• In 1968, AT and T proposed concept of cellular telephony to Federal Communications Commissions (FCC).

• The basic idea of a cellular system is to divide the coverage zone into smaller cells served by low power transmitters. And the channel frequency is reused at a sufficient distance to increase the spectrum efficiency. These low power transmitters are called as Base Station (BS).

• The portable Tx/Rx units used by the subscribers are known as Mobile Station (MS).

• Initially, FCC allocated 40 MHz spectrum in the 800 MHz band for the US Advanced Mobile Phone System called as AMPS technology. It is the first mobile technology and is analog in nature. Bandwidth of a simplex channel in AMPS was 30 KHz. Bandwidth of a duplex channel was thus 60 KHz. Hence, total no. of duplex channels formed were $\frac{40 MHz}{60 KHz}$ i.e 666 channels.

• As the number of customers increased, number of frequency channels were not sufficient to serve the users lots of congestion issues were seen. Then in 1989, 10 MHz more spectrum was allotted i.e. additional 166 channels were made available.

• Hence, in all 832 channels were available for cellular communication.

• The communication link from MS to BS (BS to MS) is known as up link (down link) and channel used on this link are called Reverse (Forward) channel.

Frequency Channel Allocation

• Up Link : 824-849 MHz (Reverse channel spectrum or Uplink Bandwidth)
• Down Link : 869-894 MHZ (Forward Channel spectrum or Downlink Bandwidth)

These Formulae are empirical Formulae to find center frequency for any forward and reverse pair of channel, where N is the channel number.

Spectrum for forward channel is always higher than the reverse channel in mobile communication technology. It is due to the fact that higher frequency needs higher processing power. Base Stations are fixed; they have proper battery backups and can afford to operate at a higher power rating. Hence a higher frequency is allotted to Base Stations for the Down Link transmissions.

The Channel numbers 800-989 does not exist. It is because the center frequency formula are actually the formula on which frequency synthesizer operate. For the channel numbers, 800- 989, the synthesizer’s output was a frequency outside the allotted band by FCC which was not allowed. It can be understood in better way by following example.

Example: Draw the Frequency Spectrum allocation for US Cellular, find the centre Frequencies for the following forward and reverse channels.

a. Channel 999 b. Channel 701 c. Channel 102 d. Channel 816

a. Channel 999 The formula to be used for Channel 999 is

Reverse channel: 0.030 (N-1023) + 825 = 0.030 (999-1023) + 25 = 824.28 MHz

Forward Channel: 0.030 (N-1023) + 870 = 0.030 (999-1023) + 870 = 869.28 MHz

b. Channel 701 The formula to be used for Channel 701 is

Reverse channel: 0.030 (N) + 825 = 0.030 (701) + 825 = 846.03 MHz

Forward Channel: 0.030 (N) + 870 = 0.030 (701) + 870 = 890.03 MHz

c. Channel 102

Reverse channel: 0.030 (N) + 825 = 0.030 (102) + 825 = 828.06 MHz

Forward Channel: 0.030 (N) + 870 = 0.030 (102) + 870 = 873.06 MHz

d. Channel 816

Reverse channel: 0.030 (N) + 825 = 0.030 (102) + 825= 849.48 MHz

Forward Channel: 0.030 (N) + 870 = 0.030 (102) + 870 = 894.48 MHz

Both of these centre frequencies are outside the spectrum allotted to Cellular Radio service. Hence Channel Numbers 800 -989 do not exist in US cellular standards.

Initially, 800 MHz was allocated for cellular radio service. The basic reason behind this was, Cellular concept was proposed in 1968. By this time all the other communication services were already operating on most of the frequency Bands.

• FM broadcasting was allotted 88-108MHz..
• TV channels were being transmitted on 41-960MHz.
• Air to ground services used 118-136 MHz.
• Military aircraft used 225-400 MHz.
• Also mobile transmission cannot be applied at 10 GHz and above due to severe propogation losses and multipath fading.

There was an educational TV channel which got shut down during the same era. This frequency band was released to Federal Communication Committe (FCC). FCC allotted it to AT & T Labs for planning of cellular system.