GB2364853A

GB2364853A - Channel diversity

Info

Publication number: GB2364853A
Application number: GB0017113A
Authority: GB
Inventors: Leif Geert Petersen
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-07-13
Filing date: 2000-07-13
Publication date: 2002-02-06
Also published as: GB0017113D0

Abstract

Spare communication channels within a cell are allocated to mobile communication devices receiving a weak signal. The mobile communication device subsequently selects the channel received with the greatest signal strength. The average received signal strength may be used to make the selection. The receiver may be frequency, time or code division multiplexed.

Description

<Desc/Clms Page number 1> COVERAGE IMPROVEMENT BY USAGE CAPACITY OVERHEAD IN RADIO SYSTEMS FIELD OF THE INVENTION The present invention relates generally to radio communication systems, and more particularly to a method of improving the coverage of such systems by usage of capacity overhead. BACKGROUND OF THE INVENTION Wide area radio communication systems are well known. Such systems typically include a set of remote transceiver sites which serve a number of subscriber units, or radios, in the system. Subscriber units may be portable radios, mobile radios, consoles, or radiotelephones. In cellular systems it is well known to employ diversity reception of signals propagating between a mobile or portable radio and a cell site to overcome the effects of multipath fading. Multipath propagation of a signal occurs when the signal is reflected or diffracted by different obstacles. This leads to reception of several copies of the signal that are shifted in time in relation to each other. Copies in counter-phase with each other tend to cancel each other leading to a very weak received signal. Alternatively, copies in phase with each other adds to generate a strong signal. For example, a moveable radio receiver traversing a multipath environment will experience received signal strengths that change very rapidly as the radio moves. This phenomenon is termed multipath fading. In order to overcome problems relating to multipath fading, it is known to provide a radio with two antennas that are separated in space by a distance sufficient to un- correlate the multipath fading of the signals on the antennas. Only one antenna is connected to the radio receiver at one time. If the received signal falls below a threshold level, the receiver is switched to the other antenna.

Such a solution is expensive and space consuming. SUMMARY OF THE INVENTION It is an object of the present invention to provide a radio system, such as a cellular radio system, e.g. a GSM system, etc, a trunked cellular system, e.g. a TETRA system, etc, etc, employing diversity techniques without a requirement for additional receiver hardware.

It is a further object of the present invention to provide a radio system, such as a cellular radio system, e.g. a GSM system, etc, a trunked cellular system, e.g. a TETRA system, etc, etc, that facilitates diversity reception utilising idle system resources.

According to a first aspect of the invention, a diversity receiver with an antenna is provided for reception of signals in a radio system that provides communication of signals within a set of communication channels. The receiver comprises a communication channel selector for selecting a specific communication channel of the set of communication channels, a detector for determination of a parameter of a signal of the selected communicat ion channel that is received by the antenna, and a controller that is operatively coupled to the detector and the selector and that is adapted to compare a present value of the parameter to a reference value and to provide a switch signal to the communication channel selector in response to results of the comparison activating the communication channel selector to select another communication channel of the set of communication channels. According to a second aspect of the invention, a radio system is provided for communication of signals within a set of communication channels and including a plurality of moveable radios with diversity receivers for enhancing communications between the moveable radio and a specific cell site. The radio system comprises a system controller for, upon request, allocating a first communication channel of the set of communication channels for communicating signals between a selected

moveable radio and a selected cell site. The system controller is further adapted to assign a second communication channel of the set of communication channels for communicating signals between the selected moveable radio and the selected cell site whereby diversity reception is facilitated.

The radio system may be any radio system that provides communication of signals within a set of communication channels, for example a cellular radio system, e.g. a GSM system, etc, or a trunked cellular system, e.g. a TETRA system, etc, etc.

A communication channel is an identified portion of an interface. A radio system may comprise a plurality of communication channels that may be used simultaneously for communication of information between selected respective users of the system.

