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LTE FDD and TDD related

LTE FDD and TDD related


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  • 1. Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 1467 LTE-FDD and LTE-TDD for Cellular Communications A. Z. Yonis1 , M. F. L. Abdullah1 , and M. F. Ghanim2 1 Faculty of Electrical and Electronic Engineering, Department of Communication Engineering University of Tun Hussein Onn Malaysia, Johor, Malaysia 2 Computer Engineering Department, College of Engineering, University of Mosul, Mosul, Iraq Abstract— 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 traffic 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 flexibility 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 (fU L ) 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 different, sufficiently separated, frequency bands, while TDD implies that downlink and uplink transmission take place in different, non overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum. Also the required flexibility and resulting requirements to support LTE operation in different paired and unpaired frequency arrangements are discussed in this Paper. This paper focuses on the main difference 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.1. INTRODUCTIONWith full coverage in the 3 GPP Release 8 specifications of both TDD and FDD modes of operation,LTE can effectively be deployed in both the paired and unpaired spectrum. LTE TDD and FDDmodes have been greatly harmonized in the sense that both modes share the same underlyingframework, including radio access schemes OFDMA in downlink and SC-FDMA in uplink, basicsubframe formats, configuration protocols, etc.. As clear indication of the harmonization, theTDD mode is included together with the FDD mode in the same set of specifications, includingthe physical layer where there are just a few differences due to the uplink/downlink switchingoperation. In terms of architecture there are no differences between FDD and TDD and thevery few differences in the MAC and higher layer protocols relate to TDD specific physical layerparameters. Procedures are kept the same. Thus there will be high implementation synergiesbetween the two modes allowing for efficient support of both TDD and FDD in the same networkor user device. Coexistence would of course still require careful analysis. Another key feature ofthe LTE-TDD mode (known also as TD-LTE) is the commonality with TD-SCDMA. In this paper,the detailed aspects of LTE-TDD that differ from the LTE-FDD mode are introduced. Further,information related to both the link and system performance of the LTE TDD mode of operationis given [1].2. SPECTRUM FLEXIBILITYA high degree of spectrum flexibility is the main characteristic of the LTE radio-access technology.The aim of this spectrum flexibility is to allow for the deployment of LTE radio access in differencefrequency bands with different characteristics, including different duplex arrangements and differentsizes of the available spectrum [2].2.1. Flexibility in Duplex ArrangementOne important part of the LTE requirements in terms of spectrum flexibility is the possibility todeploy LTE-based radio access in both paired and unpaired spectrum. Therefore, LTE supports
  • 2. 1468 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012both frequency- and time-division-based duplex arrangements. FDD as illustrated on the leftin Figure 1, implies that downlink and uplink transmission take place in different, sufficientlyseparated, frequency bands. TDD as illustrated on the right in Figure 1 implies that downlink anduplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate inunpaired spectrum, whereas FDD requires paired spectrum [1]. Operation in both paired and unpaired spectrum has been supported by 3GPP radio-accesstechnologies even before the introduction of LTE by means of FDD-based WCDMA/HSPA incombination with TDD-based TD-SCDMA radio. However, this was then achieved by means of,at least in the details, relatively different radio-access technologies leading to additional effort andcomplexity when developing and implementing dual-mode terminals capable of both FDD andTDD operation. LTE, on the other hand, supports both FDD and TDD within a single radio-access technology, leading to a minimum of deviation between FDD and TDD for LTE-based radioaccess. In the case of differences between FDD and TDD, these differences will be explicitly indicated.Furthermore, the TDD mode, also known as TD-LTE, is designed with coexistence between TD-LTE and TD-SCDMA in mind to simplify a gradual migration from TD-SCDMA to TD-LTE. LTE also supports half-duplex FDD at the terminal (illustrated in the middle of Figure 1). Inhalf-duplex FDD, transmission and reception at a specific terminal are separated in both frequencyand time. The base station still uses full-duplex FDD as it simultaneously may schedule differentterminals in uplink and downlink; this is similar to, for example, GSM operation. The main benefitwith half-duplex FDD is the reduced terminal complexity as no duplex filter is needed in theterminal. This is especially beneficial in the case of multi-band terminals which otherwise wouldneed multiple sets of duplex filters.3. DUPLEX SCHEMESSpectrum flexibility is one of the key features of LTE. In addition to the flexibility in transmissionbandwidth, LTE also supports operation in both paired and unpaired spectrum by supporting bothFDD- and TDD-based duplex operation with the time–frequency structures illustrated in Figure 2. Although the time-domain structure is, in most respects, the same for FDD and TDD, there aresome differences, most notably the presence of a special subframe in the case of TDD. The specialsubframe is used to provide the necessary guard time for downlink–uplink switching.3.1. Frequency-division Duplex (FDD)In the case of FDD operation (upper part of Figure 2), there are two carrier frequencies, one foruplink transmission (fU L ) and one for downlink transmission (fDL ). During each frame, there Figure 1: Frequency and time-division duplex [1]. Figure 2: Uplink/downlink time-frequency structure for FDD and TDD [2].
