|Friday, 15 September 2006|
Radio communication systems have been developed which provide two-way radio communication of voice and data between a mobile station and fixed base stations of the system. A two-way radio is a communication system commonly used to communicate between and among a plurality of mobile or portable transmitter/receiver stations and one or more fixed base transmitter/receiver stations, with the aid of a repeater station. Two-way radios, sometimes referred to as walkie-talkies, handie-talkies, or acknowledge-back paging systems, generally broadcast directly from one communications device (typically a portable radio) to another, or they may use a repeater to amplify their signals. These devices generally use allocated channelized spectrum. These channels can be frequency bands around a central carrier. These two-way radios generally allow users to communicate with one another without experiencing interruptions caused by other users, such as the case with citizen band radios. Two-way radio communications are generally carried out using an associated pair of frequencies within one or more bands. Thus, when a transceiver operates in a particular channel, it must generate at least one frequency for transmission and at least one frequency with which to mix a received signal to produce a fixed intermediate frequency signal. In two-way radio communications systems, communications are typically performed via individual calls between two users or via group calls where users communicate to a number of other users in a particular communication group. Two-way radio communication systems such as land mobile radio (LMR) have been in use for many years. With land mobile radios, users typically communicate with each other using hardware devices comprising at least one base station or dispatching unit and one or more transmitting/receiving (TR) devices, such as mobile radios (MR) which typically comprise hand-held portables, vehicle-installed radios, and the like, and to communicate with each other, users of the system select a predetermined frequency over which to transmit and receive messages via the base station.
Two-way radio systems operate either in the duplex mode in which communications in the two directions can take place simultaneously on two different frequencies, or can include a control whereby transmission takes place in only one direction at a time and a single frequency is used. The duplex mode is similar to a telephone system where the receiving and transmitting paths are both open and both parties can speak to each other with no other requirement. Two-way radios use what is known as squelch mode in which the radio speaker is silenced when there is no signal being received on an available radio frequency carrier. Squelch is the phrase used for muting a speaker. For example, two-way radios generally squelch the speaker until minimum signal strength is detected, so that the radio is not constantly humming with background noise. By using squelch codes, the radio only un-squelches the speaker when it detects the squelch code. Two-way radio messaging systems utilize low-power transmitters in the portable subscriber units to transmit messages and acknowledgments to the base receivers of the system using an inbound channel. In a number of modern two-way radio and cellular telephone products, zero intermediate frequency (ZIF) technology is used in the radio receiver. The ZIF operates using the incoming radio frequency signal where it is mixed or heterodyned with an internal local oscillator (LO) at approximately the same frequency. A resultant or intermediate frequency signal is created by this process andl is then passed through a respective low pass filter, demodulator, and audio processing circuits where it is output in the form of a recovered audio signal available to a user. This process is generally referred to as direct conversion. In a two way radio communication system there are often a number of communication resources, for example frequencies, time periods, coding schemes, that are available for allocating to communication system user's to enable the users to set up a communication link. In some communication systems, the radio users transmit and receive on the same radio frequency. In such a system, assuming no geographical reuse of the frequency, only one call can be established between at least two radio units at any one time.
Communication devices, such as portable radios, are undergoing increasing levels of miniaturization. A typical communication device usually includes an antenna used for receiving and transmitting signals. The communication device requires an antenna of an appropriate length to enable reception or emission of such radio waves. Two-way radios typically include microphones. These microphones contain an externally actuated push-to-talk (PTT) switch that activates the microphone and simultaneously enables a transmission mode in the two-way radio. Generally, the PTT switch assembly includes an external actuator, such as a lever, and is designed to provide good tactile response, so that the user receives positive feedback indicating that the switch has been activated. Generally, two-way radio messaging systems comprise a radio communication terminal and a plurality of portable SCTs (selective call transceivers) for communicating messages thereto. The radio communication terminal uses frequency reuse methods for maximizing message capacity to portable SCTs. Frequency reuse is accomplished by assigning a frequency pattern to a cluster of communication zones, and repeating the frequency pattern across other clusters. When the number of communication zones in a cluster is decreased, frequency reuse is increased, thereby increasing system capacity. As a means for tracking selective call transceivers in the messaging system, each SCT is programmed to request registration from the radio communication terminal each time the SCT enters a new zone. By knowing the location of each selective call transceiver, the messaging system is able to optimally use the frequency reuse plan. Two-way communication radios incorporate a hand-held microphone having a press-to-talk transmission switch that must be manually activated to transmit a message. Such radios also typically include a scanning feature. In the scan mode of operation, the radio scans through all or a programmed group of the multiple channels so as to allow the monitoring of communications taking place. Manufacturer of this type of two-way radios include a number of companies under brand names including: Motorola, General Electric, Uniden, Kenwood, Regency, Wilson and Bearcat.
