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Cell Phone Networks – What can we learn?

cellular-systems

The magic of the cellular system is the division of a coverage area into small cells. This allows for low power and extensive frequency reuse across the cells, so that many users can use their cell phones simultaneously.

Prior to cellular networks people used "car phones". Typically, these were high power VHF two-way radios that could cover about 30 miles and limited to perhaps ten channels. The systems I saw in the UK did not have any way of passing users between base stations, so if you drove out of range of one station there was no choice but re-try and hope you reached a new base station.

Before cell phones at about the time I was starting my radio career the Pye Vanguard was the standard two way radio used for mobile communications, it was VHF, had perhaps six channels and produced about 25 watts from a quick heat power tetrode. It typically used a 5/8 vertical and communicated with a split frequency bases station, often with diversity receivers and a hundred watts of power. Not unlike our current VHF and UHF repeater systems today. The car phone used similar technology but has a tone based selective calling squelch that would only alert the user if there was a call for them.

It is hard to explain the difficult culture shift that occurred moving to a cellular network. Prior to cell systems the design strategy was base stations in high locations and high-power mobiles capable of communicating in sheltered locations but always with a 20–30-mile range. That was the though process of the system engineers. The new generation of engineers talked about powers of less than one watt and ranges much less than a mile. The old school could not comprehend that cell users could be switched automatically and seamlessly between base stations.

The move to cellular was both a pardine shift and a disruptive technology. Just like the telephone replaced the telegraph, cell phone technology is now the norm, and no serious commercial radio system uses the base station mobile model (well not strictly true, many US Police and Emergency Services still use these systems), however most now use private services piggy-backed on the cellular networks.

The growth of cellular systems has spawned significant technological advances. It is hard to believe the original 1G systems were analog FDM systems with handsets the size of a one or two bricks.It is worth looking at the development of cellular technology:

1980's – AMP or 1G – An analog frequency division multiplexed system using 30KHz at 2.4kbps.

1990's – GSM or 2G - Bandwidths of 30KHz to 200KHz included SMS and MMS messages, migrating to GPRS & EDGE technology often called 2.5G with data-rates up to 144kbps, allowing e-mail and web browsing.

2000's UMTS & CDMA2000 allowed data-rates up to 14Mbps, enabled users to make video calls, share files, and view TV online. Finally, being shut down in the US as I write this in early 2022.

2010's LTE or 4G – The start of the smartphone with theoretical download speeds of between 10Mbps and 1Gbps, and better latency. The system is a packet switched and IP based, treating voice as just another service.

2020's – 5G – The ICU approved specification will enable near-real time response rates; and connection densities of 1000 devices per square kilometer (100 times more than 4G) will support the growing numbers of IoT devices and sensors. To get the higher speed new higher frequency spectrum has been allocated, potential co-location interference particularly in the US is hampering roll out and social media sources constantly claim COVID and big brother surveillance are the real reason 5G is being developed. The higher mm wave signals have a range of a few hundred meters so creating a new cellular network of Pico cells.

An interesting overview? perhaps we can say there has been a new generation of cellular technology every ten years. This summery has not done justice to the technological developments in packet switching, codec development and the chip/radio technology needed to support this growth. What I find surprising is how little of this technology has transitioned into Ham Radio.

As the champion of the Emergency Ham Net, I was motivated by the gap between current telecommunications practices and where ham radio is today. I see parallels between using small adjacent cells of a Cellular Network and using small low power systems by radio operators to link together to form a digital network. The theory suggest that four channels could be sufficient to create the mesh and that there are many candidate digital modulations that could provide reliable low bandwidth communications. I do not expect to see voice quality bandwidths in my lifetime, but a low data rate off grid (no Internet) network could be built. What do you all think?

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Thursday, 29 September 2022