LTE battery life

In the grand scheme of things LTE is at its infancy, but it is interesting to look at how battery life will be impacted by LTE. Information sources on the subject are rare, but I did come across the following figures on the Samsung website for the Galaxy SII LTE.

As you can see LTE talk time is not mentioned as the current voice solution of LTE is fallback to 3G or 2G. More on this subject can be found on a previous post here.

LTE standby time is mentioned and as you can see it is worse than 2G and 3G by a considerable margin. The actual figures are pretty irrelevant as manufacturers measure these assuming the largest idle mode DRX cycle possible which you will never find in a real network.

The relative difference however, almost 30% reduction in battery life, indicates that the idle mode tasks (listening to the paging channel, measure serving cell and neighbour cells, etc) of the LTE part of the baseband and RF chain require a lot more power that the 2G/3G ones. I expect this will improve over time, but at least in the near term early adopters of LTE smartphones/tablets should have a charger handy.

The full specifications of the Samsung Galaxy SII LTE, including the extract above can be found here.

Nokia Lumia 900 Dual Cell smartphone and much more!

Nokia's recently announced Lumia 900 got good reviews in terms of its design, ease of use, Windows OS etc, but for me this phone represents a truly feature packed device from a radio point of view.

Having a look at its specifications we can see it supports the 3GPP release 8 HSPA+ Dual Carrier/Cell feature with a theoretical DL speed of 42Mbps. Even though there have been a few DC smartphones out already, these have been launched in the US and to my knowledge this is the first European DC smartphone.

Furthermore the Nokia Lumia 900 also has Rx diversity meaning that it has two receive antennas and can combine the energy received from both of them resulting in a much better Eb/N0. This will mean that the gap between the theoretical and real performance of HSPA will be smaller than a traditional single antenna device.

Naturally the Lumia 900 also supports HSPA+ 64QAM single carrier with a theoretical top speed of 21Mbps.

In addition to all of the above the Lumia 900 also supports DTM (Dual Transfer Mode) which allows the simultaneous data transfer of speech and data in 2G, HSUPA category 6, WB-AMR (a.k.a. HD Voice) and all this is powered by Qualcomm's MDM9200 baseband processor.

A truly radio feature packed phone which makes the iPhone4S and even the more recent Samsung Galaxy SIII look poor in comparison.

LTE efficiency @ 10MHz

A recent 4G throughput test campaign from PC World in the US, produced the graph above. The whole report can be found here but the interesting point to note is that essentially it is a comparisson between the efficiency of LTE Vs HSPA+ using the same bandwidth. Allow me to explain..

Both AT&T and Verizon are currently operating LTE networks using 10MHz of spectrum in the 700MHz band. T-Mobile on the other hand is operating an HSPA+ network using the Dual Cell/Carrier feature which it labels 4G (or fauxG if you are a purist). As the Dual Cell/Carrier feature essentially aggregates two 5MHz carriers the total amount of spectrum is the same (i.e. 10MHz).

Looking at the results it is clear that LTE performs better than HSPA+ but the difference is not that big. Both technologies are using 10MHz of spectrum and capable of 64QAM. LTE has the advantage of MIMO which in certain radio conditions can double the data rate.

The difference between T-Mobile and AT&T is 3.6Mbps and the difference between T-Mobile and Verizon is 1.8Mbps.

But what about the difference between the theoretical peak and the actual average rate? LTE @ 10MHz should deliver 73Mbps. In the tests the best network delivered 9.12Mbps. That is 12.5% of the max.

HSPA+ Dual Cell/Carrier should deliver a theoretical peak of 42Mbps. In the tests T-Mobile achieved 5.53Mbps. That is 13.2% of the max, which is a little bit better than LTE. We also need to remember that T-Mobile's HSPA+ network is a mature network with a lot of existing traffic. As LTE devices become more popular we can expect the LTE networks to slow down due to resource sharing.

One last point to mention, is that it is possible to combine Dual Cell/Carrier with MIMO. Although T-Mobile doesn't offer this capability it would have been interesting to see how close the results would have been. Maybe even HSPA+ would out-perform LTE?!

LTE Default Bearer concept

If you have started looking into LTE you will not doubt soon come against the concept of the default bearer. Even though in the begining I was a bit confused about it, it turns out it is quite simple to understand it, especially if you have some background in UMTS or GPRS as it is very similar to a Primary PDP context.

The picture above highlights the differences in terminology quite well, and as you can see besides the naming conventions the concepts are identical.

