Gemstar Communications GSPS-ACT-02 User Manual TDMA principles of operation mod

Gemstar Communications Inc TDMA principles of operation mod

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TDMA Activator Principle of Operation Supplementary Manual

TDMA Activator Principle of OperationTDMA ActivatorPrinciple of OperationSupplementary ManualMay 4, 2000
TDMA Activator Principle of OperationTABLE OF CONTENTSINTRODUCTION ...................................................................................................................................................... 31-TYPE OF UNITS..................................................................................................................................................... 31.1-AUTONOMOUS................................................................................................................................................ 31.2-MASTER............................................................................................................................................................ 31.3-SLAVE ............................................................................................................................................................... 32-TIME SLOTS .......................................................................................................................................................... 33-SYNCHRONIZATION........................................................................................................................................... 44-RE-SYNC and LOSS OF SIGNAL........................................................................................................................ 45-USE OF TIME BLOCKS ....................................................................................................................................... 46-HARDWARE........................................................................................................................................................... 67-RF DATA FORMAT............................................................................................................................................... 77.1-ENCODING...................................................................................................................................................... 77.2-SLAVE AND AUTONOMOUS ACTIVATOR DATA SEQUENCE........................................................... 77.3-MASTER ACTIVATOR DATA SEQUENCE............................................................................................... 88-KNOWN LIMITATION......................................................................................................................................... 89-INSTALLATION..................................................................................................................................................... 99.1-ANTENNA TYPES........................................................................................................................................... 99.2-POWER SETTING AND ANTENNA GAINS............................................................................................... 99.2.1-MAX POWER FOR MONO-POLE........................................................................................................ 99.2.2-MAX POWER FOR PATCH................................................................................................................... 99.3-POWER SETTING AND ADJUSTING......................................................................................................... 9
TDMA Activator Principle of OperationINTRODUCTIONThe TDMA (Time Domain Multiple Access) mode activator system uses a synchronised timekeeping system to permit various activators to transmit their signals without causing interferenceto other activator. This is done by defining time slots and permitting any given activator totransmit only within selected time slots. By insuring no two overlapping activator fields have thesame time slot, interference should not be an issue.1-TYPE OF UNITSThere are 3 basic modes the activators can operate in:• 1.1-AUTONOMOUS Operates without regards to time slots. This was the original modeof operation. It has uses where no time base signal can be presented and transmits at alltimes • 1.2-MASTER There is usually only one master unit in a system. It sends the timebase count based on it own internal clock, starting at 1 when reset. The Master onlytransmits during its selected time blocks. • 1.3-SLAVE This is the most used mode. When the unit operates as a slave itfirst must receive a valid time base signal from the Master unit to synchronize its internalcounter to match the master unit. Once that has happened, it then transmits during itsselected time blocks and received time base sync. signals when not in transmit mode. 2-TIME SLOTS Time is split into 6 units, each unit being 40 seconds long. The system repeats the timeloop cycle every 240 seconds (4 minutes) That is, the system starts in time block (slot) 1 doingwhat is programmed for this time block, then moves to time block 2 and so on. At the end ofblock 6, the system resets back to beginning of block 1. Block 1 Block 2Block 1 Block 2 Block 3 Block 4 Block 5 Block 6Repeat loop The Duration of each block was selected based on the fact that the current postal tagwakes up every 32 seconds. By using a time block of 40 seconds, we insure that the
