Multiplex communications – Wide area network – Packet switching
Patent
1994-11-17
1996-09-24
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
375297, H04J 316
Patent
active
055598072
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a time division multiple access (TDMA) trunked radio system and it relates to a frame structure for communication between fixed, mobile and portable radios in such a system.
BACKGROUND OF THE INVENTION
currently proposed spectrally efficient radio systems such as the proposed European standard for digital trunked radios require highly linear power amplifiers to minimize the transmission of modulation products or "splatter" on adjacent channels. A technique for achieving the necessary linearity performance in power amplifiers involves the use of a linearization training sequence such as is described in U.S. Pat. No. 5066923 of Motorola Inc.
A problem with this method of linearizing of power amplifiers is that a finite amount of time needs to be allocated for the training sequence to be transmitted. In time division multiple access systems, the allocating of a finite amount of air time for training is a particularly costly overhead which reduces overall system throughput.
One method for avoiding the need to allocate excessive air time for training is described in UK patent application No. 9204496.5 of Motorola Limited. In the arrangement described in that patent application, training is achieved off-air by use of a circulator and an antenna switch and a load, in which the training sequence is transmitted into the load by selective operation of the antenna switch.
There is a perceived need in the art for a manner of achieving linearization of the power amplifier without the use of substantial additional hardware and with a minimum of air time being allocated for training. Other requirements that need to be addressed are a minimum delay between a requirement to transmit and the achievement of linearity prior to transmission and/or the minimisation of transmissions while not fully linearized to an acceptably low percentage of all transmissions.
SUMMARY OF THE INVENTION
According to the present invention, a radio system is provided comprising a base station and a plurality of remote radio units for communicating over time slots of a time divided radio channel, the base station having means for transmitting a synchronization signal for defining the timing of the time slots, and each of the remote units having a power amplifier with linearization means, characterized in that the channel comprises predetermined linearization time slots for transmission of linearization training signals, interspersed between other time slots and each remote radio unit comprises means for generating a linearization training signal during a linearization time slot and linearizing its power amplifier during that time slot in advance of and independent of transmission of signals during one of said other time slots.
Throughout this description the word "channel" is used to refer to a physical radio channel, i.e. a given bandwidth of radio spectrum. The expression "logical channel" is used to refer to a predetermined series of timeslots on a physical channel which together form a communication link.
The predetermined linearization timeslots can be considered to be a logical channel and the term "common linearization channel" or "CLCH" will be used to refer to this logical channel.
The invention has the advantage that all users on the system turn to the common linearization channel to train the linear transmitters. It is of no consequence if high power training signals are transmitted on this CLCH, because all units on the system are synchronized to use this CLCH for no other purpose.
Thus, transmitter linear training overhead is not required in every timeslot (traffic or control signalling).
It is preferred that a CLCH is provided on every physical channel.
A problem remains where a unit that wishes to transmit is granted a new physical channel. Unless the CLCH timeslots are provided on a very frequent basis (which would result in a high overhead signalling cost and would be contrary to the aims of the invention), there will be a delay between receiving a command to tune to the new c
REFERENCES:
patent: 5066923 (1991-11-01), Gailus et al.
patent: 5440544 (1995-08-01), Zinser Jr.
Boscovich Dragan
Valentine Stephen T.
Van den Heuvel Anthony P.
Lukasik Susan L.
Motorola Inc.
Olms Douglas W.
Ton Dang
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