Abstractâ LTE-Advanced (Long Term Evolution-Advanced) is used on fourth generation (4G) in mobile phone technology as many providers are beginning to ...
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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. INTRODUCTION
With 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 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, configuration protocols, etc.. As clear indication of the harmonization, the TDD mode is included together with the FDD mode in the same set of specifications, including the physical layer where there are just a few differences due to the uplink/downlink switching operation. In terms of architecture there are no differences between FDD and TDD and the very few differences in the MAC and higher layer protocols relate to TDD specific physical layer parameters. Procedures are kept the same. Thus there will be high implementation synergies between the two modes allowing for efficient 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 differ 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]. 2. SPECTRUM FLEXIBILITY
A 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 difference frequency bands with different characteristics, including different duplex arrangements and different sizes of the available spectrum [2]. 2.1. Flexibility in Duplex Arrangement
One important part of the LTE requirements in terms of spectrum flexibility is the possibility to deploy LTE-based radio access in both paired and unpaired spectrum. Therefore, LTE supports
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both frequency- and time-division-based duplex arrangements. FDD as illustrated on the left in Figure 1, implies that downlink and uplink transmission take place in different, sufficiently separated, frequency bands. TDD as illustrated on the right in Figure 1 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 [1]. Operation in both paired and unpaired spectrum has been supported by 3GPP radio-access technologies even before the introduction of LTE by means of FDD-based WCDMA/HSPA in combination 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 and complexity when developing and implementing dual-mode terminals capable of both FDD and TDD operation. LTE, on the other hand, supports both FDD and TDD within a single radioaccess technology, leading to a minimum of deviation between FDD and TDD for LTE-based radio access. 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 TDLTE 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). In half-duplex FDD, transmission and reception at a specific terminal are separated in both frequency and time. The base station still uses full-duplex FDD as it simultaneously may schedule different terminals in uplink and downlink; this is similar to, for example, GSM operation. The main benefit with half-duplex FDD is the reduced terminal complexity as no duplex filter is needed in the terminal. This is especially beneficial in the case of multi-band terminals which otherwise would need multiple sets of duplex filters. 3. DUPLEX SCHEMES
Spectrum flexibility is one of the key features of LTE. In addition to the flexibility in transmission bandwidth, LTE also supports operation in both paired and unpaired spectrum by supporting both FDD- 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 are some differences, most notably the presence of a special subframe in the case of TDD. The special subframe 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 for uplink 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].
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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 transmission can occur simultaneously within a cell [3]. Isolation between downlink and uplink transmissions is achieved by transmission/reception filters, known as duplex filters, and a sufficiently large duplex separation in the frequency domain. Even if uplink and downlink transmission can occur simultaneously within a cell in the case of FDD operation, a terminal may be capable of full-duplex operation or only half-duplex operation for a certain frequency band, depending on whether or not it is capable of simultaneous transmission/reception. In the case of full-duplex capability, transmission and reception may also occur simultaneously at a terminal, whereas a terminal capable of only half-duplex operation cannot transmit and receive simultaneously. Supporting only half-duplex operation 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 only half-duplex operation in certain frequency bands while being capable of full-duplex operation in the remaining supported bands. It should be noted that full/half-duplex capability is a property of 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-duplex and half-duplex FDD, a single cell may simultaneously support a mixture of full-duplex and halfduplex FDD terminals. Half-duplex operation has an impact on the sustained data rates that can be provided to/from a single mobile terminal as it cannot transmit in all uplink subframes, but the cell capacity is hardly affected as typically it is possible to schedule different terminals in uplink and downlink in a given subframe. Since a half-duplex terminal is not capable of simultaneous transmission and reception, the scheduling decisions must take this into account and half-duplex operation can be seen as a scheduling restriction. If a terminal is scheduled such that downlink reception in one subframe immediately precedes a subframe of uplink transmission, a guard time is necessary for the terminal to switch from reception to transmission. This is created in such cases by allowing the terminal to skip receiving the last OFDM symbol(s) in the downlink subframe, as illustrated 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 only and uplink and downlink transmissions are separated in the time domain on a cell basis [4]. As seen in the figure, some subframes are allocated for uplink transmissions and some subframes for downlink transmission, with the switch between downlink and uplink occurring in the special subframe (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 the frequency 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 the bandwidth is shared between uplink and downlink and the maximum bandwidth is specified to be 20 MHz in Release 8, the maximum achievable data rates are lower than in LTE FDD. This way the same receiver and transmitter processing capability can be used with both TDD and FDD modes enabling 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 uplink and downlink directions, known as the duplex separation. In a FDD UE implementation this normally requires a duplex filter when simultaneous transmission and reception is facilitated. In a TDD system the UE does not need such a duplex filter. The complexity of the duplex filter increases when the uplink and downlink frequency bands are placed in closer proximity. In some of the future spectrum allocations it is foreseen that it will be easier to find new unpaired allocations than paired allocations with sensible duplex separation thereby increasing further the scope of applicability for TDD.