postal tag will have two reading every time slot. The control and determination of time blocks or slots is determined by internal timekeeping clocks that count from 1 to 240 seconds. Where block 1 is 1 to 40 seconds,block 2 is 41 to 80 seconds and so on. The selection of time block for transmit for each activator is done by a 6 position dipswitch. Each pole of the switch represents a single time block. The activator’s processorreads the switch every few seconds and depending if the switch is open or closed for agiven time block, the software will either activate the transmitter or receive modes. 3-SYNCHRONIZATION For the system to work, all slaves must have same time count as the master unit. Theslave units must not transmit until they have synchronized their internal time count to themaster unit, and the correct time block is active. The slave units turn on their receiver at reset time and clear their valid time flag. Theythen receive signals until a valid time base signal is found. Once that occurs, the softwaresets the valid time flag and the units begins to compare the current time block numbers(time base count divided by 40) to the setting on the time block selection dip switch. 4-RE-SYNC and LOSS OF SIGNAL Once the activator is synchronized, it constantly adjusts it internal count to match themaster time base signal during times it is not in transmit mode and the master time base ispresent. A time stamp is made of each re-sync point. If the unit fails to re-sync for aperiod of 1 hour, the unit shall return to the reset state and halt any further transmissionuntil it can find the time base again. This is done to prevent the unit from getting too faroff target time, as its internal clock is subject to about 1 min per day max error. 5-USE OF TIME BLOCKS The following chart shows 1 master and 4 slave units each set to various time blocksetting. block 2block 1block 6block 5block 4block 3block 2block 1settingunit 100000Master 010000Slave 1 001000Slave 2 000100Slave 3 010011Slave 4 =TRANSMITTING
Master unit It is set to send in block(slot) 1. There is no specific requirement to have themaster operate in this slot, but for this chart is set here. Thus during time block 1's, themaster activator sends its signal. During the other 5 blocks of time it sits idle. Slave 1This unit is set to operate in block 2. Thus it transmits during block 2 and receives duringthe other 5 time blocks Slave 2This unit is set to operate in block 3. It would most likely be a neighbour of Slave 1 Slave 3This unit is set to operate in block 4. It could neighbour on slave 1 and 2 withoutinterference. Slave 4This unit is set to transmit in blocks 2, 5 and 6. It would not be a neighbour of slave 1due to the block 2 transmit time, but could neighbour on the other two slave units. Itsends signal ½ the time and therefore would be used in areas where a higher chance ofbeing received by a postal tag is required such as a receiving area etc.
6-HARDWARE The above diagram relates the core components of the activator unit. • The AM receiver and its external antenna pick up the signal from a MasterActivator (916 MHz) which is an Amplitude Modulated signal. The Receiverdemodulates this and present it to the timing control cpu • The TIMING CONTROL CPU performs a number of tasks • Decode the data from the receiver and look for time base signal• maintain time count using a crystal control timing reference• load and compare time/slot switch data and determine when to transmit• provides control commands to the data encoding CPU • The DATA ENCODING CPU under the control of the timing control cpu readsthe contents of the external ID/command switch and combines this with data fromthe control cpu. When so directed, it turns on the AM transmitter and formatesthe data as required for reception by the postal tags • The AM transmitter is a hybrid unit operating at 916.5 MHz. This unit generatesa low power RF signal (0 dBm or less). Operation is On-Off-Keying (OOK) modewhere the digital signal either turns carrier full on (logical 1) or fully off (logical0)
• The RF ATTENUATOR allows the installer to reduce the output RF signal levelto a much lower level to reduce the coverage area of the unit 7-RF DATA FORMAT The activator system transmits data using a low power OOK (on-off-keying) AmplitudeModulation system. The format of the data and data rate were determined by thecharacteristics and limitations of the RF transmitter/receiver components. 7.1-ENCODING The data is encoded in such a manner that the overall signal has a 50% duty cycle(within a few percent) regardless of data content. This is required to maintain DCbalance of the signal as presented to the data slicer internal to the receiver module. To provide the objective of DC balance, the data format includes both true andcomplement forms of the original data, plus additional padding bits. Thecomplement data is redundant and thus provides a means of simple error checkingbased on comparing the true and complemented data received. 7.2-SLAVE AND AUTONOMOUS ACTIVATOR DATA SEQUENCE Dotting Dotting DottingID dataID data6 mS 6 mS 6 mS10 mS 10 mS The slave and autonomous activators use the simplest format of data transmission. The sequence starts with 6 mSec of dotting. This is a simple 5 KHz on/offsequence which allows the receiver to sync up to the incoming data stream. Thenormal data stream will never contain this long of sequence of on/off cycle in arow. The dotting is followed by 10 mSec of data, again at the same bit rate, andincludes 48 bit elements for the 8 bit transmission (true and complement data pluspadding). The cycle continuously repeats the cycle about 60 times per secondwhen the activator is in active transmit mode.
