Dynamic Spectrum Access (DSA) techniques for Small Cells and Wi-Fi

Picture Source: Analysis Mason

There is a lot of spectrum which is used sparingly or is kept reserved for unlicensed or shared access. Any party that wishes and is allowed to use this spectrum has to co-ordinate with the license holders or others in similar situations. Hence we have different access mechanisms which are collectively called as Dynamic Spectrum Access (DSA) techniques.

An article by Analysis Mason on this topic suggests the following:

The term DSA has come to encompass a number of different approaches and techniques that aim to increase the utilisation of the radio frequency spectrum. At its most ambitious, it is hypothesised that cognitive and software-defined radios could intelligently choose when to transmit, so as to avoid other radio transmissions and also to avoid causing undue interference to fellow frequency users. Short-term propositions include near-real-time spectrum assignment in certain bands and greater use of long-term secondary spectrum leasing to authorised spectrum partners.

DSA is therefore all about making better use of radio spectrum through re-use of 'idle' bandwidth, being either frequencies that are not used in all locations, in which other systems could be deployed, or frequencies that are only used intermittently, and which could therefore be re-used outside these times. These 'gaps' in utilisation, which provide opportunities for DSA, arise for a number of reasons.

• Coverage: a licence holder might not be using its allotted licence in a specific region.
• Time: an area of spectrum might by less-frequently required at different times during a day (or on longer timescales).
• Lack of service users: there may be a limited number of subscribers taking advantage of a service.
• Licence technical parameters: the regulator may have mandated that a piece of spectrum can only be used for a specific purpose, while other technologies emerge during the life of a licence that can use the same spectrum.
• Pragmatic under-utilisation to prevent interference: empty guard bands are placed between spectrum bands to stop transmission leakage to prevent interference, which could be re-used by systems that have the appropriate characteristics to avoid interference.

One of the overarching drivers for DSA is to help overcome spectrum shortages – particularly noting that under-utilised bands may exist across a relatively wide range of the spectrum. Even in the economies where wireless communications have developed the most and usage restrictions have been removed, thus making spectrum use as flexible as possible, spectrum under-utilisation is still considered to be widespread.

The Cisco vision on the other hand seems far too optimistic and suggests the following:

TV White Spaces (TVWS) are spectrum allocated to TV broadcasts, but not being used in a given geographic location. TVWS radios allow for use of white space spectrum for unlicensed wireless access.

Authorized Shared Access (ASA) or Licensed Shared Access (LSA) allow a secondary licensee to use the “shared” spectrum when the primary licensee is not using it.

The United States Federal Communications Commission (FCC) has proposed a three-tier model for shared access in the 3.5-GHz band. Tier 1 would be for incumbent federal agencies, including military radar users. Tier 2 would be authorized prioritized access similar to ASA and LSA. Tier 3 would be generalized authorized access, which is similar to unlicensed access.

A number of DSA technologies already exist or are in exploration.

Geo-location, database-based spectrum sharing techniques have the most traction as a practical approach to spectrum sharing. Devices that want to use shared spectrum must geo-locate themselves and consult a database to determine what spectrum is available.

The geo-location database manages the spectrum resource allocation based on predefined policies and availability to ensure the primary licensee is not impacted. An enhanced version of the geo-location database system—called a Spectrum Access System (SAS)—is the basis for the FCC spectrum-sharing proposal in the 3.5-GHz band.

A second technology is cognitive radio, which senses and monitors the radio environment. This includes knowing the location and policies for self-regulation. Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) are cognitive radio techniques that allow co-existence with radar and satellite systems.

Another technology is Software-Defined Radio (SDR), which allows devices to adapt to local radio conditions and use the appropriate radio frequencies.

I came across this very interesting whitepaper by MIT that details all the DSA techniques and its progress. Paper embedded below:

New Models for Protected Shared Access - Toward More Efficient Spectrum Management from Zahid Ghadialy

We will discuss in the future post how the DSA techniques could be useful for using Small Cells in the unlicensed spectrum (a.k.a. LTE-U).

Small Cells as a Service (SCaaS) using NFV

If you have not read my earlier post on SCaaS, then check this out. If you have no idea what Network Function Virtualization (NFV) is, check this out.

In a recent presentation in CW, Interdigital suggested that they are working on some ideas where they can virtualize the small cells network in such a way to have multi-tenancy (multiple operators sharing pool of small cells resources). Here is the complete presentation embedded. Let me know what you think.

Interdigital: Virtualization – You Know More Than You Realize! from Zahid Ghadialy

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.

Small Cells Research Bible

Here is a detailed research on Small Cells from Mehdi Bennis and Walid Saad presented in IEEE Dyspan 2014, this month. Feel free to add any comments for questions you may have.

Recent Advances in Wireless Small Cell Networks from Zahid Ghadialy

AT&T: Small Cells advertisement and Cell On Wheels (COW)

Interesting Small Cells advert from AT&T:

In the recent Boston marathon on the weekend, they also used their COW (Cell on Wheels) to boost the service. Here is a picture from their official tweet account. To know more, check their tweet out below.

Gearing up with a COW (Cell on Wheels) to boost service at the @BostonMarathon. #itsourboston http://t.co/FaiNQizpup pic.twitter.com/3bWHo5RChG
— AT&T (@ATT) April 19, 2014

You can also see whats inside the COWs in the video below:

Unlocking Small Cells for the Enterprise - EE

Another excellent presentation by the UK operator EE in our Cambridge Wireless SIG. There was a discussion about the different options during deployment, which options are more preferable than others, etc.

Another point worth mentioning is the potential of Small Cells in emergency and disaster situations. We have seen similar deployments happening in Japan during the tsunami that occurred a few years back. The complete presentations as follows:

EE: Unlocking small cells for the enterprise from Zahid Ghadialy

Related links:

• Summary of CW Small Cell SIG event 'Stretching the Limits' by ThinkSmallCell
• Operator plans for the ultra-dense network by Caroline Gabriel
• LTE Deployment Dilemma - Quiz

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

Smart ANDSF by NSN

I have blogged about Access Network Discovery and Selection Function (ANDSF) in great detail on the 3G4G blog here. Earlier this week I was hearing from NSN about their Smart ANDSF solution which is the standard ANDSF with enhancements to handle unpredictable load conditions. Here is their presentation from the webinar.

NSN: Grow Capacity from Zahid Ghadialy

HetNet and LTE Trends and Challenges

Metrocells and C-RAN has been a frequent area of discussion on this blog. A good post about Metrocells is available here and about C-RAN is available here and here.

I found a small and useful presentation in my collection that highlights the trends, opportunities and challenges with the HetNets. The presentation is embedded below:

HetNet and LTE trends from 3G4G Blogs

3G / 4G Small Cells Mobility Scenarios - 3GPP Technical Report

3GPP has an excellent Technical Report (TR) 37.803 that covers different mobility scenarios and enhancements for 3G HNB and 4G HeNB. Its an interesting read if you are involved in this activity. Embedded below for reference.

Mobility enhancements for Home Node B (HNB) and Home enhanced Node B (HeNB) from Zahid Ghadialy