In one embodiment of the invention, the radio system may utilise Frequency Division Multiple Access (FDMA) whereby the frequency band used by the system is divided into a plurality of sub-bands usually of identical bandwidth. Each sub- band constitutes a communication channel. A specific channel is assigned for communication of transmission between a transmitter and a receiver by setting the transmitter and the receiver to transmit and receive radio signals within the frequency band of the assigned channel.

In another embodiment of the invention, the radio system may utilise Time Division Multiple Access (TDMA) wherein each of the channels of the system is assigned a specific time slot for transmission of information.

In still another embodiment of the invention, the radio system may utilise Code Division Multiple Access (CDMA) wherein each channel of the system is assigned a specific code for transmission of information.

It should be noted that the above-mentioned channel allocation schemes may be combined. For example, GSM and TETRA use a combination of Frequency Division Multiple Access and Time Division Multiple Access.

The receiver may reside in a moveable radio or at the cell site.

In order to detect occurrence of multipath fading, the controller of the receiver may be adapted to repetitively measure the parameter of the received signal of the selected communication channel. The parameter may be any suitable parameter for indicating the quality of the received signal, such as received signal strength, received signal-to-noise ratio, received bit error rate, etc. The measured parameter value may be compared with a fixed reference value. When the measured value falls below the reference value, the controller may command the channel selector to switch the receiver to another channel.

Alternatively, the reference value may be adjusted in accordance with the characteristics of the transmission environment of the receiver, such as averaged received signal strength in the vicinity of the receiver. Thus, the controller may be further adapted to calculate the reference value as a running average value of the measured parameter, such as received signal strength, received signal-to-noise ratio, received bit error rate, etc, in response to a predetermined number of prior measurements.

In a radio system, the number of available communication channels is much smaller than the total number of potential users and therefore, channels enabling bi-directional communication are only assigned at need by the system controller. Often, some of the communication channels are not in use for communicating information. According to the present invention, the system controller is adapted to assign such idle channels as second channels for duplicate transmission of information that is transmitted on a previously assigned first channel. As already mentioned, the receiver, i.e. the receiver of a moveable radio, or the receiver of a base site, receiving information on the first channel in question are adapted to

determine a parameter, such as received signal strength, received signal-to-noise ratio, received bit error rate, etc, of the received signal and, upon fulfilment of a predetermined criteria, to switch from the first channel to the second channel, i.e. switch from receiving a signal on the first channel to receiving a signal on the second channel.

Preferably, the system controller is adapted to monitor the parameter at the moveable radios or at the cell sites receiving signals from the moveable radios and to assign second channels to those moveable radios that relate to the weakest parameter values. The monitored parameter values may be averaged parameter values.

As already mentioned, when a moveable radio moves in an electromagnetic field, signal strength may vary rapidly because of multipath fading. The phase relationship between various copies of a signal typically changes rapidly when the radio moves. Thus, if a radio experiences a very weak signal, switching to another channel changes the above-mentioned phase relationships abruptly with a high probability of increasing signal strength significantly. For example, if the radio system is using FDMA, switching channel means that the signal frequency is changed whereby multipath phase relationships are changed completely. Likewise, if the radio system is a Time Division Multiple Access System, switching channel means changing time of reception of the signal in question which again means that the radio shifts position of reception of the signal and thus, again multipath phase relationships are changed completely.

It is an important advantage of the present invention, that the improvements in received signal strength is obtained without adding hardware to the moveable radio. The channel selector, the detector, and the controller, are already there. There is no need for adding an extra antenna, an extra receiver, etc.

Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the

embodiments thereof, from the claims and from the accompanying drawings in which the details of the invention are fully and completely disclosed as a part of this specification BRIEF DESCRIPTION OF THE DRAWING Fig. 1 schematically illustrates a radio system 100 in accordance with the present invention, and Fig. 2 is a flow diagram of a method in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 schematically illustrates a radio system 100 that includes a fixed cell site 10, 30 of a conventional variety used in connection with cellular mobile radio telephone service. The cell site 10, 30 includes a number of transmitting and receiving channels R, T,,. The cell site controller 10 assigns and re-assigns communication channels R, T. for communication between a mobile receiver 11, 12 as needed by sending control messages to the mobile receiver 11, 12 on a control channel as is well-known in the art. In the illustrated example, a mobile receiver 11 communicates with the cell site 10, 30 on channel 23. Further, mobile receiver 12 communicates with the cell site 10, 30 on a first channel 20, and a second channel 21 is also assigned for duplicate communication between the mobile receiver 12 and the cell site 10, 30. Typically, multipath fading on the first channel 20 and on the second channel 21, respectively, will not correlate, i.e. received signal strength on the two respective channels will be different, and the controller of the mobile receiver 12 or the controller of the corresponding receiver at the cell site 10, 30, respectively, can choose the best signal in accordance with the flow diagram in FIG.2.

