topic Re: LTE network fundamentals in Get Support

LTE network fundamentals

sheytoon — Sun, 26 Mar 2023 01:01:44 GMT

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!

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

Additional topics:

Which frequencies/channels/bands are used for LTE by Public Mobile (TELUS)
Overview of 700 MHz (B12, B13, B17, B29)
5G and LTE-A Pro
What frequencies does my phone need to work on Public Mobile?

NEW:

Overview of 5G

Additional 5G topics:

What does the 5G icon mean?

Re: LTE network fundamentals

zhadj030 — Fri, 13 Jan 2017 03:25:05 GMT

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

Re: LTE network fundamentals

Vickel — Fri, 13 Jan 2017 03:33:04 GMT

This is great!

thanks for putting this together

looking forward to the posts

Re: LTE network fundamentals

Korth — Fri, 13 Jan 2017 03:58:28 GMT

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

will13am — Fri, 13 Jan 2017 04:32:57 GMT

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

Re: LTE network fundamentals

ShawnC13 — Fri, 13 Jan 2017 05:02:46 GMT

@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.

Re: LTE network fundamentals

sheytoon — Sat, 14 Jan 2017 02:03:12 GMT

@Korth

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

Re: LTE network fundamentals

sheytoon — Sat, 19 Dec 2020 14:06:23 GMT

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 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:
http://www.sharetechnote.com/html/Handbook_LTE_SMS.html

Re: LTE network fundamentals

sheytoon — Sat, 19 Dec 2020 16:42:03 GMT

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 transmitted radio signals 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.

Re: LTE network fundamentals

Korth — Sat, 14 Jan 2017 23:56:51 GMT

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.

Re: LTE network fundamentals

sheytoon — Thu, 04 Aug 2022 01:27:59 GMT

Topic #3: How can I achieve very fast speed test results?

In order to get peak download speeds on your phone, you need to make sure your signal is good and the network is not congested.

The main indicator for a good signal for download speeds is your phone's Signal to Noise (and Interference) Ratio, more commonly known as SNR (or SINR). This shows the cell's power relative to every other signal at the same frequency / channel in decibels (dB). The higher your SINR, the higher speeds you can expect to see. An excellent value for SINR would be 25 - 30 dB.

Number of bars are not related to SINR. You could have full bars with a poor SINR. The bars are showing a different measurement, which doesn't take interference into account. This value is known as RSRP, or the Reference Signal Received Power. RSRP is used for performing handovers, among other things.

For uplink, there's no way to check the SINR at the eNodeB, so the next best indicator would be RSRP. It's not perfect, but it's the best indicator available. An excellent value for RSRP would be -85 dBm or higher (Note: higher means less negative, and -75 dBm would be an example of a higher value).

In terms of network congestion, the Big 3 operators all have excellent core and backhaul networks, so it would be safe to assume there is no limitation there. The only limitation would be at the RAN, but again there's no way to measure this from the phone. As a rule of thumb, networks will be very close to zero utilization after midnight.

Here's an example of some tests I did last year in Toronto on Bell using a Samsung phone. B2 + B4 CA with a total aggregated bandwidth of 35 MHz (20 + 15), using 2x2 MIMO. Peak network capability is 260 Mbps. My results would have been even better after midnight.

For comparison, here are the results from a lab environment, where 260 Mbps is the MAC layer limit. Application layer limit seems to be 249 Mbps.

~~From what I have seen using a Public Mobile SIM, there appears to be some limit imposed from the core network. See HERE and HERE for more info. Koodo users are not impacted:~~

~~http://www.howardforums.com/showthread.php/1777615-KOODO-LTE-speeds-post-em-if-you-got-em?p=16808019#post16808019~~

UPDATE: As of May 26, 2017, Telus has resolved the speed issues at the core network and Public Mobile customers on "4G speed" plans are no longer limited in speed. See HERE and HERE.

Re: LTE network fundamentals

sheytoon — Sat, 09 Sep 2017 01:45:58 GMT

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

The most popular 3G technology globally is known as UMTS. It was a new technology developed to increase efficiency and data capacity relative to older 2G GSM networks. UMTS uses the same core network as GSM, but the RAN is new. UMTS allowed RAN sharing to be implemented for the first time, so Bell and Telus cooperated in building a RAN to catch up with Rogers.

