Heterogeneous networks, in which small cells are most tightly-coordinated with large cells, are vital to accommodate escalating data traffic demands. HetNets, including LTE Advanced technologies standardised in 3GPP Releases 10 and 11, spread traffic loads optimally, maintain performance and service quality at cell edges by managing radio interference, while reusing spectrum most efficiently.
Some frustrations at the recent Consumer Electronics Show in Las Vegas illustrate the enormous challenges in providing fast, consistent and reliable mobile communications services in densely-populated places. The BBC’s popular technology correspondent and gadget geek, Rory Cellan-Jones, highlighted this in a wrap-up radio broadcast describing his visit to the show this year. He was wowed by the many new products and credited smartphone developments with the emerging phenomenon of “wearables,” for example; but was also bemused by his connectivity problems there. With significant communications needs as a journalist who also prolifically tweets, blogs and presents TV features, he revealed his persistent anxieties about whether or not he would be able to connect satisfactorily while in competition with 100,000 or so others attending. And yet, CES is the epicentre for advanced mobile communications with abundant fibre, distributed antenna systems and small cells with Wi-Fi and cellular, including LTE. There is plenty of motivation and money to make things work there, but serving high demand remains a struggle. CES is not unique. Sports venues and transportation hubs provide similar demands, as do many other places where people live closely or gather en-mass. Adoption and usage of smart devices, including new and bandwidth-intensive services, is escalating with data traffic up 80 per cent in the year to September 2013 and set to continue increasing at similar rates for many years, as indicated in Ericsson’s quarterly mobility reports. The need for network upgrades increases correspondingly.
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More is still not enough
Until now, operators have met network demand growth with new radio spectrum, simple cell splitting, adding multi-antenna techniques, implementing more efficient modulation and coding schemes. Spectrum is scarce: it is generally very costly, and in some cases additional frequencies may not be available any time soon. Cell splitting “densifies” an existing macro-network of large cells, while maintaining it as a homogeneous network by adding more radio sectors per eNB base station or deploying more macro eNBs. However, reducing the site-to-site distance in the macro-network can only be pursued to a certain extent because implementing new macro-sites becomes increasingly difficult and can be expensive, especially in city centres, due to planning constraints and high real estate costs.
The alternative is to introduce small cells through addition of low-power base stations (e.g. LTE’s eNBs, HeNBs or Relay Nodes or Remote Radio Heads to existing macro-eNBs). Site acquisition is easier and cheaper with this equipment, which is also correspondingly smaller. Small cells are primarily added to increase capacity in hot-spots with high user demand and to fill in areas not covered by the macro network–both outdoors and indoors. They also improve network performance and service quality by offloading from the large macro-cells. The result is a heterogeneous network with large macro-cells in combination with small cells providing increased bitrates per unit area.
Densification by adding small cells
Figure 1: Illustration of a heterogeneous network, with large and small cells.
Small cells have been in widespread use for more than five years, including femtocells providing coverage at home; but these can either be rather inefficient in spectrum reuse or cause interference with the macro network. Heterogeneous network planning was even used in GSM; with which the large and small cells are separated through the use of different frequencies. This solution is still possible in LTE, but it is inefficient in use of radio spectrum. LTE networks mainly use a frequency reuse of one to maximize utilization of the licensed bandwidth. It is most desirable to preserve this efficiency while creating HetNets.
New antenna, modulation and coding technologies alone are also insufficient in the most crowded environments and at cell edges where performance can significantly degrade. Operators are therefore also adding new kinds of small cells that are more tightly-coordinated with their macro networks. These spread traffic loads optimally, widely maintain performance and service quality while reusing spectrum most efficiently.
Enter LTE Advanced
While LTE is successfully providing faster services at lower cost in new spectrum, LTE Advanced takes things further by maximising capacity in hot-spots, improving coverage and service quality–particularly at the cell edges. LTE Advanced, with features standardised in 3GPP Releases 10 and 11 optimises the coexistence of large and small cells by coordinating them. These HetNets squeeze as much as possible out of the scarce spectrum resources by enabling frequency reuse of one with minimal interference. They can mitigate the high interference levels at cell edges, and across entire small cells in the case of femtocells serving only closed subscriber groups.
Innovative new techniques included in Releases 10 and 11 include:
- Relay Nodes. Each of these is connected to an eNB. The RN provides improved coverage, the capacity for which is provided via the eNB connection. From an eNB perspective, RNs appear to be User Equipment. From a UE perspective, RN’s appear to be eNBs. RNs provide overall improvements in coverage and capacity.
- Cell Range Extension. This increases the use of the small cells by effectively expanding them. This is achieved by determining cell boundaries and handover on the basis of uplink path loss rather than downlink signal strength. A negative effect of this is the increased interference on the downlink experienced by the UE located in the CRE region and served by the small cell base station. This and other interference effects can be mitigated by the following technique.
- Enhanced Inter-Cell Interference Coordination. ICIC was introduced in Release 8 to mitigate inter-cell interference for UEs at the cell edge. The eNBs communicate using ICIC via the X2 interface between eNBs to optimize traffic scheduling for UEs at cell edges. With eICIC, interference is especially controlled in downlink control channels through use of Almost Blank Subframes. Power is reduced by including only control channels and cell-specific reference signals, and no user data.
- Carrier aggregation with cross-carrier scheduling. CA increases the total bandwidth available to UEs and hence their maximum bitrates. With cross-carrier scheduling it is possible to map the physical downlink control channels onto different component radio carriers in the large and small cells. PDCCH may be transmitted with higher power than the traffic channels. Consequently, using different carriers for the PDCCH in the large and small cells reduces PDCCH interference.
- Coordinated multipoint. Macro-cells and small cells can both be involved in data transmission to and from one UE. For example, data can be transmitted at the same time from more than one transmission point to one UE, or data can be received from one transmission point in one subframe and from another transmission point in the next subframe.
I have coauthored a paper for 3GPP which provides detailed descriptions of the above and can be accessed via its site.
Release 12, set to be completed in 2014, will further improve HetNet and hyper-dense network capabilities by enhancing some of the above and by adding new features such dual connectivity with large and small cells, and techniques specifically to improve TD-LTE performance.
As a leading implementer of LTE, standardised in Releases 8 and 9, with around 90 per cent of the U.S population already covered, AT&T is eager to upgrade its networks to include these LTE Advanced capabilities. Home femtocell deployments aside, it plans to deploy 40,000 LTE Advanced small cells by the end of 2015. These will be predominantly indoors, at least initially, to provide improved capacity and coverage. It is also experimenting outdoors by placing the radios on lamp posts, telephone poles and elsewhere. AT&T’s priority is to employ the techniques described above in particular locations where capacity is currently most constrained. This crucially depends upon its local spectrum holdings, including 700 MHz and the 1700 MHz AWS band where it has it. In other words, these techniques are, unsurprisingly, needed most where demands are high and spectrum holdings are short.