FIG. 2 is a flow diagram of the steps of a method in accordance with the present invention for diversity reception on multiple communication channels. The method

illustrated in the flow diagram of FIG.2. is carried out by the controller of the receiver in question. Initially, a first channel 20 is assigned by the cell site controller 10 for communication of signals between a mobile radio 12 and the cell site 10, 30. A received signal parameter at the cell site 10, 30 and/or at the mobile radio 12 is monitored by the cell site controller 10 and if an average value of the parameter falls below a certain level and if an idle channel is available, an idle channel 21 is assigned as a second channel 21 for duplicate communication between the mobile radio 12 and the communication site 30. Thus, the answer to the question at step 230 of the method will be "yes". Then, at step 240 and 250, respectively, of the method, S., i.e. a parameter determined by the receiver in question as a quality indicator for the received signal of the first channel 20, is determined, and likewise S., i.e. a parameter determined by the receiver in question as a quality indicator for the received signal of the secondary channel, is determined, and the determined values are compared. If the S. is greater that SP, the channels are switched in step 260, and the receiver continues reception on the second channel 21 at step 270. If all channels of the cell site 10, 30 are busy when a new call request is received by the cell site controller 30, the controller may re-assign a diversity channel to the new call. The cell site controller 30 may not know which of the diversity channels 20, 21, the receiver of the mobile radio 12 has selected for reception of signals, e.g. when the cell site transmits signals on both channels. During duplex communication, the mobile unit will transmit signals on the selected channel and thus, the cell site controller 30 knows the identity of the selected diversity channel. Thus, when the cell site controller 30 knows which one of the first and the second channel 20, 21 that is idle when the call request is processed, the idle channel 20 may be re-assigned to the new call so that the answer at step 290 of the method will be "no" and the receiver will continue reception of signals on the second channel 21. If the assignment of the first channel 20 is maintained, then, at step 300 and 310, respectively, of the method, SP and SS are compared. If the S. is less than SP in step 320, the channels are switched and the receiver continues

reception on the first channel 20 at step 210. Again, if all channels of the cell site 10, 30 are busy when a new call request is received by the cell site controller 30, the controller may re-assign an idle diversity channel to the new call, Le. the second channel 21 may be re-assigned to a new call. If so, the receiver will continue reception of signals on the first channel 20.

If the cell site controller 30 does not know which of the diversity channels 20, 21, the receiver of the mobile radio 12 has selected for reception of signals, the cell site controller selects one of the diversity channels for reassignment to the new call. A corresponding control message is sent to the mobile radio 12 and in response to the control message the selector selects the diversity channel that has not been reassigned for continued reception of signals. Thus, at the corresponding step 230, 290, respectively, the answer will be "no" until a new channel becomes available for diversity reception.

Figure 3A illustrates component parts of a mobile receiver 11, 12 according to one embodiment of the present invention, to illustrate diversity in the case of different channels on different frequencies. The mobile receiver includes an antenna 421 connected to receiving circuitry (and transmission circuitry not shown). The receiving circuitry comprises two receivers 430 and 431 tuned to different frequency channels, each providing a received signal strength indicator (RSSI) to a controller 425. The outputs from the two receivers 430 and 431 are coupled via a switching element 423 to a digital signal processor (IDSP) 440 (or alternatively an application-specific integrated circuit), which provides appropriate outputs to a man-machine interface (MMI) 450 (e.g. an audio output or a screen).

