The demand for mobile broadband is growing rapidly in both mature and emerging markets. Nokia Siemens Networks (NSN) estimates 5 billion internet connections and a hundredfold increase in traffic on networks by 2015.

NSN’s Mobile Broadband Study 2010 reported a 40 per cent increase in the average monthly expenditure on mobile broadband in 2010 over the previous year.

According to the equipment provider, mobile internet traffic will increase to 23 EB by 2015, which implies that 6.3 billion people will be downloading a digital book every day. The increasing demand for internet services is driving mobile operators to invest significantly in additional spectrum. Operators are looking at unpaired spectrum as a complement to paired spectrum.

The catalyst for this is long term evolution (LTE), which was designed to harmonise networks for unpaired and paired spectrum. LTE is designed for network efficiency and constitutes a flat, all-IP architecture with an orthogonal frequency division multiplexing air interface. LTE and system architecture evolution (SAE) are standardised by the 3rd Generation Partnership Project (3GPP).

The LTE standard supports both frequency division duplex (FDD) (for paired spectrum) and time division duplex (TDD) (for unpaired spectrum) operation modes. LTE TDD enables global roaming in an otherwise fragmented spectrum landscape.

LTE TDD on the global stage

Historically, the global mobile industry has not favoured TDD technologies due to the voice-centric nature of networks and abundant availability of FDD spectrum.

Interest shown by major markets such as India, Russia, Japan and the US has put LTE TDD on every operator’s plan.

Europe’s major operators have typically deployed FDD networks but recent trends indicate that service providers are increasingly becoming interested in TDD bands. LTE TDD makes these bands an attractive asset with a more realistic pricing structure and the ability to deliver similar performance and coverage to the FDD version.

It is a futureproof technology with strong industry support from Next Generation Mobile Networks; the LTE SAE Trial Initiative; and terminal, chipset and infrastructure vendors.

Spectrum in LTE TDD

Global spectrum frequency bands that are likely to be used for LTE deployments are the cellular, personal communications service, advanced wireless services and digital dividend bands.

FDD-paired spectrum bands are expected to be the most common spectrum blocks. Besides, spectrum blocks that could be used in an unpaired way are also available, as LTE TDD is a 3GPP standard. Today, TDD spectrum bands allocated in most parts of the world are technology agnostic. The key TDD spectrum bands are 2.3 GHz and 2.6 GHz. However, in October 2010, 3GPP defined nine TDD bands for supporting LTE operations.

LTE TDD has been standardised in 3GPP Release 8. 3GPP Release 9 provided minor feature enhancements to the LTE standard. LTE evolves with 3GPP Rel. 10 to LTE Advanced in 3GPP Rel. 10. Like LTE FDD, LTE TDD supports scalable bandwidth. With the expected data traffic growth, higher bandwidths will be required to achieve optimum performance.

Status of LTE TDD

LTE is the next evolutionary step for mobile networks like GSM/EDGE, WCDMA/HSPA, CDMA/EVDO and Wi-Max. LTE is best suited to 3GPP and 3GPP2 operators, which makes it a global roaming technology. LTE TDD is also an evolutionary path for TD-SCDMA and Wi-Max networks as well as operators which have unpaired spectrum.

The technology also resolves issues like interworking, coexistence and roaming across different technologies. Moreover, it helps Wi-Max operators to integrate the large 3GPP ecosystem, and leverage the benefits of highest economies of scale, roaming and network sharing.

The shift from TD-SCDMA and Wi-Max to LTE TDD will be a gradual process, thereby making it important for the two technologies to coexist.

The global potential and the initial definition of higher-frequency bands have also made LTE TDD the ideal capacity option for communication service providers with LTE FDD deployments. Reuse of sites and equipment on large LTE TDD frequency allocations allows cost-efficient provision of data to consumers. Moreover, it eliminates congestion on the FDD band.

SAE is required in the core network architecture (defined by 3GPP) in order to avail of all advantages of LTE. SAE provides a flat, fully IP-based network architecture, consisting of only one node in the user plane of the core network, thereby guaranteeing optimal scalability and reduced cost per bit. Since a circuit switched network is not available on LTE TDD, voice services will be supported as VOIP.

Terminal ecosystems

Several leading vendors are developing terminals with LTE TDD capabilities. There have also been several announcements from chipset and platform vendors about the future availability of multi-mode LTE (FDD and TDD) offerings. Commercial-scale shipments of such devices are expected to be in sync with operators’ commercial service rollout requirements. s

Based on a white paper on LTE TDD by Nokia Siemens Networks