International telecommunication union

1 Introduction

consumers and corporate/business users increase over the next few years. The TTD technology thus provides operators with an opportunity to be able to deploy sufficient capacity and capability in order to increase their ARPU (Average revenue per user) through offering bandwidth-hungry and asymmetric data services.

TDD is based on the concept of transmit and receive on the same frequency which means that both uplink and downlink channels experience more or less the same radio channel conditions. This reciprocity in the uplink and downlink can be used to the best advantage to introduce new and innovative techniques where most of the intensive signal processing can be carried out at the base station or the user terminal and the base station will be able to utilize information from the channel to the best advantage of the system. These techniques (described in the following sections) can be used to improve both coverage and capacity.

TDD is also cost-efficient for network deployment as it can leverage the infrastructure of an FDD-only roll-out to offer scalable capacity for “hotspots” where combined voice and data traffic will be supported through a multi-tier architecture of macro-, micro- and picocells.

Overall, TDD offers a platform for systems beyond IMT 2000. This is further described in the following section.

2 Enhanced TDD – A key platform for systems beyond IMT 2000

2.1 High data rate support

TDD technology is enhancing towards higher and higher data rate support. This is in line with the increasingly data centric usage projected. Higher order modulations combined with fast link adaptation will provide adaptive modulation and coding techniques that reduce the signal to noise ratio requirements and allow a more efficient data communications, in effect increasing system data transport capacity. In addition, improved diversity techniques and smart antenna techniques will help support high rate data traffic. Furthermore, techniques such as hybrid ARQ and its variations will make data transmission more efficient from the air interface point of view. These features and others are being introduced to allow TDD support ever-increasing data rates.

TDD capability to provide both time as well as code multiplexing, make it more attractive for high speed access schemes, because it allows more flexible and efficient use of physical channel resources, which consent to better integration of different service types (i.e. voice, data, etc.). This in turn provides higher capacity as well as higher spectral efficiency.

In addition, the inherent reciprocity in uplink and downlink in TDD would make it more feasible to deploy techniques such as smart antennas and diversity techniques to be able to support higher data rate traffic.

2.2 Higher spectral efficiency

In addition to the signal processing techniques and modulation schemes mentioned earlier, very efficient resource allocation algorithms make TDD spectrally efficient, especially when supporting asymmetrical data such as for Internet related services. The so called slow and fast dynamic channel allocation algorithms make sure that resource units are allocated optimally for uplink and downlink transmission.

2.3 Improved cell planning and coverage

TDD provides a second “dimension” in cell planning when it is deployed together with FDD systems. When addressing hotspots, splitting cells, and serving data centric usage areas, TDD and FDD systems could be laid out in a way that takes advantage of the strengths of each. When doing the cell planning, FDD and TDD coverage areas can be treated almost independent and provide the operator with a new plane of coverage maps, given the coexistence practices are followed.

2.4 Flexible and IP data centric deployment

As data rates demanded become higher and higher, cell sizes shrink and evolve towards micro and picocells. TDD technology can also be offered in picocell format and address high data rate users in support of IP-centric applications requiring high bandwidth. With TDD, the architecture of base stations and user terminals are planned to take full advantage of multi user detection algorithms, diversity and antenna processing techniques required for such environments.

2.5 Integrated multi-mode offerings

Dynamics of the standards bodies and their activities so far indicate that more than one standard is expected to emerge as systems beyond IMT 2000 with multiple evolution paths. TDD technology will be offered as part of the multiple-solution products that support more than one standard. Inherent features of the enhanced TDD offer the opportunity for cost-effective common architecture alongside other systems beyond IMT 2000.

3 TDD current and emerging system innovation

TDD offers an excellent opportunity for future systems innovation through its inherent features. One of its key features is the ability to transmit and receive on the same frequency thus allowing innovative techniques to take advantage of the reciprocity of the uplink and downlink channels. Key examples of innovative techniques that are being considered are as follows.

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