In operation, the receivers 430 and 431 receive the primary and secondary channels described with respect to FIG. 2. Each provides a received signal strength indicator (RSSI) to the controller 425. These RSSI indicators inform the controller 425 of the qualities of the signals on the respective channels and the controller 425 issues appropriate signals to the switching device 435 to select one or other of the receivers for passing signals to the IDSP 440. The controller 425 also operates to select the channels to which the receivers 430 and 431 are tuned,

so as to tune one to the primary channel and the other to a vacant channel when possible for diversity reception. Selection of a vacant channel for selection follows normal trunked radio channel selection procedures, except that the channel is designated as a secondary channel and can be reassigned at any time as the primary channel for another receiver.

Preferably the controller 425 is connected to a transmitter (not shown) for transmitting signals to the system controller 10 (FIG. 1 ), including signals that contain an indication of the RSSI from at least the primary receiver (e.g. receiver 430).

Optionally, when the mobile radio 11, 12, switches from its primary channel to its secondary channel, it informs the system, and the system reassigns the secondary channel as the primary channel and vice-versa.

Although the communications device may include separate receivers 430 and 431 tuned to different channels for diversity reception, a single receiver Rx1/2 could be used as shown in Figure 3B. In this case, the controller 425 controls a synthesizer 437 to tune to more than one channel (e.g. in different time slots).

Alternatively, in a TDMA system, a single receiver is provided that receives more than one time-divided channel and the controller controls the timing of acceptance of signals from the channel and the transfer through of these signals through the DSP 440.

The above description has been given by way of example only. The reader will appreciate that modifications can be made within the scope of the invention.

Claims

CLAIMS 1. A diversity receiver with an antenna for reception of signals in a radio system providing communication of signals within a set of communication channels, comprising: a communication channel selector for selecting a specific communication channel of the set of communication channels, a detector for determination of a parameter of a signal of the selected communication channel that is received by the antenna, and a controller that is operatively coupled to the detector and the selector and that is adapted to compare a present value of the parameter to a reference value and to provide a switch signal to the communication channel selector in response to results of the comparison activating the communication channel selector to select another communication channel of the set of communication channels.
2. A frequency multiplexed receiver according to claim 1.
3. A time division multiplexed receiver according to claim 1.
4. A code division multiplexed access receiver according to claim 1.
5. A receiver according to any of the preceding claims, wherein the controller is further adapted to repetitively measure the parameter of the signal of the selected communication channel and calculate the reference value as a running average value of the measured parameter in response to a predetermined number of prior measurements.
6. A radio system providing communication of signals within a set of communication channels and including a plurality of moveable radios with diversity

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receivers for enhancing communications between the moveable radio and a cell site and having a system controller for, upon request, allocating a first communication channel of the set of communication channels for communicating signals between a selected moveable radio and a selected cell site, and wherein the system controller is further adapted to assign a second communication channel of the set of communication channels for communicating signals between the selected moveable radio and the selected cell site whereby diversity reception is facilitated.
7. A radio system according to claim 6, wherein the system controller is further adapted to determine received signal strength at a plurality of the moveable radios, and to assign an idle communication channel as a second communication channel to a radio receiving a weak signal.
8. A method of communication of signals within a set of communication channels, comprising the steps of: selecting a specific communication channel of the set of communication channels, determination of a parameter of a signal of the selected communication channel, and comparing a present value of the parameter to a reference value and selecting another communication channel of the set of communication channels in response to results of the comparison.
9. A method according to claim 8 further comprising the steps of: repetitively measuring the parameter of the signal of the selected communication channel, and

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calculating the reference value as a running average value of the measured parameter in response to a predetermined number of prior measurements.
10. A method according to claim 8 or 9, further comprising the steps of: allocating a first communication channel of the set of communication channels for communicating signals, and assigning a second communication channel of the set of communication channels for communicating signals selected cell site whereby diversity reception is facilitated.
11. A method according to any of claims 8-10, further comprising the steps of: determining received signal strength at a plurality of moveable radios, and assigning an idle communication channel as a second communication channel to a radio receiving a weak signal.