for the occurrence of a signal;

(h) transmitting said first digital message over said digital signal bus beginning upon elapse of said window and slot delay time if no signals are detected by said monitoring step (g) prior to the time said window and slot delay times elapse; and

(i) discarding said first digital message if said monitoring step (g) detects the occurrence of a signal on said digital signal bus between time T.sup.O and elapse of said window and slot delay times.

5. A method as in claim 4 wherein said transmitting step (h) includes transmitting said first message over a duration longer than said window delay time.

6. In an RF trunking system of the type including at least a first receiving site including digital RF receiving means for receiving digitally encoded RF signals transmitted by mobile/portable RF transceivers and means for decoding said received signals to generate corresponding digital messages, a method comprising:

(a) receiving a digital message provided by said means for decoding included in said first receiving site;

(b) calculating a bit error rate in response to said received digital message;

(c) timing a delay the duration of which is responsive to said calculated bit error rate;

(d) testing whether a service request line is inactive;

(e) if said testing step (d) reveals said service request line is active, discarding said digital message received by said receiving step (a) and inhibiting a transmitting step (f); and

(f) if said testing step (d) reveals said service request line is inactive, transmitting said first digital message over a signal bus beginning at a time determined at least in part in response to said timed delay.

7. A digitally trunked RF communications system comprising:

a first RF communications site including RF transceiving means for transmitting RF signals to and receiving digitally encoded RF signals from mobile/portable RF transceivers, said first RF communications site further including control channel transceiving means for transmitting synchronization and control signals over an RF communications channel and for receiving control signals over said RF communications channel;

at least one further RF communications site geographically distant to said first site, said further site including RF receiving means for receiving digitally encoded RF signals from said mobile/portable RF transceivers, said further site further including a control channel monitoring means connected to said RF receiving means for monitoring said synchronization signals transmitted by said first site on said outbound control channel frequency and for synchronizing said RF receiving means with said received synchronization signals;

first communications link means connecting said further site RF receiving means to a voting location for communicating received digital signals from said satellite RF receiving means to a voter means at said voting location;

second communications link means connecting said first site RF transceiving means to said voting location for communicating received digital signals from said first site RF transceiving means to said voter means;

said voting means disposed at said voting location and connected to said first and second communications means for determining which of said signals communicated thereto are redundant, for selecting the version of said redundant signals having the lowest error rate, and for communicating said selected signal version to said first site RF transceiving means, said voting means including a bus, and delaying means for delaying application of signals to said bus in response to the magnitude of said error rate.

8. In an RF trunking system of the type including plural spatially separated receiving sites S1-SN each including digital RF receiving means for receiving digitally encoded RF signals transmitted by mobile/portable RF transceivers over a slotted inbound RF channel, and means for decoding said digitally encoded RF signals received by said digital RF receiving means to provide corresponding digital messages and for communicating said digital messages to a digital voter apparatus, said digital messages being organized into frames, said digital voter apparatus comprising:

receiving means operatively connected to each of said means for decoding for receiving and temporarily storing a frame of said digital messages communicated thereto from each of said plural sites;

error rate computing means for calculating an indication of the reliability of said stored frames of digital messages; and

selecting means operatively connected to said error rate computing means for selecting and outputting a message frame received by said receiving means having the lowest calculated error indication associated therewith and for discarding all other stored message frames; and

wherein each of said frames comprise a certain number of bits;

said error rate computing means calculates said reliability indication based on less than said certain number of bits; and

said selecting means selects said messages on a frame-by-frame basis.

9. In a digitally trunked RF communications system which assigns RF channels for temporary use by RF transceivers, said system being capable of communicating digital signals transmitted by said RF transceivers, a digital voter for selecting between different versions of said digital signals, said voter comprising:

a digital signal bus;

reliability determining means for determining an indication of reliability of a version of said digital signals; and

delay means, coupled to said bus and to said reliability determining means, for delaying application of said version of said digital signals to said bus by a duration responsive to said determined reliability indication.

10. An RF communication system digital signal voter comprising:

reliability indicator determining means for determining an indication of the reliability of a frame of digital signals; said frame including a certain number of bits, and said reliability determining means including error rate computing means for calculating said reliability indication based on less than said certain number of bits of said frame; and

selecting means for selecting said frame of digital signals in response to said reliability indicator determination;

wherein said digital signal voter votes on said digital signals on a frame-by-frame basis.

11. A voter as in claim 10 wherein said reliability indication includes bit error rate.

12. In a trunked RF communication system, a method of voting on digital signals received over an RF channel comprising the following steps:

(a) calculating bit error rate for each of a plurality of redundant digital signal frames; and

(b) scheduling application of each of said redundant digital signal frames to a common digital signal bus at times responsive to the bit error rates calculated for said frames.