  • 3. Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 1469 Figure 3: Guard time at the terminal for half duplex FDD [2].are thus ten uplink subframes and ten downlink subframes, and uplink and downlink transmissioncan occur simultaneously within a cell [3]. Isolation between downlink and uplink transmissions isachieved by transmission/reception filters, known as duplex filters, and a sufficiently large duplexseparation in the frequency domain. Even if uplink and downlink transmission can occur simul-taneously within a cell in the case of FDD operation, a terminal may be capable of full-duplexoperation or only half-duplex operation for a certain frequency band, depending on whether or notit is capable of simultaneous transmission/reception. In the case of full-duplex capability, transmis-sion and reception may also occur simultaneously at a terminal, whereas a terminal capable of onlyhalf-duplex operation cannot transmit and receive simultaneously. Supporting only half-duplexoperation allows for simplified terminal implementation due to relaxed duplex-filter requirements.This applies especially for certain frequency bands with a narrow duplex gap. Hence, full duplex support is frequency-band dependent such that a terminal may support onlyhalf-duplex operation in certain frequency bands while being capable of full-duplex operation inthe remaining supported bands. It should be noted that full/half-duplex capability is a propertyof the terminal; the base station is operating in full duplex irrespective of the terminal capabilities.Hence, as the relevant transmission structures and timing relations are identical between full-duplexand half-duplex FDD, a single cell may simultaneously support a mixture of full-duplex and half-duplex FDD terminals. Half-duplex operation has an impact on the sustained data rates that canbe provided to/from a single mobile terminal as it cannot transmit in all uplink subframes, but thecell capacity is hardly affected as typically it is possible to schedule different terminals in uplinkand downlink in a given subframe. Since a half-duplex terminal is not capable of simultaneoustransmission and reception, the scheduling decisions must take this into account and half-duplexoperation can be seen as a scheduling restriction. If a terminal is scheduled such that downlinkreception in one subframe immediately precedes a subframe of uplink transmission, a guard time isnecessary for the terminal to switch from reception to transmission. This is created in such casesby allowing the terminal to skip receiving the last OFDM symbol(s) in the downlink subframe, asillustrated in Figure 3.3.2. Time-division Duplex (TDD)In the case of TDD operation (Upper part of Figure 2), there is a single carrier frequency onlyand uplink and downlink transmissions are separated in the time domain on a cell basis [4]. Asseen in the figure, some subframes are allocated for uplink transmissions and some subframesfor downlink transmission, with the switch between downlink and uplink occurring in the specialsubframe (subframe 1 and, in some cases, subframe 6). Like FDD, LTE TDD supports bandwidths from 1.4 MHz up to 20 MHz but depending on thefrequency band, the number of supported bandwidths may be less than the full range. For example,for the 2.5 GHz band, it is not likely that the smallest bandwidths will be supported. Since thebandwidth is shared between uplink and downlink and the maximum bandwidth is specified to be20 MHz in Release 8, the maximum achievable data rates are lower than in LTE FDD. This way thesame receiver and transmitter processing capability can be used with both TDD and FDD modesenabling faster deployment of LTE. The TDD system can be implemented on an unpaired band (or in two paired bands separately)while the FDD system always requires a pair of bands with a reasonable separation between uplinkand downlink directions, known as the duplex separation. In a FDD UE implementation thisnormally requires a duplex filter when simultaneous transmission and reception is facilitated. Ina TDD system the UE does not need such a duplex filter. The complexity of the duplex filterincreases when the uplink and downlink frequency bands are placed in closer proximity. In some ofthe future spectrum allocations it is foreseen that it will be easier to find new unpaired allocationsthan paired allocations with sensible duplex separation thereby increasing further the scope ofapplicability for TDD.
