LTE-LAA updates - Jan 2015

There has been quite a few updates on LTE LAA in the past month. First, there is this good video from Ericsson explaining what it is:

The 3GPP chairman recently presented a status update about developments on unlicensed spectrum at an IEEE 802 meeting last month. His presentation is embedded below:

3GPP & unlicensed spectrum from Zahid Ghadialy

IEEE 802 group have their own presentation on the co-existence lessons learned. This is embedded below:

Ericsson has also got a recent presentation on this topic. As can be seen, they expect a solution to be available in 2016-17. Presentation embedded below (download link here)

Finally, if you watched the video by Ericsson, they mention that one of the key milestones of 5G is to be able to combine licensed and unlicensed technology. One of the technologies being proposed in 5G is called Multi-Stream Aggregation (MSA). MSA allows multiple access technologies over licensed and unlicensed bands effectively. The picture above shows how it would work in theory. It may be more difficult in practise though. 

Wi-Fi and Cellular: Authentication & Security

While in the past WiFi was considered not as secure as the cellular counterpart, things have been changing slowly and steadily. While the cellular is able to offer authentication using the EPS-AKA, Wi-Fi is now able to offer EAP-AKA and EAP-SIM. 

In fact the above table is quite interesting to know about. Recently I was following a discussion that talked about the lack of QoS support in WiFi. As we can see, its supported but not guaranteed. 

Devices that contain the USIM card can use EAP-AKA while that without one can offer EAP-SIM. EAP-AKA works in a similar way to the cellular authentication protocol EPS-AKA. For anyone who is interested in reading more details about the authentication and how it works, including signalling, see this whitepaper here.

If you haven't seen our whitepaper on Cellular and WiFi integration, please check it out here.

3GPP Definitions of Small Cells

While the Small Cell Forum defines the different types of Small cells clearly and these Small Cells can be said to contain the complete/partial functionality of the eNodeB, 3GPP definitions of Small Cells can be a bit fuzzy sometimes.

Generally, in the 3GPP documentation, there is a reference to Femtocells and Picocells. Femtocells are Small Cells that are defined as Closed Access (see my old post here) by 3GPP. The open access small cells are referred to as Picocells. Sometimes remote radio heads (RRH's) are also referred to as Small cells, open access type.

Relays, even though not referred to as Small Cells by 3GPP, is also referred to as Small Cells by some people.

Do you know of anything else?

X2: The necessity for Interoperability and Interference management in HetNets

Recently I wrote an article in the TMN magazine here about HetNet co-ordination. One important point that I mentioned here (and in several of the trainings that I do) is that X2 can be quite a useful interface, especially when you want to manage Interference between just macro cells or between different types of cells, as in HetNet environment. From the TMN article:

The initial deployments of LTE did not pay too much emphasis on X2 interface being present. Inter-operability was another issue for which X2 didn’t work very well. One of the major benefits of having an X2 interface is that different base stations or eNodeB’s (hereafter referred to as just eNB’s) can talk to each other and coordinate to make sure interference is kept at the minimum, especially on the cell edges. The Macro-cell’s had the provision for the X2 interface from the beginning, which formed the basis for Inter-Cell Interference Coordination (ICIC). Initially, the Small-cell’s didn’t have a facility for an X2 interface amongst them or with the Macro-cell. The support of X2 for Small-cell’s was added in 3GPP Release-10 and 3GPP Relese-11 (hereafter referred to as just Release-10 and Release-11) as seen in the picture. X2 for Small-cell’s is crucial for Interference management in HetNets.

The Small Cell Forum just published a whitepaper called "X2 interoperability in multi-vendor X2 HetNets". This paper is quite interesting with just the right amount of details. In their own words:

A key area of standardization needed to support small cells relates to the procedures to assist co-ordination with macrocells over LTE’s ‘X2’ interface. This document surveys standards currently in place to support frequency domain interference coordination,time domain interference coordination, mobility robustness and mobility load balancing. 

The whitepaper is available to download from here and is embedded below:

Small Cell Forum Release 4: X2 Interoperability from Zahid Ghadialy

Small Cells for the 'Connected Car'

Couple of weeks back I was in an event where Connected cars were a big focus. A few discussions centred around Small Cells in the cars. It may be a bit of a challenge but it should still be possible to have Small cells in the cars. The biggest challenge would be the backhaul. You cant have the standard backhaul for cars, especially as its moving, generally at high speeds. 

