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Why TDD is the smarter way to deploy Broadband Services

Why TDD is the smarter way to deploy Broadband Services?

With the advent of B3G** technologies, the older method of deploying networks with paired FDD spectrum loses it charm. In the post-Voice centric era of network expansion to capture a market share of data hungry devices, it becomes very essential to squeeze the most out of spectrum. 4G systems are all IP system, but the RF allocations follow the voice-centric approach of earlier generations. While LTE standards allow for either Frequency Division Duplexing (FDD) or Time Division Duplexing (TDD), all initial LTE equipment uses FDD. FDD requires two separate blocks of spectrum—one for each direction. FDD makes perfect sense for bi-directional voice traffic. It makes no sense for data. With the exception of peer-to-peer file sharing, data traffic is very asymmetric. Sending data via FDD means one block of spectrum is fully utilized and the other, equal sized block, is dramatically underutilized, which is an inefficient way for capacity planning.

LTE and TD-Ecosystem

The LTE ecosystem supports both FDD and TDD operation. Fifteen paired (FDD) and eight unpaired (TDD) spectrum bands have already been identified by the 3GPP for LTE. This means an operator can introduce LTE in new spectrum bands, where it is easiest to deploy 10 MHz or 20 MHz to carriers and eventually deploy LTE in all bands.

Operators can launch LTE to match their existing networks, spectrum and business objectives for mobile broadband and multimedia services. LTE in FDD spectrum bands are being deployed in the US by Verizon and AT&T. Verizon is launching LTE in the 700 MHz spectrum and is among the first in the world to launch LTE, starting with 25 to 30 markets in 2010, covering approximately 100M people; and extending to cover its current 3G footprint in 2013. Additionally, there will be TD-LTE to be deployed by operators in fragmented TDD spectrum bands by Greenfield operators like Clearwire who have a lot of available 2.5 GHz spectrum available.

LTE ecosystem will be deployed in vast economies of scale from being a 3GPP technical specification that will be a combined LTE FDD & TDD standard. TD-LTE will ensure high-speed mobile broadband connectivity across a wide range of end-user devices and applications in networks with unpaired frequency bands. Another key benefit of TD-LTE is the 3GPP evolutionary approach from TD-SCDMA to LTE, which will increase the overall LTE ecosystem and scale by including a seamless integrated option for TD-SDMA operators such as China Mobile to migrate to TD-LTE.

TD-LTE devices must provide compatibility with legacy 3GPP systems, and a series of handover scenarios are specified to ensure conformance. These aim to ensure service continuity for the user, and check everything from idle mode and in-call intra-frequency TDD-TDD handovers, through inter-frequency changes and TDD-FDD handovers, handovers to 3G W-CDMA and HPSA systems, and finally to handing over from TDD to GSM.  3GPP has been successful in fulfilling its goal to achieve a single radio-access specification equally applicable to paired and unpaired spectrum. From a specification perspective, differences between FDD and TDD mode are on the physical layer and, particularly the frame structure. The differences found on higher layers are limited and are related to configurability of the physical layer and slightly different timing relations due to the discontinuous nature of uplink and downlink.

TDD and Interference

To avoid severe interference between uplink and downlink transmissions despite the fact that the two links use the same frequency, the cells in a TDD network typically use the same uplink downlink configuration together with inter-cell synchronization to a common time reference to align the switch-points among all the cells. This avoids interference between the two links as uplink and downlink transmissions do not occur at the same time. This is especially important in macro deployments with antennas placed above rooftops with possible line-of-sight-like propagation conditions between base station antennas. In this case base station-to-base station interference may otherwise severely degrade uplink reception of base stations.

Due to the propagation delay, a downlink transmission from a distant base station is still propagating at the base station trying to receive uplink transmissions even though all base stations switched from downlink to uplink at the same time. This causes interference at the base station at the beginning of the uplink period. Even though it is expected to be highly scenario dependent, it may be noted that for base stations with elevated antennas and little downtilt. This will require interference mitigation techniques like downtilts, adding filters at transmitters and receiver, use adaptive antennas, Implement transmitter power control and utilize antennas with low side lobes.

Generally TDD has worse coverage for a given data rate, due to the inherent discontinuous uplink transmission and the fact that both FDD and TDD terminal have the same uplink transmission. This is true for any TDD system; however, for LTE, the subframe structure for both FDD and TDD are the same and the users are scheduled on a subframe basis.

TD-LTE Spectrum

Around the world several spectrum bands have been allocated to support TD-LTE. Many countries throughout the world have TDD spectrum available and it is expected that these spectrums will trade at a much lower price per MHz/population than their FDD equivalents. Most likely 2.3GHz and 2.6GHz will be the most prominent TD spectrum bands used for TD-LTE.

Advantages of TDD over FDD:

Real-time adaptation provides highest transport efficiency

  • Millisecond real-time adaptation
  • 35% improvement over FDD/TDMA

TDD enables 100% use of available spectrum

  • Well-suited for wide, single block allocations and narrow, dual block allocations
  • Minimal latency variation enables prioritization of preferred subscribers and critical applications

Spectral Efficiency:

  • Minimize guard band
  • Change symmetry on the fly depending on subscriber’s needs
  • Adaptive downstream/upstream ratio allows for emerging new applications without the need for spectrum re-farming
  • Enables advanced technologies such as mesh network and adaptive antenna arrays
  • Highly effective for bursty data traffic while still supporting voice

In the U.S., the Broadband Radio Service (BRS) and the Educational Broadband Service (EBS) are in the 2496-2690 MHz band. While this band was previously known as Microwave Multipoint Distribution Service (MMDS), the band plan was recently rearranged and some additional rules changes were adopted.

While a network operator could gain enough spectrum to create a pairing and thus make the use of FDD a possibility, this approach has not happened yet. Even if an FDD approach was used, it would likely require equipment and devices with small economies of scale. Currently, most systems in the BRS and EBS bands have been based on TDD (e.g. UMTS TDD & WiMAX). As LTE becomes more prevalent in the marketplace and TD-LTE equipment becomes available, it will be an option for use in the BRS and EBS bands.

Future of TD-Ecosystem

Clearwire paved the way for LTE in US when they submitted a proposal to adopt the 2496MHz-to-2690MHz frequency band in the US for TD-LTE, and it was accepted by 3GPP meeting earlier this year. The acceptance is significant because it will enable Clearwire and other spectrum holders to deploy TD-LTE, which is the time division duplex (TDD) version of LTE, in the US. Clearwire was not alone in asking for the 2.6GHz spectrum to be defined as a TDD band for LTE. Indeed, there was broad industry support for the proposal from other companies, including: Sprint Nextel Corp. , NII Holdings Inc. , China Mobile Communications Corp. , UK Broadband Ltd. , Motorola Inc. Huawei Technologies Co. Ltd. , TD Tech Ltd. , WiChorus Inc. , ZTE Corp. , Chinese Academy of Telecommunications Technology , Nokia Siemens Networks , Cisco Systems Inc., Sequans Communications , Alcatel-Lucent , Alcatel Shanghai Bell Co. Ltd. , Rohde & Schwarz GmbH & Co. KG and not to forget – Qualcomm which has won a chunk of spectrum in India, where it plans to deploy TD-LTE.

** B3G is a terminology for Beyond 3G technologies and has been coined by Martin Sauter – http://mobilesociety.typepad.com/mobile_life/

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