13. A method as in claim 12 further including the steps of:

(c) detecting whether there is activity on said bus; and

(d) inhibiting application of at least one frame of digital signals to said bus in response to detected bus activity.

14. In a trunked RF communication system of the type capable of communicating digital signals, a method of voting on digital signals received over an RF channel on a frame-by-frame basis, said method comprising:

calculating the bit error rate of a frame of digital signals; and

applying said frame of digital signals to a voter bus beginning at a time responsive to said calculated bit error rate.

Description Submit all comments and votes

CROSS-REFERENCES TO RELATED APPLICATION

This application is related to commonly-assigned copending application Ser. No. 07/364,907 filed on Jun. 12, 1989 now abandoned in the name of Brown et al entitled "DIGITAL VOTER FOR MULTI-SITE PST RF TRUNKING SYSTEM". The entire disclosure of that copending application is hereby incorporated by reference herein as if expressly set forth.

FIELD OF THE INVENTION

The present invention relates to digital trunked radio communications systems, and more specifically to such communications systems including multiple receiving sites. Still more particularly, the invention relates to an arrangement for receiving several identical incoming digitally-encoded RF messages from different radio receiver sites, for "voting" on (and thereby selecting) one of those several messages, and for passing the selected message on to a main site.

BACKGROUND AND SUMMARY OF THE INVENTION

Modern trunked radio communications systems typically include geographically distributed "satellite" receiving sites in addition to one or more main transmitting site. Consider a simple system including only one main transmitting/repeater site. The main transmitting/repeater site is typically located at a relatively high elevation (e.g., on the top of a hill, mountain or tall building) and generally is provided with relatively high powered RF transmitters to permit the site to "cover" a large desired geographical service area. The main site transmitter output power and other factors contributing to the "effective radiated power" (ERP) are selected so that signals transmitted by the main site can be received at acceptable signal strength throughout the desired service area.

Unfortunately, most or all of the mobile and portable RF transceivers served by the main site cannot provide the same high effective radiated power as is provided by the main site because of several limiting factors.

While the main site transmitter power output can be in the range of several hundreds or thousands of watts RF, a mobile transceiver may be capable of providing only 5 to 25 watts of RF at its output and portable (e.g., hand held) transceivers may be capable of generating even less power (e.g., on the order of 1 watt or even less). Size and cost limitations to a large degree dictate the limited RF power outputs of mobile and portable units, but power source limitations are perhaps the most critical factor. A portable transceiver using a few small nickel-cadmium battery cells as its power source can provide only low power output levels. Mobile transceivers can obtain additional power from a vehicle electrical system, but even this power source imposes serious constraints on the maximum power supply current the transceiver can draw.

Since the various mobile and portable transceivers within a radio communications system have vastly lower effective radiated power outputs than do the transmitters at the main site, all mobile/portable transceivers within the service area can typically receive the strong transmissions from the main site but the receivers located at the main site may not receive the weaker transmissions from the mobile and portable transceivers (or may receive the transmissions at signal strengths which are too low to provide useful, reliable communications). In other words, the "talk in" range of the main site is typically less than its "talk out" range.

As is well known, multiple receiving sites have been employed in the past to help solve this problem. Typically, so-called "satellite" receiving sites are provided at various geographical locations within the service area. The main site is provided with a full complement of receivers, and similar receivers are provided at each satellite receiving site. When a mobile or portable receiver transmits within the service area, some or all of the satellite receiving sites and the main site may receive the transmission. Depending upon where in the service area the transmitting mobile/portable happens to be at the time it transmits, some sites will receive the transmission at high signal strength levels, other sites will receive the transmission at lower levels, and some sites may not receive the transmission at all, for example, if an obstruction or very long signal path exists between the mobile/portable and the site.

When digital RF communications are involved, it is desirable to select only the "best" version of the received message and to discard or ignore redundant versions of the received message (this process can be called "redundancy removal"). The main site need only process a single version of the message--and to reduce error it should process the "best" version of that message (e.g., the version with the least "bit error rate" or other similar standard relating to the correctness of digital message reception. As the various receiving sites receive a given mobile/portable transmission at different average signal strengths and under different noise and other conditions, it may, for example, be desirable in some systems to select the version of the transmission received with the best quality (e.g., highest average signal strength or lowest bit error rate) since that version is most likely to have carried the communicated information reliably and correctly (noise, fading and other effects can degrade reception of weak signals).