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However, since uplink and downlink share the same frequency band, the signals in these two transmission directions can interfere with each other. This is illustrated in Figure 4, with the use of TDD on the same frequency without coordination and synchronization between sites in the same coverage area. For uncoordinated deployment (unsynchronized) on the same frequency band, the devices connected to the cells with different timing and/or different uplink/downlink allocation may cause blocking 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 Uses Frequency-Division Duplex
TDD-LTE 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 stations. Because FDD base stations use different frequencies for receiving and transmitting, they effectively do not hear each other and no special planning is needed.
With TDD, special considerations need to be taken in order to prevent neighboring base stations from interfering with each other.
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 two antennas that require spatial separation and, therefore, cannot reuse the resources. The result is more costly hardware [5].
In TDD, both the transmitter and receiver operate on the same frequency but at different times. Therefore, TDD systems reuse the filters, mixers, frequency 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 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 channels.
two
spectrum
more
interference-free It is requires only one interference-free channel.
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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 using, for example, satellite based solutions like GPS or Galileo or by having another external timing reference shared by the LTE TDD base stations within the same coverage area. LTE FDD does not need the base station synchronization. There is no interference between uplink and downlink in FDD due to the duplex separation of the carriers. 4. SUMMARY AND COMPARISON
The two versions of LTE are very similar. In fact, they differ only in the physical layer and, as a result, the version implemented is transparent to the higher layers. This means that UEs will be able to support both TDD-LTE and FDD-LTE with one chipset with only minor modifications required. The Table 1 shows the main comparison between FDD-LTE and TDD-LTE. 5. CONCLUSION
The uplink coverage with respect to a specific data rate in TDD-LTE is generally worse than FDDLTE due to the fact that the uplink transmission is not continuous. The percentage of coverage for control and data channels is, however, very similar to that of FDD-LTE. In terms of spectrum efficiency, 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, TDDLTE 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 next generation wireless networks. TDD is more flexible than FDD in meeting the need to dynamically reconfigure the allocated upstream and downstream bandwidth in response to customer needs. In summary, TDD is a more desirable duplexing technology that allows system operators to receive the most from their investment in spectrum and telecom equipment, while meeting the needs of each individual customer ACKNOWLEDGMENT
The Authors are grateful to University of Tun Hussein Onn Malaysia, Faculty of Electrical and Electronic Engineering, Communication lab for their valuable suggestions and help in carrying out this 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¨old, 4G LTE/LTE-Advanced for Mobile Broadband, 100– 137, Elsevier Ltd., UK, 2011. 3. Dahlman, E., S. Parkvall, J. Sk¨old, and P. Beming, 3G Evolution: HSPA and LTE for Mobile 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 Communications, Vol. 4, No. 3, 146–153, Apr. 2009. 5. Progri, I., Geolocation of RF Signals: Principles and Simulations, 115, Springer, USA, 2011.
