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Deputy Mayor / Adjoint au Maire

LTE network fundamentals

In response to the positive feedback on my first post about network sharing last weekend, I thought it would be a good idea to expand to other topics that people might be interested in learning about.


Here are the topics previously discussed:


These are the topics I am hoping to cover. If there is a preference for what you would like to see first, let me know!


  1. How do voice calls work? What is CSFB and VoLTE? Why does SMS work on LTE, but not voice?
  2. What is MIMO and how does it work?
  3. How can I achieve very fast speed test results?
  4. 3G vs 4G vs LTE. What is the difference? What are the paths to 1 Gbps, and how are peak network speeds calculated?
  5. What does a cell site look like in real life?
  6. Basics of LTE roaming and why I don't need to change my APN (disclaimer: This is not a strong point for me).
  7. How is a SIM card used to help a phone attach to the network?
  8. Who are the main players in providing network equipment for the cellular operators like Telus?

Additional topics:

Mayor / Maire

Re: LTE network fundamentals

Sounds like a lot of great reading is to be done this weekend

I would like to have more info on point 4

*3G vs 4G vs LTE. What is the difference? What are the paths to 1 Gbps, and how are peak network speeds calculated?

This might lead to understanding the question before of:

*How can I achieve very fast speed test results?

Many thanks in advance

Model Citizen / Citoyen Modèle

Re: LTE network fundamentals

This is great!

thanks for putting this together 

looking forward to the posts

Mayor / Maire

Re: LTE network fundamentals

An in-depth look at exactly which frequencies/channels/bands are used for LTE by Public Mobile (TELUS), along with their performance limits and capacities?


Re: LTE network fundamentals

OP these are all great, geeky topics.  I have no preference for order of dicussion.  I am ready to learn.


Re: LTE network fundamentals

@Jeremy_M  we seem to have a very valuable asset in @sheytoon not sure if anything like this is possible or been done before but a section where all of sheytoon's posts and explanations could be put in one central location for everyone to view easily as it is great info.

* I am happy to help, but I am not a MOD please do not include any personal info in a private message to me, click here to private message a Moderator *
Deputy Mayor / Adjoint au Maire

Re: LTE network fundamentals


Very good, I will try to cover this as well.

Deputy Mayor / Adjoint au Maire

Re: LTE network fundamentals

Topic #1: How do voice calls work? What is CSFB and VoLTE? Why does SMS work on LTE, but not voice?


Voice calls on 2G and 3G networks are circuit-switched (CS). This means an end-to-end communications link is established between the two parties and the circuit for that cellular channel is reserved for the call. Whether there is conversation or not, the voice channel uses a flat-rate amount of resources.


LTE is an all-IP network, which is also packet-switched (PS). There are no circuits and voice calls are handled by a technical standard known as Voice Over LTE (VoLTE - prounounced voltee). VoLTE is a type of Voice Over IP (VOIP). With PS voice calls, the voice data is converted to packets and sent to an IP address, similar to regular internet data. If the phone call is silent, no voice packets need to be sent, and the utilization of the channel is dynamically reduced as a result. Packets can also take different routes to reach the destination. To ensure voice calls have high quality, VoLTE packets are prioritized in the network. If the LTE channel becomes congested, VoLTE calls will not be impacted. Advantages of VoLTE include: extremely fast call setup times, better call quality due to higher rate wideband codecs, LTE data speeds while on a call, more efficient use of network resources, and longer battery life for users who make lots of calls.


VoLTE requires the implementation of an additional core network, known as IMS.


When LTE first launched, coverage was poor, operators did not have IMS networks deployed, and phones were not VoLTE-ready. As a result, a temporary mechanism was required to handle voice calls. This technique is known as Circuit-Switched Fallback (CSFB). When an LTE phone registers on the LTE RAN, it also informs the core network that it needs to be simultaneously registered with LTE and 3G core networks. The LTE core network informs the 3G core network and the user is registered in both cores. For an incoming call, the 3G core informs the LTE core to instruct the phone to switch to 3G and receive the call. For an outgoing call, the phone informs the LTE RAN that it will be switching to the 3G RAN to initiate a call, and then it proceeds to do so. During a phone call, the phone stays on 3G. It can only go back to LTE once the phone call has ended.


If you force your non-VoLTE phone to "LTE Only" mode, you will not be able to make or receive any phone calls.


