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Cell edge speed?

The Shannon curve shows that as the SNR is worse (Signal to Noise Ratio), you have less speed. You can get more speed by increasing base power or having a better modem aerial.

If you increase base power the modems at another mast with same channel of sector experience interference as the aerials for mobile devices must be omnidirectional. Typically the aerial gain is -2dBi to 0dBi. The base power is thus set to give about -2dB SNR at the Cell edge, which is the edge also of the next cell. Thus the speed is as low as 1Mbps for a sole user (QPSK with lots of error correction). Closer to mast the SNR can be over 20dB, allowing 21Mbps and little overhead (regular HSDPA is 16QAM 14.4Mbps with zero error correction.). Modem/Handset limits cell size as it must use a USB port or a battery. 300mW is typical peak power.

Fixed Wireless uses a directional aerial of 6dBi to 30dBi gain and can have more Modem power as it can use mains. Inter-cell interference on same channel is avoided at the cell edge power allowing over 20dB SNR, 22Mbps @ 64QAM in 5MHz because the users aerial points at the Base. Fixed wireless thus only operates at Shannon limit at cell edge for Receive, but turns down modem power on uplink as you as closer to mast to keep same SNR for the Mast’s receiver from all Modems.

Mobile systems thus must operate at more than 25dB lower SNR at cell edge than fixed Wireless systems to avoid inter-cell interference. This means on average cell or sector throughput the Fixed system is always x10 to x20 better than equivalent spectrum Mobile System. As 100Mbps LTE is mobile, it can’t break these rules. The Cell edge speed for a sole user has to be about 400kbps.

Also in a dense Urban 250m radius cell, the same Shannon curve applies, the cell base and modems must use much less power. The cell edge is still -2dB SNR and only 6% area have 14.4Mbps speed. Otherwise the cells interfere. The only way to have more speed is more spectrum so that the same channel is many cells away and then more power in Urban cells can be used. In rural cells you run out of modem power.

Indoor use reduces the signal by up to 20dB depending on type of glass and direction of mast.


© Michael Watterson 2009 watty at eircom dot net