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LTE carrier aggregation requires careful consideration when valuing mobile spectrum

Janette Stewart Principal, Consulting
Mark Colville Principal, Consulting

"Using carrier aggregation can create problems for operators, which raises interesting questions when considering the incremental value of adding different frequency bands into a 4G network."

SpectrumCarrier aggregation (CA), a key feature of LTE-Advanced (LTE-A), enables carriers at multiple frequencies to be used together to provide improved data rates for users of 4G networks. However, these increased data rates may come at the cost of exacerbating coverage and capacity problems, depending on which carriers are being aggregated. This article reveals that operators will therefore need to take careful account of the benefits, but also the pitfalls, of carrier aggregation when valuing spectrum ahead of a licence acquisition.

CA enables operators to offer higher peak throughputs

The rapid growth in the levels of data traffic being carried by mobile broadband networks in recent years has resulted in mobile operators deploying various technologies to improve the throughput provided for mobile data transmission. For example, operators are migrating mobile broadband users from 3G to 4G networks in countries where LTE and LTE-A networks have been launched. However, customer demand and the need for a compelling service proposition are driving a need to provide continual improvement in data throughput and achieve higher peak downlink speeds in particular.

Most mobile operators own spectrum in multiple frequency bands (spanning sub-1GHz bands up to 2.6GHz and beyond), and they are turning to CA as a means of combining fragmented spectrum holdings to form wider carriers, which allow them to offer increased data rates. CA can take one of two forms:

  • intra-band – two or more carriers in a single band are aggregated
  • inter-band – combines carriers from different bands.

CA implementations are typically limited to combining two or three 10MHz or 20MHz carriers, although CA of up to five component carriers is envisaged within imminent releases of the 3GPP specifications for LTE-A. The CA configurations that an operator can deploy depend largely on its spectrum holdings (which could be in various bands, covering 700MHz, 800MHz, 850MHz, 900MHz, 1800MHz, 1900MHz, 2.1GHz, 2.3GHz and 2.6GHz based upon currently defined CA combinations). CA can also occur between different modes of 4G transmission – that is, between the time-division duplex (TDD) and frequency-division duplex (FDD) schemes used in LTE. By transmitting the aggregated carriers in parallel to or from the same mobile device, higher peak throughputs in the downlink, or in both the downlink and the uplink, can be achieved. Each of these options has its own merits, and in some cases different options can provide similar outcomes for operators. However, the real benefits of different approaches depend on what spectrum the operator uses to deploy CA.

CA offers benefits, but it can also create problems for operators

As noted above, the theoretical objective of combining carriers together using CA is to enable higher data throughputs to be provided (either in the downlink only, or in both downlink and uplink directions). However, the benefits of deploying CA depend on the extent to which CA helps operators improve the coverage of higher-speed services across wider cell areas, beyond the central area of a base station (where speeds are typically highest).1 The coverage improvement for higher-speed services that CA can offer is closely linked to the spectrum that an operator has available, and the bands chosen for CA deployment.

Two scenarios are emerging for CA – CA that combines low and high 4G bands (for example, 800MHz with 2.6GHz, or 700MHz with AWS2 ), and CA that combines mid- and high-band 4G spectrum (for example, 1800MHz and 2.6GHz). In both cases, CA provides benefits by improving the peak speeds available to compatible devices within the coverage area of the two bands. However, the combinations of bands that are more closely matched in frequency (for example, mid- and high-band spectrum) offer a better synergy for operators from a coverage perspective. In particular, there is likely to be less coverage mismatch between mid- and high-band spectrum than there is between low- and high-band spectrum. This can be highly beneficial for operators because CA can be used in this scenario as an effective means of overcoming so-called 'cell edge' problems in 4G networks (when users located in the outermost areas of a cell receive a lower speed).

By contrast, when low- and high-band spectrum is combined, there is a greater risk of coverage mismatch occurring, such that the higher throughput provided by the combined carriers is only available in practice within the coverage area of the higher band (see Figure 1).

Figure 1: Coverage area mismatch in LTE-A CA

Figure 1: Coverage area mismatch in LTE-A CA

CA has implications for both operators and regulators when valuing mobile spectrum licences

The implications of coverage mismatch occurring when using different CA combinations is that the higher throughputs that the CA provides will only be available in certain areas of the network, leaving pockets of network coverage where there is no CA. This not only increases the potential differential between the 'best' and 'worst' speed provided to users depending on where they are located, but also potentially further lowers the capacity available to users in areas where a lower strength of signal is being delivered (that is, towards the cell edge). This is because the available capacity within a low frequency band (for example, 800MHz) may be exhausted by users closer to the centre of the cell receiving higher speeds through use of the 800MHz in a CA carrier combination. This mismatch in user experience is a potential downside of using CA that operators need to carefully manage.

In practice, overcoming this coverage mismatch relies on mobile operators deploying careful traffic and load balancing within their networks, such that the valuable low-frequency spectrum (which propagates further) can be reserved for users who require the extra range that this spectrum can provide. Alternatively, operators might naturally favour different CA combinations that allow for more closely spaced frequencies within their 4G portfolios to be combined.

For operators, and for regulators, these considerations raise interesting questions when considering the incremental value of adding different frequency bands into a 4G network. For operators, it is clearly important to value potential spectrum licence assets on the basis of the real benefits that they can provide to the business. Regulators worldwide are considering various additional bands to meet the growing needs for mobile data traffic, and therefore need to carefully assess the network coverage and capacity benefits offered. The availability of high-speed data is becoming the foundation of better user experience within 4G networks, and Analysys Mason's view is that using CA to achieve this may depend on which carriers different operators are aggregating.

Analysys Mason has helped to shape spectrum policy around the world with our wide-ranging studies helping regulators and operators to develop spectrum strategy, determine spectrum policy, formulate spectrum licence conditions, value spectrum and prepare for spectrum awards. For more information, please contact Janette Stewart, Principal, at

1  Due to the physical properties of radio waves and the design of the LTE standard, users typically receive higher speed services when located closer to the base station, or while using devices within cell areas where the density of other simultaneous use is low.

2  Advanced wireless services, referring to spectrum in the 1.7GHz–2.1GHz bands used in several countries, including the USA.