What happens if you start a VoLTE call and move to an area with no LTE coverage? CSFB will not help in this scenario, because CSFB works by having the voice call use the 3G RAN for the entire duration of the call. It cannot transfer an existing VoLTE call from LTE to 3G. For this scenario, another mechanism is needed, and it's called SRVCC (Single Radio Voice Call Continuity). SRVCC is the mechanism that allows an existing PS session (VoLTE voice + LTE data) to be split into 2 separate CS (3G voice) and PS (3G data) sessions. Once SRVCC is triggered, the voice call remains on the 3G network until it ends. The user can then go back to LTE after the call has ended.


SMS is handled a bit differently. Even in "LTE Only" mode, SMS works in both directions and doesn't need the 3G RAN. SMS messages are sent to the MME (one of the nodes in LTE core), and the MME sends it to the MSC (one of the nodes in 3G core). This works in the reverse direction as well. The MME sends and receives SMS to/from the phone using the LTE control plane. For more details, refer to this note:

Deputy Mayor / Adjoint au Maire

Re: LTE network fundamentals

Topic #2: What is MIMO and how does it work?

As the demand for mobile data increases, new ways of delivering more data are needed. The amount of available spectrum is limited, and buying new spectrum is extremely expensive for operators. Another way of boosting capacity is by deploying MIMO.

2G and 3G networks broadcasted each channel on 1 antenna. The phones also had 1 antenna, and the radio-frequency (RF) transmission was quite simple. This implementation is known as SISO.
SISO = single input, single output
SIMO = single input, multiple output
MISO = multiple input, single output
MIMO = multiple input, multiple output
With the launch of LTE, the eNodeB is able to transmit and receive signals for a channel using 2 antennas. The phones have 2 antennas as well for receiving (though only 1 of them can be used for transmitting). So the eNodeB is MIMO, but the phone is MISO.
Under good signal conditions, half of the DOWNLINK data is sent on one antenna, and the other half is sent on the other antenna. Because the phone also has 2 receive antennas, it can receive 2 streams of data simultaneously. Thus, the overall throughput is doubled. This technique is known as spatial multiplexing. The picture below is for Wi-Fi, but the concept is the same.
For UPLINK, the phone sends on one antenna, and eNodeB receives on 2 antennas.
In the example above, downlink is 2x2 MIMO, because transmitter (eNodeB) has 2 ports, and receiver (phone) also has 2 ports. Uplink is not MIMO because transmitter is only 1 port. This is one of the reasons why uplink speeds are slower than downlink. Other reasons include lower modulation and lack of CA.
What happens if the phone is at the edge of coverage and has a weak signal? Instead of sending half the data per antenna, the entire data is sent on both antennas, for redundancy. The receiver can selectively combine the best of each signal path to reconstruct the original signal. This technique is known as transmit/receive diversity and effectively increases the cell coverage.
eNodeB dynamically adjusts between diversity and spatial multiplexing for all users under its coverage based on signal conditions to maximize throughput.
Can we use 4x4 or 8x8 MIMO to increase downlink speeds even more?
Yes, higher-order MIMO has already been standardized, and conceptually we can go even higher than that with massive MIMO, but there are practical limitations to consider. Phones are quite small already, and doubling the number of antennas while keeping a certain separation between them is challenging. Phone manufacturers are only now starting to release 4-antenna models capable of downlink 4x4 MIMO, and this is only on certain bands.
For upcoming 5G networks, massive MIMO is currently being developed and tested, and it is common to see 64 transmit ports on the base station side.
Mayor / Maire

Re: LTE network fundamentals

Other upper limits also exist.  Maybe there's no limit to how many active antenna can get stuffed into each phone; maybe people won't mind hauling around ever-larger devices, maybe engineers will always invent ways to decrease component sizes and increase technological densities.


But (FCC) regulatory limits cannot be circumvented.  Useable portions of the EM spectrum are limited, and allowable portions even moreso.  Allowable transmission power is strictly limited, so mobile data can't just multiply and multiplex across more and more antenna elements without consequence.  And every Watt (well, milliWatt) of transmitted power has to come from an electrical power source - mobile batteries impose fundamental limits on how much energy they can store and on how much energy they can (safely) discharge - batteries in today's "ultra-power-efficient" devices can already get surprisingly (almost dangerously) hot and run charges down "into the red" after short activity sessions.


Many people and many organizations already protest radio noise for a variety of health, safety, aesthetic, and technical reasons.  It's increasingly difficult for a ham radio operator to erect a radio tower without being shut down by concerned neighbours.  People already campaign (sometimes successfully!) to shut down cellphone towers/repeaters in their neighbourhoods, and I'm sure that more antenna arrays to service more frequencies would attract even more attention as "dangerous" "noisy" "eyesores".


Just saying that many "real" limits are not technical, they're entirely different arenas - legal, financial, social, political, etc.