Similar to LTE, UMTS has 2 different implementations. Most operators use FDD, also known as W-CDMA (wideband CDMA), but China Mobile uses TDD, also known as TD-SCDMA. The 2 systems are generally incompatible with each other and phones will usually support one or the other, not both. See here for a quick overview of FDD and TDD.

HSPA+ is an evolution of the data part of UMTS, which allows faster data speeds than basic UMTS. HSPA+ and UMTS are both 3G technologies, though they are incorrectly identified as 4G.

The confusion began when Verizon was launching 4G (LTE) in the US and T-Mobile USA didn't want to be seen as having an inferior 3G (HSPA+) network. They decided to call their existing network 4G, and eventually other operators followed along. The United Nations International Telecommunication Union (ITU) normally defines what technologies can be called 3G, 4G, etc, and many in the technical industry were surprised when ITU allowed UMTS/HSPA+ to be called 4G. True 4G is a technology that can provide 1 Gbps (1000 Mbps), and even today we are only starting to see commercial networks offer these speeds.

So depending on what network you're on, and what software your phone has, you might see 3G, H+ or 4G for a UMTS/W-CDMA/HSPA+ network. For LTE, you might see 4G, LTE or 4G LTE.

Some manufacturers like Samsung have started displaying LTE+ when a Secondary Component Carrier (SCC) for CA is added and total aggregated bandwidth is greater than 20 MHz. There is no universally defined technology that is known as LTE+.

As mentioned above, 1 Gbps is one of the main goals of true 4G networks. This can be achieved through a combination of these enhancements:

CA up to 100 MHz bandwidth
4x4 or higher MIMO
256 QAM

Peak throughput values are defined as follows for SISO and 64 QAM:

18 Mbps for 5 MHz bandwidth
36 Mbps for 10 MHz bandwidth
55 Mbps for 15 MHz bandwidth
75 Mbps for 20 MHz bandwidth

For 2x2 MIMO, double the above values. For 4x4 MIMO, quadruple the above values. For 256 QAM, increase the peak throughput by ~33%.

Let's look at an example. Using a combination of these advancements, an operator can deploy 3-CA with 55 MHz of spectrum (20 + 20 + 15), 4x4 MIMO on all 3 bands and 256 QAM on all 3 bands. This would reach peak speeds of 1 Gbps. The calculation is:

3-CA = 75 + 75 + 55 = 205 Mbps

4x4 MIMO = 205 x 4 = 820 Mbps

256 QAM = 820 x 1.33 = 1091 Mbps

Once the network is ready, a compatible phone must be available to take advantage of all of the enhancements. Upcoming flagship phones are only starting to support 4x4 MIMO or 256 QAM for certain bands. They will not be able to do 3-CA and 4x4 MIMO and 256 QAM at the same time on all bands. It will take some time before we see phones on the market that can achieve 1 Gbps.

Re: LTE network fundamentals

sheytoon — Sun, 02 Jan 2022 00:17:45 GMT

Topic #5: What does a cell site look like in real life?

A cell site (base station) is generally made up of a few essential parts:

Transport router to connect to backhaul network (backhaul can be microwave, fiber, satellite, etc)
Baseband unit for processing
Radio unit to transmit/receive RF signal
Antenna

With 2G networks, the Baseband and Radio units were located in a weatherproof cabinet or small shelter at the base of the tower (or on the roof of a building). Long RF cabling was used to connect to antennas. RF cabling is expensive, bulky, results in signal attenuation, and it wasn't the best way to design a network.

With modern 3G and 4G networks, the Radio units have been weatherproofed and mounted outdoors, directly beside the antennas, making the RF cabling very short. Now instead we have long fiber cables to connect the Remote Radio Units (RRU) to the Baseband Units (BBU). Fiber is practically lossless for this type of transmission. RRUs are generally built to support one band per radio, but can usually support multiple technologies.For example, a Band 2 radio (1900 MHz) can usually be used for UMTS (3G), LTE, or both at the same time!