In many prior art systems, all sites which receive the transmission generate an indication of the quality of the received signal (e.g., based on received signal strength and/or other factors). The overall communication system then typically may "vote" based on the quality indications reported by the different receiving sites to select a single version of the received signal for use. Commonly assigned U.S. Pat. No. 4,317,218 to Perry (1982) describes in detail one example of this type of prior art voting circuit within a repeater station control system. See also, for example, U.S. Pat. No. 4,013,962 to Beseke et al (1977). It is also generally known to calculate the "BER" (bit error rate) of a received digital signal as an indication of the quality and reliability of the received signal.

Things happen very rapidly in state-of-the-art digital trunking systems such as The General Electric Company 16-PLUS Public Service Trunking (PST) digitally trunked radio communications system. The architecture and operation of this system is disclosed in much greater detail in the following co-pending commonly assigned U.S. patent applications (which are incorporated by reference herein):

Ser. No. 07/056,922 Childress et al filed Jun. 3, 1987, issued Feb. 29, 1990 now U.S. Pat. No. 4,905,302

Ser. No. 07/057,046 Childress et al filed Jun. 3, 1987; and

Ser. No. 07/085,572 Nazarenko et al filed Aug. 14, 1987, issued May 30, 1989 now U.S. Pat. No. 4,835,731.

Briefly, GE's PST system communicates digital data at 9600 baud in "slotted" message frames each having a duration of 30 ms. These "slots" are closely (although not exactly) synchronized in time across all channels (for both inbound and outbound communications) and follow the "slot" timing of the digital control channel. Some messages use only a single slot, while other messages occupy two slots.

A mobile transceiver requiring a channel assignment will in GE's preferred system transmit a channel assignment request message in one or more successive predefined time slots on the inbound control channel and then wait for a responsive two-slot channel assignment message to be transmitted by the main site on the outbound control channel. The mobile transceiver expects to receive a response (of some sort) to its request message within a relatively short time period (e.g., so the mobile can rapidly determine if its request was ignored and retransmit the request). In configurations including multiple satellite receiving sites, it is necessary during this short time period for the system to select a single version of the received message and pass it to the main site for processing and response. In a system having a main site and two satellite receiving sites, for example, the following may occur:

(a) the main site receives and decodes the message;

(b) satellite receiving site (1) receives and decodes the message;

(c) satellite receiving site (2) receives and decodes the message;

(d) satellite receiving sites (1) and (2) and the main site communicate the message versions they received (assuming they each received the message) to a centralized voter (e.g., located at the dispatch console);

(e) the voter "votes" on the versions of the messages received by the main and satellite sites to select a single version of the message (and preferably also discards all non-selected versions while somehow ensuring that the messages being discarded are in fact redundant versions of the same message rather than different messages);

(f) the system processes the selected version of the message and generates an appropriate responsive message; and

(g) the generated responsive message is transmitted over the main site outbound control channel for reception by the mobile.

The term "system latency" refers to the amount of time it takes for a message to propagate through the system. For example, one measure of system latency is the delay from which a mobile transceiver user keys his microphone (e.g., thereby generating a working channel assignment request) to the time the mobile transceiver receives a responsive message back from the system. Obviously, it is desirable to minimize system latency since rapid access and rapid system response provide great advantages in terms of user friendliness, system throughout, and the like. In the GE PST system, this particular system latency parameter has a maximum of 90-100 milliseconds--and the mobile will automatically retransmit its request if such a time period elapses and no response has yet been received. Thus, it is generally necessary for steps (a)-(g) described above to be performed within 100 ms or less in the GE PST system.

For such minimal system latency to be achieved, each of steps (a)-(g) must be performed as rapidly as possible. The delays introduced by some of the steps cannot be significantly reduced because of practical considerations and the laws of physics (e.g., it takes a certain finite amount of time to receive and decode an RF message being transmitted, it takes a finite amount of time to transmit a received message over a landline from a satellite site to a central location, and it takes a certain finite amount of time to transmit a responsive RF message over the control channel). The time required by step (e) to "vote" on one received message and to discard redundant versions of the same message should therefore be minimized in order to reduce overall system latency.

FIG. 1 is a schematic block diagram of a prior art digital voter architecture 50 used in the past by GE in its Voice Guard Digital Voter (described in greater detail in GE Publication LBI-31600). Voter 50 includes a digital selector 52 and plural digital receivers 54 connected together via a bus 56. In the embodiment shown, digital receiver 54(1) receives messages in digital form from receiver site 1, digital receiver 54(2) receives messages in digital form from receiver site 2, . . . , and digital receiver 54(N) receives messages in digital form from receiver site N. Each receiver 54 stores the messages it receives in a temporary buffer for selection by digital selector 52.