There are two basic types of communication required between a base station (BS) and a mobile station (MS) in a radio communication system. The first is user traffic, for example speech or packet data. The second is control information, required to set and monitor various parameters of the transmission channel to enable the BS and MS to exchange the required user traffic. Using a mobile station in such a system, a subscriber can initiate calls and receive calls while located in the service area adjacent one or more base stations of the system. The service area may extend a distance of 50 km or more from the base station and transmissions within the system, both from the base station and from mobile stations, are relatively high power. By a process of registration, the system maintains location information for the mobile station so that pages may be sent to alert the mobile station. The mobile station may register with any base station in the system when transported in the vicinity of the base station. Ongoing radio communication with a given mobile station may be handed off among base stations to provide full mobility within the system. Examples of such systems include cellular telephone systems and trunked radio systems. In many communication systems one of the functions of the control information is to enable power control. Power control of signals transmitted to the BS from a MS is required so that the BS receives signals from different MS at approximately the same power level, while minimising the transmission power required by each MS. In a two-way radio communication system power control is normally operated in a closed loop manner, whereby the MS determines the required changes in the power of transmissions from the BS and signals these changes to the BS, and vice versa. To enhance user convenience, mobile stations are conventionally powered by a depletable energy source such as a battery. Battery power provides true portability for a limited period of time while the battery retains sufficient charge to operate the mobile station.
Portable two-way radios are becoming more advanced in both form and function. Modern communication systems require more efficient use of the limited and congested radio spectrum, and therefore utilize high-speed data and elaborate signaling schemes to achieve greater message signal throughput. Today, portable two-way radios are being used to communicate with extremely complicated communication systems which automatically switch operating frequencies and signaling protocols. These systems typically provide display message capabilities as well as automatic interface to land-based telephone interconnect systems. Mobile radio communication systems using transmission trunking are an improvement over conventional mobile radio communication systems in that trunked systems enable two or more users to communicate on the system at the same time. Transmission trunked communication systems group a number of channels for the same coverage zone together into a single audio communication system, with each channel accessible to each user in the system. Because each user will only need to communicate over the trunked system part of the time, and because it is unlikely that all users will need to communicate at precisely the same time, the number of channels assigned to a trunked system group can always be less than the number of users allocated to that trunked system group. Trunked radio systems utilize a central control unit to organize and control the operation of the entire system. Users of the system communicate with the central control unit over a dedicated data channel to request and receive channel access information. During the course of a long conversation, the user may actually switch between several frequencies, as determined by the central control unit. The central control unit also couples to land-line telephone interconnect service to provide users of the trunked radio system access to telephone service via radio.
Two-way radios have become popular communication tools where phones or other communication methods are not cost effective or simply not efficient. These two-way radios generally allow users to communicate with one another without experiencing interruptions caused by other users, such as the case with citizen band radios. Modern selective call systems are rapidly evolving into two-way radio messaging systems. Two-way systems advantageously provide confirmation of message delivery, as well as automatic retry upon a failure of a message delivery attempt. In addition, two-way systems allow users of portable subscriber units to originate as well as receive messages. Two-way radios are frequently used by police and fire departments, rescue workers, paramedics, power and telephone company field technicians, municipalities, and other mobile groups that require immediate communication with other members of their respective groups. Typical users of mobile radio communication systems include, for example, police departments, fire departments, taxi services and delivery services. Present mobile radio communication systems can be configured to provide for radio communications between the control station and all vehicles in a fleet, between the control station and selected vehicles in the fleet, or between different individual vehicles or groups of vehicles within a fleet.