In Topic #2, I talked about MIMO and how it requires multiple antennas. It's important to note that multiple antenna elements can be manufactured in a single antenna panel. Here is a picture of an antenna with four ports (four internal antennas). This one antenna can be used to provide 4x4 MIMO:

Here's an example of a Sprint USA rooftop site where you can see the RRUs mounted to the back of the antenna pole:

Rooftop cabinets housing BBU, power equipment, cooling equipment, and transport router:

All of the pictures I am sharing from Canadian sites are from public Google searches.

Rogers (left) and Bell (right) towers side by side in Toronto:

Sometimes operators will share towers to save space. Tower owner will put their equipment higher up for better coverage and lease the lower spots to competitors.

Inside of a shelter will have the same equipment as the cabinets, but more spaced out. Here's an example of a site not in Canada (though they are all similar):

In the last few years, operators have been getting creative in hiding RRU and antenna equipment in order to keep the sites looking clean. Sometimes these sites will be disguised as flagpoles, or simply "empty" poles that have a cover hiding the equipment. Here are some examples:

What if we combine the antenna and RRU into a single product? That could reduce RF cabling even more and increase efficiency. In fact, the major vendors are already doing this with integrated antenna products. For upcoming 5G products that utilize massive MIMO, the radios and antennas will always be integrated.

Ericsson's AIR:

Nokia's RAS:

Huawei's AAU:

What about micro / pico cells? These are just smaller base stations (eNodeB for LTE), with less power and capacity, and a combined RRU + BBU. Here's an example of a pair of Telus outdoor microcells:

Microcells can be installed indoors or outdoors.

Picocells are smaller versions of microcells, and are mainly installed indoors. They look similar to public WiFi access points.

Femtocells are even smaller versions of picocells, purchased directly by end customers to install in their homes/small offices for improved cellular coverage. They use the customer's own internet to route traffic to the operator's network via a secure tunnel. Femtocells are currently not offered by any operators in Canada.

Re: LTE network fundamentals

Watoko — Wed, 18 Jan 2017 21:41:33 GMT

Very interesting! Thanks for putting this together. It may not be for the average PM customer, but I certainly learned a lot!

Great work!

Re: LTE network fundamentals

sportymi — Thu, 19 Jan 2017 15:00:02 GMT

You can almost write a textbook for these things. lol~ Thanks for the info!

Re: LTE network fundamentals

MVP — Thu, 19 Jan 2017 15:03:13 GMT

@sheytoon wrote:
Topic #3: How can I achieve very fast speed test results?

In order to get peak download speeds on your phone, you need to make sure your signal is good and the network is not congested.

The main indicator for a good signal for download speeds is your phone's Signal to Noise (and Interference) Ratio, more commonly known as SNR (or SINR). This shows the cell's power relative to every other signal at the same frequency / channel in decibels (dB). The higher your SINR, the higher speeds you can expect to see. An excellent value for SINR would be 25 - 30 dB.

Number of bars are not related to SINR. You could have full bars with a poor SINR. The bars are showing a different measurement, which doesn't take interference into account. This value is known as RSRP, or the Reference Signal Received Power. RSRP is used for performing handovers, among other things.

For uplink, there's no way to check the SINR at the eNodeB, so the next best indicator would be RSRP. It's not perfect, but it's the best indicator available. An excellent value for RSRP would be -85 dBm or higher (Note: higher means less negative, and -75 dBm would be an example of a higher value).

Thank you, OP, for the illuminating presentations!

With Public Mobile I have RSRP - -100 dBm or less at home, SINR = 10 dB or less (often even goes below zero 🙂 ), and still am able to achieve 20d/5u Mbps speed.

With Fido I have SINR = 20 dB, and can reach the speeds of 100d/20u.

So, it does make sense to speed test only of SINR > 20 dB.

I use Network Signal Guru app that shows a lot of info (SINR, RSRP, CA etc), sometimes even difficult to digest. Will try to test PM in a different spot, with higher SINR, to see what it gives..

Re: LTE network fundamentals

MVP — Thu, 19 Jan 2017 17:14:54 GMT

P.S. Just tested Public Mobile at a different spot with SNIR 15 to 20 dB,

and immediately got 50d / 15u Mbps!

Thanks for clarifying OP! Now it seems to me that Public Pobile is NOT throttled.

