Collocated FDD/TD-LTE raises interference issues

Phil Marshall/Tolaga Research
The global market for LTE is taking hold with vengeance. Current LTE initiatives span 76 countries, where there are 21 commercial networks launched, 23 pre-commercial networks, and 35 active trials already initiated in 2011.
 
The lion’s share of LTE deployments use the FDD-LTE variant, which requires paired radio spectrum for uplink and downlink connectivity. China Mobile has fueled global interest in TD-LTE by adopting it as the upgrade technology for TD-SCDMA. TD-LTE is well suited for unpaired radio spectrum in the 2.3 and 2.5GHz bands, with early deployments and trials in markets outside of China, including Sweden, India, Malaysia, Germany and USA. With growing support, TD-LTE has a burgeoning ecosystem, which is being driven by players including Alcatel-Lucent, Ericsson, Huawei, Nokia Siemens Networks, Qualcomm, and ZTE.
 
As service providers deploy TD-LTE, they must pay close attention to the coordination between FDD and TDD systems. Since TDD radios both transmit and receive in the same spectrum band, excessive interference issues can be created amongst the transmitted and received signals of collocated TDD/FDD systems. To avoid interference, TDD and FDD systems can either be separated physically or with adequate spectrum guard bands.
 
In Sweden, Hi3G has deployed a collocated TDD/FDD system and demonstrated that while collocation can be achieved, it requires careful interference management. Hi3G benefits from effectively having guard-bands in excess of 10 MHz between its TDD and FDD systems. We believe that collocated TDD/FDD systems will experience excessive interference in cases where there is less than 10-MHz guard-band separation.
 
This will add constraints to future infrastructure collocation and sharing arrangements, and complicate network consolidation in cases where TDD and FDD networks are involved. It may also cause conflict between competing service providers in cases where interference occurs (or is believed to occur) between adjacent TDD and FDD systems.
 
Service providers globally are progressively embracing TD-LTE, and recognize its potential role in bolstering network capacity. For example, in a recent German spectrum auction, the TDD and FDD licenses in the 2.5-GHz band fetched comparable prices. This contrasts earlier auctions that saw FDD licenses carrying a significant price premium relative to TDD licenses.
 
As TD-LTE takes hold, we anticipate a variety of strategies will emerge to ensure adequate coordination between TDD and FDD systems. Strategies beyond traditional interference management might include:
· Using TD-LTE for underlay micro and pico cells and FDD-LTE for macro-cells
· Coordinated spectrum allocations that allow for shifting guard bands in anticipation of future network deployment scenarios
· Introducing intelligent interference management schemes capable of simultaneously managing FDD and TDD systems.
 
Phil Marshall, PhD is Chief Research Officer at Tolaga Research. For more information contact: philip.marshall@tolaga.com or go to www.tolaga.com/

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