  • 4. 1470 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012 However, since uplink and downlink share the same frequency band, the signals in these twotransmission directions can interfere with each other. This is illustrated in Figure 4, with the use ofTDD on the same frequency without coordination and synchronization between sites in the samecoverage area. For uncoordinated deployment (unsynchronized) on the same frequency band, the devices con-nected to the cells with different timing and/or different uplink/downlink allocation may causeblocking for other users. In LTE TDD the base stations need to be synchronized to each other at Table 1: Comparison between FDD-LTE and TDD-LTE. FDD-LTE TDD-LTE Uses Frequency-Division Duplex Uses Time-Division Duplex Generally better suited for applications Is better at reallocating traffic than like voice calls that have symmetric FDD-LTE such as Internet or other data traffic, because traffic in both directions is centric services. always constant. It requires paired spectrum with different Does not require paired spectrum since frequencies with guard band. transmit and receive occurs in the same channel Is appears when planning sites for base With TDD, special considerations need to stations. Because FDD base stations use be taken in order to prevent neighboring different frequencies for receiving and base stations from interfering with each transmitting, they effectively do not hear other. each other and no special planning is needed. Allows for easier planning than TDD It is cheaper than FD LTE since in LTE. TDD-LTE no need of duplexer to isolate transmission and receptions. FDD LTE is full duplex this means that TDD LTE is half duplex as either upload both the upload and download are always or download can use the channel but not available. at the same time. With FDD, the bandwidth cannot be TDD can allocate more time for the part dynamically reallocated and the unused that requires more bandwidth, thereby bandwidth is wasted. balancing the load FDD-LTE every downlink subframe can TD-LTE the number of downlink and be associated with an uplink subframe uplink subframes is different and such association is not possible. An FDD system uses a duplexer and/or In TDD, both the transmitter and receiver two antennas that require spatial operate on the same frequency but at separation and, therefore, cannot reuse the different times. Therefore, TDD systems resources. The result is more costly reuse the filters, mixers, frequency hardware [5]. sources and synthesizers, thereby eliminating the complexity and costs associated with isolating the transmit antenna and the receive antenna. FDD cannot be used in environments TDD utilizes the spectrum more where the service provider does not have efficiently than FDD. enough bandwidth to provide the required guard-band between transmit and receive channels. It is requires two interference-free It is requires only one interference-free channels. channel.
  • 5. Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 1471 Figure 4: Interference from uplink to downlink in uncoordinated TDD operation.frame level in the same coverage area to avoid this interference. This can be typically done by us-ing, for example, satellite based solutions like GPS or Galileo or by having another external timingreference shared by the LTE TDD base stations within the same coverage area. LTE FDD doesnot need the base station synchronization. There is no interference between uplink and downlinkin FDD due to the duplex separation of the carriers.4. SUMMARY AND COMPARISONThe two versions of LTE are very similar. In fact, they differ only in the physical layer and, asa result, the version implemented is transparent to the higher layers. This means that UEs willbe able to support both TDD-LTE and FDD-LTE with one chipset with only minor modificationsrequired. The Table 1 shows the main comparison between FDD-LTE and TDD-LTE.5. CONCLUSIONThe uplink coverage with respect to a specific data rate in TDD-LTE is generally worse than FDD-LTE due to the fact that the uplink transmission is not continuous. The percentage of coveragefor control and data channels is, however, very similar to that of FDD-LTE. In terms of spectrumefficiency, the performances of TDD-LTE and FDD-LTE are similar for non-delay sensitive traffic.The lower performance of TDD-LTE is due to the guard periods mentioned above. Overall, TDD-LTE offers operators a great alternative to FDD. Its natural suitability for asymmetric applications,low latency, high throughput, and security make it a flexible and cost-effective solution for the nextgeneration wireless networks. TDD is more flexible than FDD in meeting the need to dynamicallyreconfigure the allocated upstream and downstream bandwidth in response to customer needs. Insummary, TDD is a more desirable duplexing technology that allows system operators to receivethe most from their investment in spectrum and telecom equipment, while meeting the needs ofeach individual customerACKNOWLEDGMENTThe Authors are grateful to University of Tun Hussein Onn Malaysia, Faculty of Electrical andElectronic Engineering, Communication lab for their valuable suggestions and help in carrying outthis study.REFERENCES 1. Holma, H. and A. Toskala, LTE for UMTS: OFDMA and SC-FDMA Based Radio Access, 267, John Wiley & Sons Ltd., United Kingdom, 2009. 2. Dahlman, E., S. Parkvall, and J. Sk¨ld, 4G LTE/LTE-Advanced for Mobile Broadband, 100– o 137, Elsevier Ltd., UK, 2011. 3. Dahlman, E., S. Parkvall, J. Sk¨ld, and P. Beming, 3G Evolution: HSPA and LTE for Mobile o Broadband, 2nd Edition, 318, Elsevier, Department in Oxford, UK, 2008. 4. Parkvall, S. and D. Astely, “The evolution of LTE towards IMT-advanced,” Journal Of Com- munications, Vol. 4, No. 3, 146–153, Apr. 2009. 5. Progri, I., Geolocation of RF Signals: Principles and Simulations, 115, Springer, USA, 2011.