N.B. I'm still able to get 15d/0.2u even at NEGATIVE SNIR = -6 dB, (RSRP = -116 dBm),

whicl is WEIRD, but I refer to this: http://s4gru.com/index.php?/blog/1/entry-308-rssi-vs-rsrp-a-brief-lte-signal-strength-primer/ saying that LTE has 100 channels, so effectively the signal is 20 dB stronger! So, out of those 100 channels there are some with positive SNIR.

Re: LTE network fundamentals

sheytoon — Sun, 29 Jan 2017 11:48:30 GMT

@MVP

Negative SINR is a perfectly valid value, as it's a logarithmic scale. A value corresponding to zero SINR on a linear scale would be (negative infinity) dB.

As for throttling, I suspect there is some limit placed at the EPC (core). See HERE and HERE.

As for LTE having 100 channels, I don't really understand what is meant by that statement. LTE channels are made up of resource blocks, which increase proportionally with channel bandwidth, up to 100 RB for a 20 MHz channel.

Network Signal Guru is amazing! I use it too. It's great for rooted phones with Qualcomm chipsets.

Re: LTE network fundamentals

sheytoon — Sun, 11 Jun 2023 00:24:42 GMT

Topic #6: Basics of LTE roaming and why I don't need to change my APN

Disclaimer: This is mainly a core topic, and not my area of expertise. I am mostly confident in the information, but there may be some mistakes.

Previously I described the LTE network with the simple diagram shown below.

LTE roaming can be achieved in 2 ways:

Home routed (traditional)
Local breakout

Before any roaming takes place, the home operator and the visited operator must come to an agreement and connect to each other's core networks. If the user doesn't have a valid account at home, which allows roaming in the visited country, then the visited core network will reject the phone from attaching to the network.

With traditional roaming, the local SGW is used, but the home PGW is used. With local breakout, the local SGW and PGW are used.

Let's look at an example:

A Telus user travels to the US and roams on AT&T. Once the AT&T core network authenticates the user's SIM card (see next topic for more info), the user is allowed to attach to the network to receive service. All of this takes place in a matter of seconds, and you may notice this short delay when you first disable airplane mode upon landing, until you have service.

With traditional roaming, the user's data is routed from AT&T's SGW to Telus's PGW in Canada, and then to the internet from there. Since the PGW is in charge of APNs, there is no need for a new APN. This is an ok approach for nearby countries, but what if the Telus user goes to Australia and is roaming on Telstra? Having to connect from Telstra's SGW to Telus's PGW will introduce noticeable delays and the LTE experience of the customer could suffer. This is especially true if he/she is trying to browse local Australian websites, which means the data travels from the phone to the RAN to Telstra's SGW, to Telus PGW, to internet, where it is finally routed back to the local Aussie web server.

In this scenario, Telus might have preferred to implement local breakout. One thing I'm not sure about here is how the APN would be configured. I believe the PGW can assign an APN to the phone via the MME during the attach procedure.

As far as I know, Telus does not implement local breakout as it is more challenging to rely on the foreign operator to keep track of data usage and other things that are more easily handled by Telus's own PGW.

Re: LTE network fundamentals

MVP — Fri, 20 Jan 2017 02:11:51 GMT

@sheytoon wrote:
@MVP
Negative SINR is a perfectly valid value, as it's a logarithmic scale. A value corresponding to zero SINR on a linear scale would be (negative infinity) dB.

As for throttling, I suspect there is some limit placed at the EPC (core). See pages 7 and 8 for more info on my tests:
http://productioncommunity.publicmobile.ca/t5/Discussions/Is-Public-Mobile-s-LTE-throttled-vs-Telus-Koodo-Who-here-has-the/td-p/80631/page/7

* Seems I lost the ability to link to direct posts with the new layout of the forums.

Thank you again , Sheytoon! yes of course negative SNIR is perfectly valid mathematically, but it would probably require a pretty tricky filtering to get a useful signal out of that. Also, we do not really know how exactly Network Signal Guru app mesures SINR, so the number may just be bogus...

The info you provide wrt throttling seems veryconvincing, however, i have a devil's advocate question: could it be possible that the speed difference/bottleneck occurs after EPC (e.g. related to the geographical/network locations of APNs vs testing servers)?