Mobile networks are groaning under the heavyweight expectations of our modern mobile lifestyles. Running hard to stay ahead of demand for capacity and coverage, many mobile service providers are finding that adding small cells to an existing LTE (News - Alert) network can be an economical and efficient way to ease the strain on the network. But relief for coverage and capacity comes with challenges in signaling.

Small Cells impact EPC signaling volume

Adding small cells to an existing LTE network can have a significant impact on the signaling volume that hits the evolved packet cor. The equation isn’t simple though. The impact varies depending on many factors, including:

• the number and type of small cells added;
• how densely concentrated those small cells are;
• whether the small cells are directly connected to the EPC or are connected through a small cells gateway; and
• whether their access mode is open, closed, or hybrid.?

Adding a small cell gateway to aggregate traffic from the small cells to the EPC is one design option to avoid some of the signaling increase. But when that is not practical and the small cells are directly connected to the EPC, the volume of signaling – especially on the mobility management entity – becomes a performance concern that demands attention.

Signaling strain on the MME

So what is going on with the MME? The MME is the control plane node in the EPC. It is responsible for:

• security procedures, which include user authentication;
• device to network session management, which involves establishing packet data contexts and QoS parameters for service requests ; and
• idle mode location management, which involves paging and tracking area management procedures.
      

As more cells (both macro and small cells) are added the signaling on the MME also increases. Studies conducted by Alcatel-Lucent (News - Alert) of dense urban areas identified as the top three contributors to MME signaling.  The bulk of this signaling is for service requests. These are procedures to set up and terminate the use of services on our mobile devices such as checking e-mail or Facebook (News - Alert), web browsing, uploading a photo or placing a call. Reducing service request signaling is challenging and requires coordination between mobile device manufacturers, application developers, and equipment vendors. Discontinuous reception/transmission and adjusting UE activity timer are examples that can help to reduce this signaling.

The second and third highest signaling volume on the MME is for the idle mode paging and tracking area management procedures. Consider the impact of the small cell on the signaling volume due to paging and tracking area updates. While a handful of small cells added to the network will have little difference to the signaling load, the numbers needed to support the service demands in dense urban areas can have a significant impact.

Striking a balance between increasing capacity and coverage with small cells and increasing signaling on the MME is not as difficult as it may seem. The best option is to reduce the volume of signaling rather than to overbuild network capacity to handle it. There are idle mode features available that can reduce paging and tracking area update signaling as well as extend mobile device battery life.
 

Turn down the signaling volume with smart paging

Paging can be reduced in small cell deployments by using Closed Subscriber Group paging optimization procedures. CSG paging optimization uses closed mode cell access and closed subscriber group memberships to limit paging. Small cells that have closed access and CSG membership are typically deployed in residential (home) and some enterprises, and it’s impractical to include them in a large paging broadcast. When the MME is directly connected to small cells, paging is initiated only to the closed mode cells that have the tracking area and the CSG ID in the UE’s list.
 

Using CSG paging optimizations does not preclude the use of other paging methods used in macrocell networks that page based on service type including voice, SMS, data, or quality of service class identifier. By combining these methods, mobile service providers can reduce the overall paging volume in any deployment scenario across both their LTE macro networks and small cell networks.
 

Optimize tracking area management

There are still more signaling savings to be had in expanding mobile networks, particularly in the optimization of tracking area management.

Tracking area management updates are triggered when the mobile device crosses the edge of one tracking area into another that’s not in its current TA list and so must notify the MME that it’s in a different area. TAU signaling can increase if the device moves along the edges of two or more traffic areas and constantly is updating the MME of its location. When a large number of small cells are added they may require a new, separate tracking area or may split the existing tracking area. While a well-planned tracking area network design can alleviate much of this signaling, there are traffic area management capabilities that can reduce the signaling associated with traffic area updates. These methods detect the movement of mobile devices between traffic areas, and the MME automatically updates the devices TA list to minimize updates.

Mobile service providers with an eye on cost control are highly motivated to optimize paging and tracking area management. Simply by reducing the signaling volume for these procedures, they can support more subscribers on their existing networks and defer additional control plane capital investment. Reducing signaling on the mobile device also extends its battery life.

That’s an economical way to boost the network for those heavyweight expectations of modern mobile lifestyles.

Dave Nowoswiat is senior manager of product and solutions marketing of the IP routing and transport division at Alcatel-Lucent (www.alcatel-lucent.com).