7.3-MASTER ACTIVATOR DATA SEQUENCE The master uses the same encoding method as used for the slave activator system. Thesame dotting and ID data fields are sent. However a second data field is attached to theend of the ID data field. This field contains the 8 bit time base count value and isencoded in the same manner as the ID field with 48 bits used. This extends the length ofthe data sequence to 26 mSec 8-KNOWN LIMITATION • The system is limited to how far the low power master activator can send itssignal to the slave activators. This is typically about 100M max. in open space.The real range is reduced by obstruction in the path such as low ceiling, metalmachines and overhead assemblies etc. • The slave units will operate for 1 hour without the master signal. It is possiblethat when there is a power failure at the master only, it may start its time basecount at a point where it transmits when one or more slave are also transmittingdue to new vs. old time base count values. During such an event, it may takesome amount of time (up to the hour in worst case) before the slave units are in areceive mode at the same time as the master is sending data, and not beingblocked by some other slave activator.
9-INSTALLATION 9.1-ANTENNA TYPES The system uses two primary types of antennas.• The non-directional mono-pole unit with ground plane. This is a low gainnon-directional unit for coverage over a large circular area.• The directional patch antenna. This is a high gain antenna that projects thesignal over a narrow coverage of about 90 degrees (45 degrees either sideof the line formed at right angles to the front surface of the antenna) 9.2-POWER SETTING AND ANTENNA GAINS To comply with radio regulations the power level must be kept below 50,000uV/M (+94 dBuV/M) measured 3 M. from the antenna, at the point of strongestsignal. For an isotropic antenna (gain=1), this happens at a power input level of-1.2 dBm for steady carrier. As we transmit only 50% of the time, we have a 3 dBmargin and can have a peak input power level of +1.8 dBm to the same type ofantenna. The isotropic antenna is not practical to construct, however it forms thebasis for comparing input power to the resultant field strength. An antenna with again of 5 dBi produces a field strength 5 dB stronger than an isotropic antenna forthe same input power. Thus if we can have an input power of +1.8 dB for anisotropic, for the +5 dB gain antenna, we must reduce our power to 1.8-5 dBm or -3.2 dBm. 9.2.1-MAX POWER FOR MONO-POLE In the case of our two antennas, the mono-pole has a measured gain of 1.3 dBiwhen attached to the normal feed port of the activator using RG-59 cable and thecable, with a F connector on one end and TNC connector on the other, where thiscable is 4 inches (10 cM) in length. This imply the maximum non-modulatedpeak carrier level can be as high as (1.8-1.2) +0.6 dBm. For the patch antenna. Iflonger cables are used, the power may be increased according. 9.2.2-MAX POWER FOR PATCH The gain is measured the patch antenna is +8.7 dBi. Thus the nominal maximumpeak carrier input should be no more than -6.9 dBm. Compensation for longercables can be made (consult details of cable for loss per meter) 9.3-POWER SETTING AND ADJUSTING The power of the unit is adjusted by the rotary control pot. near the output port.An internal pot. sets the maximum limit value for the external pot. The normalsetting of the units are to limit the output port power to -3 dBm. The externalcontrol provides 25 to 30 dB adjustment range of further attenuation. (rangedetermined by setting on internal pot.) The power level can be measured by connecting the output port with a cable ofknown loss to the input of the power meter or spectrum analyser, and if the case isopen, connect the modulator transistor’s base pin to ground to force carrier on.
The level read is the true peak power. If the signal is being measured with externally only, and a modulated signal ispresented, the determination of correct setting depends on the type of meter ordetector present. An averaging detector or meter (averages volts), results in areading with is ½ the voltage (average voltage, not average power). This is -6dBdown from the peak, or -3 dB below the true average power level. An RMSresponding meter would report the correct average power. If a peak readingdetector or meter (or Q-Peak) the results should be very close the same readingthat would be taken if the carrier was locked on and therefore directly read.

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