Digital selector 52 must determine which of digital receivers 54 have stored received messages and select one of multiple redundant messages if more than one digital receiver has stored the same received message. In the past, these steps were performed by a polling process over the bus 56. Specifically, digital selector 52 (or some other "bus controller" component) would periodically and successively send a signal over bus 56 (which could be a conventional serial or parallel data bus) to each of the digital receivers 54 in turn. This signal in effect "asked" each of the digital receivers 54, one at a time, whether they had received a message. If one or more of polled digital receivers 54 responded that it had received a digitized speech message, those receivers would typically send an indication of the quality of the signal they had received (e.g., the bit error rate of the received signal) to the digital selector 52 (e.g., either directly in response to the initial poll, or in a further communication subsequent to the initial poll). Digital selector 52 would grant the digital receiver which received the message with the lowest BER permission to transmit its received message to it over the bus 56. Contentions for bus 56 were avoided because the only time a digital receiver 54 could transmit on the bus was when it was granted permission to do so by digital selector 52--and the digital selector would only grant such permission to one digital receiver 54 at a time.

A significant problem with the polling approach is that it introduces too much delay for PST and therefore unacceptably increases system latency. Each digital receiver 54 must be polled individually, and each poll takes a certain amount of time T. If N is large (i.e., there are a large number of satellite sites), the process of polling all digital receivers will take (T*N) seconds. In the worst case w here a satellite site n is the only site to receive a particular RF message (from say a portable transceiver) and at the time this received message is communicated to corresponding digital receiver 54(n) the digital selector 52 has just finished polling digital receiver 54(n) and is about to poll digital receiver 54(n+1) (assuming a polling sequence in ascending order of 1-N), it will take the full (T*N) seconds before digital selector 52 again polls digital receiver 54(n) to determine that a message has been received. Additional time will then be required to notify digital receiver 54(n) that it has been granted permission to transmit over bus 56, and still additional time is required to actually transfer the message from digital receiver 54(n) to digital selector 52.

An alternate technique used in the past to communicate messages from the digital receivers 54 to digital selector 52 uses acknowledgements to eliminate the requirement of a bus controller (and the additional time delay a controller introduces). In this alternate arrangement, an "ACK/NACK" technique is used to resolve "bus contentions" that occur whenever two digital receivers 54 try to simultaneously transmit messages over bus 56. Using this technique, each digital receiver 54 can autonomously transmit on bus 56 as soon as it receives a message so long as no other digital receiver is already actively transmitting. Digital selector 52 receives all messages transmitted on bus 56, places a responsive "acknowledgement" (ACK) message onto the bus whenever it correctly receives a message, and places a "negative acknowledgement" (NACK) message onto the bus whenever it incorrectly receives a message. Since upon the occurrence of a bus contention digital selector 52 does not correctly receive any of the contending messages, it sends a NACK signal which causes both of the transmitting digital receivers 54 to resend their messages. This technique thus avoids the time overhead involved in polling and would appear to provide a very efficient solution to a multi-site trunked digital voter.

However, this ACK/NACK arrangement described above cannot provide satisfactory performance in a digital voter for a trunking system such as GE's PST system--because it is possible and probable that multiple digital receivers will simultaneously attempt to transmit on bus 56. This is because all satellite sites typically receive a given transmitted RF message at about the same time (slight variations in receive time are attributable to different RF path lengths between the transmitting station and the satellite receiver stations) and communicate the received messages to the voter at about the same time (variations in communication time are attributable to differences in landline distance for example). Thus, it is highly probable that several versions of the same message will arrive almost simultaneously at different voter digital receivers 54. Each of the different voter digital receivers 54 may then use "carrier sense" or some other similar technique before attempting to transfer their received message to the selector to ensure that bus 56 is not in use --but will find that the bus is not in use (since none of the other digital receivers which have received versions of the same message have yet had the chance to begin transmitting). Consequently, some or all of the digital receivers 54 will begin transmitting on bus 56 virtually simultaneously--causing a "bus collision" which prevents digital selector 54 from correctly receiving any of the messages placed on the bus. While the ACK/NACK technique typically will eventually resolve the contention (e.g., especially when used in conjunction with a "random retry" or other technique preventing further bus collisions by the contending digital receivers 54), the contention resolution takes far too much time and introduces too much delay into the voting process.

Various bus contention resolution schemes are known in the computer field for efficiently resolving contentions on a common bus. The following is a (by no means exhaustive) listing of a few examples of such contention resolution schemes:

U.S. Pat. No. 4,628,311 to Milling

U.S. Pat. No. 4,623,886 to Livingston

U.S. Pat. No. 4,395,710 to Einolf Jr. et al

U.S. Pat. No. 4,638,311 to Gerety

U.S. Pat. No. 4,644,348 to Gerety

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