LTE-FDD and LTE-TDD for Cellular Communications (2012)

LTE-Advanced (Long Term Evolution-Advanced) is used on fourth generation (4G) in mobile phone technology as many providers are beginning to augment their networks with LTE. As known, mobile phone tra-c is divided into two parts: an uplink and a downlink. This paper presents the LTE two duplexing modes: LTE-TDD (Time Division Duplexing) and LTE-FDD (Frequency Division Duplexing). Where LTE-TDD favored by a majority of implementations because of ∞exibility in choosing uplink to downlink data rate ratios, ability to exploit channel reciprocity, ability to implement in non-paired spectrum and less complex transceiver design. In the case of FDD operation there are two carrier frequencies, one for uplink transmission (fUL) and one for downlink transmission ( fDL). During each frame, there are thus ten uplink subframes and ten downlink subframes, so uplink and downlink transmission can occur simultaneously within a cell. LTE-FDD implies that downlink and uplink transmission take place in difierent, su-ciently separated, frequency bands, while TDD implies that downlink and uplink transmission take place in difierent, non overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum. Also the required ∞exibility and resulting requirements to support LTE operation in difierent paired and unpaired frequency arrangements are discussed in this Paper. This paper focuses on the main difierence between LTE-FDD and LTE-TDD in how they divide the single channel to provide paths for both uploading (mobile transmit) and downloading (base-station transmit). FDD does this by dividing the frequency band allotted into two discrete smaller channels. TDD uses the entire channel but alternates between uploading and downloading and in the case of TDD uplink and downlink communication taking place in the same frequency band but in separate non-overlapping time slots; there is typically a high fading correlation between the downlink and uplink. With full coverage in the 3 GPP Release 8 speciflcations of both TDD and FDD modes of operation, LTE can efiectively be deployed in both the paired and unpaired spectrum. LTE TDD and FDD modes have been greatly harmonized in the sense that both modes share the same underlying framework, including radio access schemes OFDMA in downlink and SC-FDMA in uplink, basic subframe formats, conflguration protocols, etc.. As clear indication of the harmonization, the TDD mode is included together with the FDD mode in the same set of speciflcations, including the physical layer where there are just a few difierences due to the uplink/downlink switching operation. In terms of architecture there are no difierences between FDD and TDD and the very few difierences in the MAC and higher layer protocols relate to TDD speciflc physical layer parameters. Procedures are kept the same. Thus there will be high implementation synergies between the two modes allowing for e-cient support of both TDD and FDD in the same network or user device. Coexistence would of course still require careful analysis. Another key feature of the LTE-TDD mode (known also as TD-LTE) is the commonality with TD-SCDMA. In this paper, the detailed aspects of LTE-TDD that difier from the LTE-FDD mode are introduced. Further, information related to both the link and system performance of the LTE TDD mode of operation is given (1).