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TDD Design for UMTS Long-Term Evolution

Rapeepat Ratasuk, Amitava Ghosh, Weimin Xiao, Robert Love, Ravi Nory, Brian Classon

Motorola Inc., 1421 West Shure Drive, Arlington Heights, IL 60004, USA Abstract-- Long-Term Evolution (LTE) will provide substantial enhancements to UMTS 3G systems including improved system capacity and coverage, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. LTE supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes to provide deployment flexibility in accordance with operator's preference and spectrum allocation. This paper presents an overview of LTE TDD design and highlights key differences with FDD. Design challenges unique to TDD are presented together with adopted technical solutions. Finally, simulation results are provided to demonstrate typical TDD system performance with data applications. I. INTRODUCTION Long-Term Evolution (LTE) of the UMTS Terrestrial Radio Access and Radio Access Network is aimed at commercial deployment around 2010 timeframe. Goals for the evolved system include support for high peak data rates, low latency, improved system capacity and coverage, reduced operating costs, multi-antenna support, efficient support for packet data transmission, flexible bandwidth operations and seamless integration with existing 2G and 3G systems. LTE supports operations in Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes to provide operators with deployment flexibility regarding spectrum allocation. Support for Half Duplex FDD is also included. Time division duplex provides several attractive features from both design and deployment perspective. For deployment in unpaired spectrum, TDD is a natural choice. In addition, TDD allows for flexible bandwidth allocation between downlink and uplink to efficiently support asymmetric traffic load. This is especially attractive as data usage over cellular networks grows since data applications such as web browsing are highly asymmetrical. Channel symmetry can also be exploited to reduce the need for feedback. Finally, no frequency duplexer is needed and several components (filters, mixers, etc.) may be shared between the transmitter and receiver, allowing for lower cost design. This paper presents an overview of LTE TDD system design and highlights key differences with FDD. Design challenges unique to TDD are presented together with technical solutions adopted in LTE. Finally, TDD performance results are provided to demonstrate typical system performance. The paper is organized as follows. In Section II, an overview of the air interface for LTE is provided. Section III highlights key differences between FDD and TDD. In Section IV, TDD frame structure is discussed. TDD design details are provided in Section V, and performance results are shown in Section VI. Finally, conclusions are drawn in Section VII. II. LTE AIR-INTERFACE OVERVIEW In the downlink, OFDM is selected as the air-interface for LTE. With OFDM, it is straightforward to exploit frequency selectivity of the multi-path channel with low-complexity receivers. This allows frequency selective in addition to frequency diverse scheduling and one cell reuse of available bandwidth. Furthermore, OFDM enables flexible bandwidth operation with low complexity. In the uplink, Single-Carrier Frequency Division Multiple Access (SC-FDMA) using DFTSpread OFDM is selected. SC-FDMA has many similarities to OFDM, chief among them is frequency domain orthogonality among users. SC-FDMA also has a low power amplifier derating requirement, thereby conserving battery life or extending range. A comprehensive overview of the airinterface may be found in [1].

Figure 1. LTE downlink subframe (normal cyclic prefix). An illustration of the LTE downlink subframe common to both TDD and FDD is shown in Figure 1. Each 1ms subframe is comprised of two slots of equal length. Each downlink subframe contains reference signals, control information, and data transmission. Downlink control signalling consists of three physical channels ­ (1) Physical Control Format Indicator Channel (PCFICH) to indicate the number of OFDM symbols used for control, (2) Physical H-ARQ Indicator Channel (PHICH) which carries downlink ACK/NACK, and (3) Physical Downlink Control Channel (PDCCH) which carries the scheduling assignments and power control commands. The downlink scheduling assignment is addressed to specific user and contains information necessary to perform data reception. The uplink scheduling grant is also addressed to specific user and contains information necessary for the user to initiate data transmission in the corresponding uplink subframe. Figure 2 shows the common uplink subframe structure. Data transmission occurs in inner band resource blocks to reduce out of carrier band emission. As a result, control

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resources are placed at the carrier band edge with inter-slot hopping to provide frequency diversity.

within one subframe, for TDD the two signals are placed in different subframes and separated by two OFDM symbols. IV. TDD FRAME STRUCTURE The TDD frame structure is shown in Figure 3. Each radio frame spans 10ms and consists of ten 1ms subframes. Subframes 0 and 5 are always downlink subframes as they contain synchronization signal and broadcast information necessary for the User Equipment (UE) to perform synchronization and obtain relevant system information. Subframe 1 is a special subframe that serves as a switching point between downlink to uplink transmission. It contains three fields - Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS) that will subsequently be explained in details. No special subframe is provisioned for switching from uplink to downlink transmission. Instead, appropriate timing advance at the UE will be employed to create the necessary guard period.

Figure 2. LTE uplink subframe (normal cyclic prefix). The Physical Uplink Control Channel (PUCCH) carries three types of control signalling ­ ACK/NACK for downlink transmission, scheduling request indicator and feedback of downlink channel quality and precoding vector. Two types of reference signals are supported on the uplink ­ demodulation reference signal, associated with transmission of uplink data or control, and sounding reference signal used mainly to aid in channel dependent scheduling. III. TDD AND FDD DIFFERENCES In LTE, TDD mode is viewed solely as a physical layer manifestation and therefore invisible to higher layers. As a result, there is no operational difference between the two modes at higher layers or in the system architecture. At the physical layer, the fundamental design goal is to achieve as much commonality between the two modes as possible. As a result, the main design differences between the two modes stem from the need to support various TDD UL/DL allocations and provide co-existence with other TDD systems. In this regard, several additional features not available for FDD were introduced. Table 1 provides a brief overview of the physical layer features available only in TDD. Details descriptions of theses features will be provided in subsequent sections. Table 1. Features only available in TDD.