In UTMS/GPRS, the UE requests the establishment of a PDP Context. This creates a logical end to end "pipe" between the UE and the GGSN. The process of establishing a PDP context also assigns the UE an IP address. Once a primary PDP context is established, secondary PDP contexts can also be established which use the same IP address but require different QoS. Even though these secondary PDP contexts have been defined in the specs since the beginning of GPRS, they were never really implemented to my knowledge.

In LTE, the UE requests the establishment of a PDN Connection. This creates a logical end to end "pipe" between the UE and the PGW. The UE is also assigned an IP address (can be IPv4 or IPv6) and the default bearer is setup. This default bearer is always best effort. Unlike UMTS/GPRS where the UE can request a PDP context at any time, in LTE the PDN connection for the establishment of the default bearer is always requested at power on. This ensures that the UE always has an IP address which in a packet based system like LTE is a necessity. If now the UE requires some QoS that is different than best effort, a dedicated bearer can be setup. This will be a necessity when things like voice services are offered over LTE but could also be used when for example a streaming session is setup, or a Skype/Facetime session etc.

Finally if you are wondering how will the network know that a dedicated bearer is needed, this is done by deep packet inspection, most likely by the PCRF (Policy and Charging Rules Function) node.

LTE worlds apart

Back in 2009, Telia in Sweden launched the first commercial LTE network. Three years down the line, looking at their website there is a little mention of some mobile broadband packages and that is about it. Looking at some other European LTE operators, things don't get much more exciting.

Jump over the pond to the USA and things are much different. LTE 4G, is plastered all over Verizon's and AT&Ts websites, there are flames, direct comparissons of download speeds between the two operators, commercials about LTE, smartphones, tablets, mi-fi, iPad, the lot!

Sure, most of it is just commercial hype, but LTE needed this American style push desperately as the European marketing style was very boring. Now it seems it has started to trickle through as a few European operators have started announcing LTE capable smartphones. Let's see how things develop..

Power consumption in different RRC states

A few posts ago I looked at the current drain of 3G Vs 2G during a voice call. For this post I thought it would be interesting to quantify the current drain of a UE in different RRC states. This helps explain the need for fast dormancy, DRX optimisation and managing RRC state transitions from a UE battery perspective.

Looking at the graph (again obtained using Nokia's energy profiler app), we start off in RRC IDLE with the current drain at approx 60mA. The web browser is launched and the UE transitions to RRC DCH via the RRC Connection Setup procedure. A PDP context is requested and following the authentication procedure the UE is assigned an HSPA radio bearer. Current drain is approx 260mA. The actual web page requested was the Google homepage so the data transfer is completed very quickly. From there on the RRC inactivity timer on the RNC is initiated and after 5s the UE is transitioned to RRC FACH. The current drain is now approx 150mA. The reduction is due to the fact that in CELL_FACH the UE can switch off its transmitter. It still has to keep its receiver on however as the RNC can schedule a data transfer at any time.

On this network the timer to transition from FACH to URA_PCH is 30s, which is quite long. This clearly shows that sometimes operators can have netwotk settings that are not very battery friendly. If the UE used fast dormancy (the specific one in question did not), the UE could have released the RRC connection and moved back to IDLE. As it didn't, after the 30s of inactivity the UE is instructed to move to RRC PCH (URA_PCH on this network). As the DRX timer is configured at the same value as the IDLE DRX timer (on this specific network), the current drain is the same at approx 60mA.

73Mbps LTE for new iPad?

I was watching the Apple presentation last week and I found it a bit odd that Philip Schiller quoted an LTE maximum speed of 73Mbps for the new iPad. It is common knowledge that Qualcomm produce category 3 LTE chipsets at the moment which support a theoretical maximum speed of 100Mbps, so why 73Mbps?

Well it turns out it is related to the messy spectrum situation in the U.S. and the fact that the two biggest carriers that will launch the new iPad (Verizon and AT&T), only have 10MHz blocks for LTE at the moment.

LTE allows for flexible carrier allocation (up to 20MHz) and the number of Resource Blocks is tied to the amount of spectrum. With a 10MHz block, 50 Resource Blocks are available.

Looking at 3GPP spec 36.213, we find a table that indicates the maximum transport block size for a given number of resource blocks per 1ms TTI.

If we look at the column for 50 Resource Blocks we find that the maximum TB is 36696 bits.

So 36696 x 1000 = 36696000 bits per second. As LTE allows for 2x2 MIMO, this is doubled to 73392000 bits per second or 73Mbps, which is the figure Apple quoted.

Of course this is a theoretical maximum speed, which requires perfect RF conditions, and the user using all the resources in the cell. Both of course are quite unlikely (unless in a lab) so real speeds can vary.