HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New MillenniumThe channel encoder packs the data of each speech frame witherror-correction information so that the data are immune to the interferencesthat can occur when signals are transmitted over the air.The data are encrypted to ensure security during cellular transmissions.Encryption uses the XORing of the data bits with a key sequencegenerated by an encryption algorithm.Equalization is used because of the multipath effects of the receivedsignal. A transmitted signal may take multiple paths to the receiverantenna which creates multiple versions of the same signal. Eachof these arrives at slightly different times and with different signalstrengths. An error-correction coprocessor operates in parallel with theDSP and accelerates the equalization and the channel coding.To assist the receiver in detecting the true signal from the multipleversions, the transmitter encodes a training sequence into the burst. Thereceiver’s equalizer knows the training sequence and uses it to identifythe transmitted signal.The trend is to integrate the DSP with analog-to-digital converters(ADCs), digital-to-analog converters (DACs), a clock synthesizer, andsystem control units that include a memory controller, an interrupt controller,and serial I/O controllers. The functionality that is usually integratedacross an entire chip set is created on a single chip.The BTS must be software upgradeable in the field so that it can beeasily reconfigured to support evolving standards over its life cycle. Onchipflash memory is used so that application software can be loadedand debugged on the spot.The protocols of narrowband personal communication services (N-PCS) include Motorola’s FLEX. The N-PCS frequencies lie in narrowbandwidths just above 900 MHz. Some of this spectrum is allocated to12.5-kHz paging response channels, allowing pagers to transmit repliesto received messages. Four-level frequency shift keying (FSK) modulationis used by FLEX. A synchronous protocol allows the pager to switchon periodically (every few minutes) and check if a message is being sent.Decoding starts with frame synchronization where the decoderaligns its receive buffer to the frames in the received bit string. Datadecoding algorithms allow the extraction of message data, error-controldata, and ID data from the frames. The error control code is used tocorrect errors in the frame data. In some protocols, such as the EuropeanRadio Messaging System (ERMES) and the Motorola FLEX paging protocol,deinterleaving is also performed.©2001 by The Fairmont Press, Inc. All rights reserved.