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Technology Stocks : VALENCE TECHNOLOGY (VLNC) -- Ignore unavailable to you. Want to Upgrade?

To: hcirteg who wrote (12145)	6/15/1999 5:12:00 PM
From: Rich Wolf	Read Replies (4) \| Respond to of 27311

OT re: GSM modulation for cellphone applications (mostly European at this time, but migrating into the US and Asia as well): Okay, I looked into the GSM modulation format. For those who want to know, read on: GSM uses both frequency and time division. The overall frequency band is separated into 63 channels. Within a given frequency channel, transmission is alternately shared by 8 users in a time-division (TDMA) format. Each user gets 0.58 milliseconds to transmit during his 'slot,' then waits while the other 7 users transmit. This is where the need arises for the phone to transmit its maximum power for only 1/8 of the time, in a regular pattern. (This was also the origin of the curve on the spec sheet for the Valence model 43D cellphone cell, showing the voltage and current characteristics for GSM pulsing.) Within the duration of the 0.58 ms pulse, there is a burst of data at a bit rate of 217 kbit/sec (using GMSK, Gaussian Minimum Shift Keying, which is the pulse shaping of the data bits). This provides 158 bits for 'data,' but because the data needs to be encoded, and because of the need for synchronization bits, the actual throughput of 'information' is less than this. For voice there are 128 bits (only the 50 most significant bits are block encoded). Bottom line is that your voice is encoded at a sampling rate of 13kHz, good enough to reproduce most human voices (and better than the 8kHz sampling rate used for some long-distance digital voice multiplexing schemes). As an aside, there are plans for expanding the capacity of such networks by reducing this sampling rate (which would allow a certain amount of degradation in sound quality). Going back to the fundamental 'time slots:' after a group of 8 users has each transmitted one set of packets for the TDMA slot, they all hop to another frequency band before doing their next transmission (not necessarily hopping together). This so-called 'slow frequency hopping' serves to move people around to different frequency slots so that if it happens that a certain user is being affected by channel propagation effects (and thereby losing part of his transmission during a given TDMA slot), he has a high probability of moving to a 'good' channel for the next packet transmission. The above is GSM modulation, which was defined by the Groupe Speciale Mobile, convened in 1982 to govern the cellular European public mobile communication system for operation in the 900 MHz band. Another poster had asked about CDMA (code division multiple access). That format can also use TDMA/FDMA slotting methods, but the fundamental CDMA modulation spreads the signal across a broader band, which is overlapping in frequency and time with transmissions for other users. In CDMA, each user is assigned a code which is mathematically nearly 'orthogonal' to the others, so that a receiver locked onto one user doesn't hear the others. What I understand is that current public CDMA systems are designed to only use CDMA on the downlink (base station to handset), but TDMA on the uplink (from the handset to the base station). If CDMA was used on the uplinks, the random delays between users would allow for possibilities of multi-user interference, and the picture gets more complicated (mitigation requires more antennas for 'diversity reception' etc). So as far as the design of the handsets in a CDMA system is concerned, I think they will also need to pulse their power 'on' for a fraction of the time, since their uplink transmissions would be in a TDMA format just like in GSM, and transmit a 'packet' of data during the 'on' interval. So the requirements for the battery cell powering the cellphone would be basically the same as for the GSM cellphones. Hope the above is helpful to some. Regards, Rich