Multiple Standard Systems

1. Deployment of new standard without impacting current services
There are many radio base stations (RBS) deployed in cities supporting different telecom standards such as 3G, 4G, and 5G. Locations of those RBS had been planned to cover the entire areas effectively. Having reserved another location for a new RBS with another standard could be expensive, thus we could incorporate the new telecom standard’s subsystem in the existing RBS as shown in the figure below.

Figure 1. An example of RBS supporting 3G, 4G, and 5G standards.

Each telecom standard has been allocated a unique frequency and protocol. Their transceiver (transmitter and receiver) could pick up appropriate signals from the antenna or sending out signals using appropriate frequency to the antenna.

Those 3G, 4G, and 5G’s subsystems are loaded on the RBS as 3 separate subsystems to lower costs of software development even though developers could eliminate redundancy or duplicate components further, because those subsystems must have common modules or interface. The plus side of it was to be able to deploy another subsystem quickly and independently; and avoid impacting functionality (bugs) in existing live subsystem. 

3G, 4G, or 5G subsystem could communicate to other external telecom components or nodes via the Interface to wireless telecom network with its protocol. This Interface module would also be able to receive incoming external signals and dispatch those to correct subsystem for processing.

The above notes were for an RBS, but this model is also applicable to a mobile phone or any equipment supporting multiple standards.

2. SIU and Police sharing telecom networks

This model was originally proposed to police (5G) telecom system. The Special Investigation Unit (SIU) of police would be able to develop their own communication standard and loaded this system on police (5G) RBS. SIU’s system would include different protocol and frequency, thus they could share the RBS’s processing power, but data would be processed separately. The police (5G) system is left intact.

Figure 2. SIU and Police Systems

3.  A common practice in development of a multi-protocol system

Figure 1 showed a strategy to introduce a new subsystem without impacting existing and working system to avoid possible interruption of services to users. However, figure 3 illustrated a common practice by system providers to implement additional features from an evolved protocol into an existing system.

Figure 3. An example of implementation of 4G and 5G subsystems into an existing working 3G subsystem.

In the figure above, the (3G, 4G, or 5G) Mobile Base Station subsystems would handle the differences in protocols by 3G, 4G, and 5G standards. The output and input of MBS subsystems would be the common protocol toward the core telecom subsystem. Usually the RBS would communicate with an MSC to handle routing and voice connections for mobile users.

4. A design of multi-frequency/protocol system using a single chip

I think, Qualcomm made a modem chip, Snapdragon X55, supporting all standards, i.e. 3G, 4G, and 5G. This is probably why Qualcomm chip is so expensive. X55 source: https://www.qualcomm.com/products/snapdragon-x55-5g-modem

Figure 4. An example of a mobile phone’s design using Snapdragon X55 modem chip

5. Sharing a frequency band by different service providers

Usually telecom providers or operators used the different frequency bands to provide wireless services.

However many operators could use "frequency modulation" or "phase shift key modulation" to provide services to their users using the same frequency band as long as they provided different amplitudes in carrier waves. The wireless receiver would extract data from wave carriers with expected amplitude, i.e. allocating different amplitudes to adjacent operators in an area.

Disclaimers:

I haven't worked in RF for telecom industry. I used to work in mobile telephony networks at Ericsson.

For multiple bands, they would need either new antennae or circuitry to branch out required data. Perhaps band pass filter would be used.

For multiple protocols, it would require technical analysis to identify the differences and common between those protocols/frequencies in order to select an optimized solution.

Some references about multi-bands in telecom

* "Changing bands or modes is done automatically by phones that support these options. Usually the phone will have a default option set, such as 1900-MHz TDMA, and will try to connect at that frequency with that technology first. If it supports dual bands, it will switch to 800 MHz if it cannot connect at 1900 MHz."

* https://www.commscope.com/Blog/How-to-Efficiently-Support-Multiple-Frequency-Bands/

* https://en.wikipedia.org/wiki/Multi-band_device