LTE Release 12: Small Cell Enhancements – Higher Layer Aspects

One of the 3GPP Rel-12 Study Items focuses on Small Cell Enhancements.from the perspective of higher layers. There are 3 scenarios under consideration:

Scenario 1:  Macro and small cells on the same carrier frequency, connected via non-ideal backhaul.

Scenario 2: Macro and small cells on different carrier frequencies, connected via non-ideal backhaul.

Scenario 3: Only small cells on one or more carrier frequencies, connected via non-ideal backhaul.

The challenges posed by these scenarios are summarized in the table below.


To address these challenges, 3GPP’s RAN Working Group 2, is currently working towards a set of solutions, with the following design goals in mind:


The idea of ‘dual connectivity’ has more or less been accepted as the way forward. This term refers to operation where a given UE consumes radio resources provided by at least two different network points connected with non-ideal backhaul. Each eNB involved in dual connectivity for a UE may assume different roles and these roles do not necessarily depend on the eNB’s power class and can vary among UEs.

Within the dual-connectivity framework, there are two potential solutions being discussed:

Inter-node radio resource aggregation: This solution is aimed at improving per-user throughput by aggregating radio resources in more than one eNB for user plane data transmission.ImageRRC Diversity: This solution is targeted at improving mobility robustness by transmitting handover related RRC signalling could from/to a potential target cell. The RRC diversity scheme could also be applied for handovers from the macro to pico cells, between macro or between pico cells.


Further details of these solutions can be found in TR36.842.

LTE Release-12 and Beyond

While current LTE deployments are mostly based on Rel. 8/9, work is ongoing in 3GPP on Release 12. The July issue of IEEE Communications Magazine has a nice article which provides an overview of the key features being developed in Release 12. It can be viewed online via the IEEE Digital Library at

Carrier Aggregation Bands

Carrier Aggregation (CA) is one of the key technologies in LTE-Advanced (Rel. 10 and beyond). In simple terms, CA allows multiple radio channels to be bonded together to create bigger bandwidth pipes. The channels may belong to the same or different spectrum bands. Release 10 supports both intra-band contiguous and inter-band CA in the FDD downlink but only intra-band CA for TDD downlink. Only intra-band CA is supported in both FDD & TDD uplink. More details regarding CA are available in the following Agilent presentation.


5G – The new buzzword

Now that the hype over 4G has somewhat reduced, 5G is slowly becoming the new thing. Lots of claims are being thrown around by vendors and silicon companies, staking claim to having achieved to having found the new holy grail, aka 5G. On one hand we have WiFi chip manufacturers referring to 802.11ac as ‘5G WiFi’ (ever heard of 3G/4G WiFi?) while there are the likes of Samsung who has tagged a new mm-wave based transmission scheme as 5G.

Real work on defining 5G (or the nearest equivalent) requirements has just started in ITU Working Party 5D. If IMT-Advanced was the equivalent of 4G, IMT-2020 technologies are expected to be branded as 5G. At the next meeting of 5D to be held in a few weeks time, various organizations will present their vision of 5G, in terms of technical requirements. Hence, the flurry of whitepapers from vendors:

5G radio access – research and vision (Ericsson)

On the road to 5G (NSN)

Various global telecom standards bodies like 3GPP, IEEE have been asked by ITU to submit their inputs for IMT-2020 requirements. Within 3GPP, there’s an ITU-R Ad-Hoc Group which is currently discussing this topic and preparing an input (Liaison Statement in 3GPP jargon). More details about that in the next blog.

Solution 3: RAN-controlled Network Selection and Traffic Steering

The key difference with respect to Solution 1 is that the 3GPP RAN decides when a UE should connect to the WLAN and how traffic should be steered to/from 3GPP network. The figure below illustrates the process.

Picture1As in Solution 1, the UE transfers its WLAN interworking capability to RAN at the time of connection setup. The following procedure is followed afterwards:

  • RAN configures a measurement for WLAN. It may provide measurement object & reporting configuration if UE supports WLAN interworking and RAN wants to interwork with WLAN
  • After receiving measurement configuration, the UE measures WLAN. If there is a WLAN that satisfies the reporting configuration, the UE reports the measured result to RAN.
  • If RAN finds that offloading is necessary  & the reported WLAN is appropriate, it transmits offloading command to steer traffic from RAN to WLAN. Alternatively, if RAN finds that the WLAN is not appropriate for offloading any more, it transmits reverse offloading command.
  • When the UE receives the forward offloading command, it connects to the indicated WLAN and continues data transmission and reception with it. If bearer info is also received from RAN, UE continues data transmission/reception of the bearer with WLAN
  • When a reverse offloading command is received by the UE, it continues data transmission/reception with RAN instead of using WLAN.

Solution 1: RAN-assisted Network Selection and Traffic Steering

This solution relies on the RAN providing relevant information that may be used by a UE to decide:

1. If and when to connect/disconnect to/from an available WLAN

2. Steer one or more data flows from/to 3GPP RAN to/from WLAN

The UE is assumed to receive network selection and traffic steering rules and/or polices either via ANDSF or user preferences or operator provisioning. The solution can be applied in both RCC_IDLE and RRC_CONNECTED modes. More specifically, the figure below, taken from doc R2-132055, illustrates the procedure when UE is idle mode.

ImageThe steps involved in this procedure are as follows:

  • When the UE establishes RRC connection with 3GPP RAN, it transfers interworking capability info to the network
  • If the UE has indicated support for WLAN interworking and 3GPP RAN also wants to interwork with WLAN, the latter may provide assistance info to UE via dedicated signalling at the time of RRC connection release.
  • In addition, RAN may broadcast System Information which includes assistance info. If UE has valid assistance information provided through dedicated signalling previously, it ignores the related broadcast information.
  • UE selects the access network based on information provided by RAN, information obtained from WLAN and rules/policies/preferences.

Note that no traffic steering happens in this case as the UE is in IDLE mode. The procedure is similar when the UE is in CONNECTED mode, as shown below.Image

  • As in the previous case, the UE transfers its interworking capability information to RAN at the time of RRC Connection establishment.
  • While the UE is connected to RAN, it receives assistance information from RAN through dedicated signalling, if the UE supports WLAN interworking and RAN wants to interwork with WLAN.
  • UE selects the access network based on the assistance info provided by RAN, acquired information from WLAN and rules.

In both cases, the assistance information may include the following:

  • 3GPP Network load
  • Resources allocation for UE
  • WLAN thresholds (e.g. RSSI)
  • RAN thresholds (e.g. RSRP)