August 18, 2014, a new HetNet paper by Keith Mallinson, WiseHarbor

LTE-Advanced  is bringing more than just carrier aggregation in 3GPP Release 12

Carrier aggregation multiplies maximum user speeds several-fold and significantly improves capacity utilization by increasing trunking efficiency as more spectrum blocks are added; but LTE-Advanced can deliver so much more including heterogeneous network capabilities in particular.

Coordinating the low-power layer of small cells with the macro network improves performance across the entire network while also further boosting efficiencies in spectrum use and power consumption, automating network configuration and optimization. The upcoming 3GPP Release 12 (due to be frozen September 2014) standardises various capabilities in these developments including dual connectivity, small cell on/off and 256 QAM.

HetNet Dual Connectivity – simultaneous connection to the macro and low-power layer

Carrier aggregation takes precedence and grabs headlines

Most mobile operators have had sufficient spectrum resources to launch their new LTE services. Now, however, their priorities are shifting and they want to increase data speeds, network capacity and spectrum utilization with carrier aggregation on their macro networks of large cells. Allocations in recent years include new spectrum at 700 MHz in the U.S., as well as 800 MHz and 2.6 GHz elsewhere. Carriers are also refarming spectrum for LTE services in previously-used bands, including 1700 MHz AWS in the U.S., 1800 MHz and others elsewhere. Before long, however, these operators will also need to implement various other LTE Advanced features--as the cells are also densified and transformed into heterogeneous network architectures (HetNets) to increase coverage and capacity.

Carrier aggregation is one of several features in LTE-Advanced that lets mobile operators bond together disparate spectrum bands to add capacity and provide faster data rates in their networks. For example, in commercial LTE-FDD networks, downlink speeds of up to 225 Mbps have been achieved by aggregating 20 MHz and 10 MHz carriers in the 800 MHz and 1800MHz bands, and 300Mbps has been delivered by combining two 20 MHz carriers in 1800 MHz and 2.6 GHz bands. One trial has combined one 20 MHz block in the 1800 MHz band with two 20 MHz carriers in the 2.6 GHz band to demonstrate peak speeds of 450 Mbps. Commercial carrier aggregation plans include LTE-TDD at 2.5 GHz. 

The best things come in small packages

Macro networks can be extended through the creation of HetNets with small cells outdoors as carriers densify their networks with new sites and associated backhaul.

Operators are also continuing to focus significant amounts of attention on small cells and Distributed Antenna Systems for improving coverage and capacity on a highly targeted basis indoors. Offices, shopping malls and sports arenas are of particular interest. Consensus among speakers at the Small Cells World Summit in London this June was that around 50 per cent of mobile network usage is indoors nowadays.

Various dedicated small-cell vendors are pursuing opportunities to improve coverage and capacity indoors. So far, indoor coverage and capacity is usually added somewhat separately from the macrocellular networks with dedicated solutions including low-power small cells, uncoordinated femtocells, DAS and Wi-Fi. These offerings boast fast deployment and low-costs by using pre-existing Ethernet wiring, Internet connections, and an ability to support multiple operators, respectively. The latter two technologies are highly desirable in public places, where it is essential to support all operators, as opposed to focusing on only one operator which might be sufficient in some enterprise buildings. But DAS precludes implementation of various LTE-Advanced capabilities.

All these low-power solutions rely on isolation from the macro network to minimize interference. This can be achieved with thick walls, or by using different frequencies with loss of efficiency in spectrum reuse.

LTE-Advanced features can coordinate outdoor or indoor small cells with nearby macrocells. A speaker at the Small Cells Summit claimed that indoor cells are often not physically isolated, and that 50% of them interfere with adjacent outdoor cells. He claimed substantial gains of up to 11dB are possible by coordinating indoor cells with the nearest macro cells.

While indoor small cell vendors also seek to be compliant with 3GPP standards, operators will typically buy macro network equipment from more than one vendor but usually deploy a single macro-RAN vendor, for any given air-interface technology, on a region-by-region basis. They do this because it tends to maximize performance with latest network features, including proprietary enhancements, while simplifying implementation and network management. Coordination of macro and small-cell environments may tend to favour single-vendor or partnership supply because coordination requires common baseband processing or tight integration across these two domains. 

Release 12 features also target small cells and HetNets

3GPP Release 12 also improves upon and introduces various LTE-Advanced capabilities which increase performance among cells of all sizes. As network loading increases and with higher cell density, improving cell-edge performance has become a priority. This is being achieved with interference management techniques such as enhanced inter-cell interference coordination, or eICIC (Rel-10, March 2011). Further eICIC (Rel-11) and coordinated multipoint, or CoMP (Rel-11) are also being introduced. Carrier aggregation improvements and these capabilities were described in detail in my previous 3GPP paper on HetNets.

Enhancements included in Release 12 also include:

• Dual connectivity: Devices maintain simultaneous connections to both macro and small-cell low-power layers to improve cell-edge throughput. Inter-node radio resource aggregation can use radio resources on a common frequency in more than one eNB. In addition, connections can be anchored to a macro cell on one frequency while boosting data-rates via the small cell on a different frequency.
• Small cell on/off: This provides energy-efficient load balancing by turning off the low-power nodes when there is no ongoing demand for data transmission. More eNBs increases air interface interference and network power consumption. Making nodes dormant can match available capacity to network traffic loading.
• 256 QAM: Close proximity of devices to small cells enables use of higher-order modulation. This is most beneficial in sparse small-cell implementations with low device mobility.
• Various other capabilities with multi-antenna beamforming, and introduction of machine-type communications for the Internet of things and device-to-device communication for use in public safety applications. 

LTE continues to advance

LTE-Advanced has arrived with the first Rel-10 / Rel-11 implementations underway and Rel-12 coming soon. As ever, the buzz with the new radio technologies is primarily about dramatically-increased peak data speeds for end users. Carrier aggregation is the primary means for achieving this because it is easiest to implement. Other features will also improve speeds, all-round performance and capacity across the entire network including small cells and in emerging HetNet implementations where low-power small cells will be coordinated with the macro networks. These will be implemented as operators overcome the practical challenges in acquiring new sites and deploying additional backhaul for cell densification outdoors, and as HetNets extend indoors.