Some tricky solutions where one of the frequencies is used for backhauling small cells while small cells would provide coverage to the passengers of the car may be doable but it may not be worth the effort. 

Generally, the focus right now is to have something like a MiFi device in the car. The device can receive the mobile network signals and create a Wi-Fi hotspot.

Another solution being discussed was the use of Mobile Relay Node (MRN). As far as I understand, MRN has been pushed out of Release-12. Another issue is that the practical gain may not be as good as expected. Most of all, small cells or relay would only be useful if all the passengers in the vehicle reply on the same mobile network operator. As far as I have seen, this is generally not the case.

In light of this, it would make sense to continue on the current solution of having Wi-Fi hotspots in the cars backhauled to the mobile network.

Your thoughts please.

Open, Closed and Hybrid Access Small Cells

A question that often keeps cropping up regularly is regarding the open access and closed access small cells. Before we look at the explanation of these cells, lets see the different types of cells (from 3GPP TS 36.304) in brief:

Acceptable cell: An "acceptable cell" is a cell on which the UE may camp to obtain limited service (originate emergency calls and receive ETWS and CMAS notifications).

Suitable cell: A "suitable cell" is a cell on which the UE may camp on to obtain normal service. It is mandatory for the UE to have a USIM card belonging to the operator to which this cell belongs.

Barred cell: A cell is "barred" if it is so indicated in the system information. Its not available for use by anyone.

Reserved cell: A cell is reserved if it is so indicated in system information. It is reserved for operator use only.

Restricted Cell: A cell on which camping is allowed, but access attempts are disallowed for UEs whose access classes are indicated as barred.

Camping on the cell: With the cell selection, the UE searches for a suitable cell of the selected PLMN and chooses that cell to provide available services, further the UE shall tune to its control channel. This choosing is known as "camping on the cell".

To keep the discussion simple, I have ignored that some UE's may belong to MVNO and the PLMN Id of the operator would be stored as Equivalent PLMN Id.

Now lets look at a simple explanation of the different types of small cells.

Open Access: All (suitable) cells are open access by default. This means that they can be accessed by any device belonging to the operator whom the cell belongs to. Some people also refer to these cells as Open Subscriber Group (OSG) cells.

Closed Access: A (suitable) cell is closed when only certain devices can camp on them. These devices form a part of whitelist stored in a database to allow camping on the cell. Devices that are not part of the whitelist are not allowed to camp on this cell, even though they belong to the same operator and this cell is a suitable cell. The devices are said to belong to ‘Closed Subscriber Group’ (CSG). The cell is said to be a CSG cell as its transmitting CSG Indication set to 'true' and the CSG Identity.

Residential Femtocells are generally Closed Access but there are exceptions. Softbank, Japan for example gives open access Femtocells that can also provide coverage to users nearby. Another example is the operator Free in France that also offers similar open access Femtocells.

Hybrid Access: A (suitable) cell can also be hybrid access, thereby allowing all devices that either belong to a CSG or non-CSG to camp onto it. A hybrid cell may offer prioritised and/or additional services to the users that belongs to the CSG it is a part of.

I am not aware of any Hybrid Access Small Cells deployment to date. Please feel free to correct me.

Operator plans for the ultra-dense network

The following is from Rethink Wireless article last month:

Large-scale deployments of public access small cells are still in their infancy, but there is already talk of 'hyper-dense' networks to cope with hotspots of intense data usage. Most of this remains just talk, but Qualcomm - on the rampage in metrocells after a hesitant start- is showing off how the approach might work in reality. 

The chip giant, never averse to a bold demonstration, is claiming the densest network ever constructed in a working environment, equating to 1,000 cells per square kilometer (a neat figure given that Qualcomm's ongoing marketing campaign revolves around the '1,000x Data Challenge', predicting an increase of that magnitude over the coming decade). 

It has put the trial together for Sprint's TDD technology, working with Airspan, the WiMAX specialist that has evolved into a small cell vendor with heavy emphasis on self-organization and integrated backhaul.

We recently heard from Caroline Gabriel in our Cambridge Wireless Small Cells SIG this (last) week. This very interesting presentation below is from that event. A very important slide is the tools that are available for achieving this ultra-dense networks. Anyway, presentation as follows:

Operator plans for the ultra-dense network from Zahid Ghadialy

Small Cells and Wi-Fi in 3GPP Releases

Here is a quick chart of features of Small Cells and Wi-Fi introduced by 3GPP in different releases. Please feel free to suggest anything that has been missed out.