Feature Frame structure TDD Implementation Introduction of a special subframe for switching from DL to UL and to provide coexistence with other TDD systems Additional short random access format available in special subframe, multiple random access channels in a subframe Multi-subframe scheduling for uplink Bundling of acknowledgements or multiple acknowledgements on uplink control channel Variable number of H-ARQ processes depending on the UL/DL allocation

Figure 3. TDD frame structure. Two switching point periodicities are supported ­ 5ms and 10ms. For the 5ms switching point periodicity, subframe 6 is likewise a special subframe identical to subframe 1. For the 10ms switching point periodicity, subframe 6 is a regular downlink subframe. Table 2 illustrates the possible UL/DL allocations. Table 2. Uplink-downlink allocations.

UL/DL Configuration 0 1 2 3 4 5 6 Period (ms) 5 Subframe 0 D D D D D D D 1 S S S S S S S 2 U U U U U U U 3 U U D U U D U 4 U D D U D D U 5 D D D D D D D 6 S S S D D D S 7 U U U D D D U 8 U U D D D D U 9 U D D D D D D

10 5

Random access Scheduling ACK/NACK H-ARQ process number

In addition to the features outlined in Table 1, FDD and TDD modes also differs in the time placement of the synchronization signals. Unlike in FDD, where the primary and secondary synchronization signals are contiguously placed

As shown in Figure 3, the total length of DwPTS, GP, and UpPTS fields is 1ms. However, within the special subframe the length of each field may vary depending on co-existence requirement with legacy TDD systems and supported cell size. Table 3 provides the supported configurations where the length of each field is given in multiples of OFDM symbols. Note that this assumes that the Node-B (base station in UMTS terminology) and UE switching time to be less than the duration of an OFDM symbol with extended cyclic prefix (CP).

Table 3. DwPTS/GP/UpPTS length (OFDM symbols).

Format 0 1 2 3 4 5 6 7 8 Normal CP

DwPTS GP UpPTS DwPTS

Extended CP

GP UpPTS

3 9 10 11 12 3 9 10 11

10 4 3 2 1 9 3 2 1

1

2

3 8 9 10 3 8 9 -

8 3 2 1 7 2 1 -

1

From Table 3, it is seen that the majority of configurations allocate at least eight OFDM symbols to the DwPTS. Therefore, data transmission capacity is generally similar to a regular downlink subframe. When there are only three OFDM symbols allocated to DwPTS, data transmission should still be possible at discretion of the Node-B so as not to waste resource, especially at high system bandwidth. B. Uplink Pilot Time Slot From Table 3, it is seen that there are only two values for UpPTS duration (one or two OFDM symbols). As a result, UpPTS usage by the UE is limited to either sounding reference signals or random access (RACH) transmission. Random access requires UpPTS length of two OFDM symbols. When one OFDM symbol is allocated to the UpPTS, only sounding reference signals transmission is possible. Random access on the UpPTS is limited by the length of the UpPTS and therefore not applicable to all deployment scenarios. An illustration of the random access transmission in the UpPTS is shown in Figure 6. Random access begins 4832×Ts seconds, where Ts = 1/(15000×2048), before the end of the UpPTS with a duration of 4544×Ts seconds. This leaves a guard period of 288×Ts seconds which allows for a maximum supported cell size of approximately 1.4 km. For larger cell sizes, RACH will have to be supported in regular uplink subframes to provide sufficient guard period.

2 -

An example of coexistence with legacy UMTS Low Chip-Rate (LCR) TDD system is shown in Figure 4 where switching point alignment between the two systems is illustrated.

Figure 4. Coexistence with LCR-TDD UMTS system. Obviously, to minimize the number of special subframe patterns to be supported, not all legacy TDD configurations can be supported. For LCR-TDD configurations, Table 3 was designed to provide co-existence with the 5DL:2UL and 4DL:3UL LCR-TDD splits which are generally viewed as the most common deployment configurations. A. Downlink Pilot Time Slot The central design philosophy is to treat the DwPTS as a regular but shortened downlink subframe. As a result, it always contains reference signals and control information like a regular downlink subframe, and may carry data transmission at the discretion of the scheduler. In addition, it also contains the primary synchronization signal (PSS) used for downlink synchronization. An illustration of the DwPTS structure is shown in Figure 5.

Figure 6. RACH in UpPTS. Within each random access region, 64 preambles are available for use. In [7], the signal-to-noise requirement for this RACH format was shown to be approximately -9.5 and -1.8dB under AWGN and Typical Urban channels, respectively. These required operating points are relatively high and therefore this format can only be supported in cells with high signal-to-noise ratios. C. Guard Period The guard period denotes the switching point between downlink and uplink transmission and its length determines the maximum supportable cell size. For LTE, cell size of up to 100 km must be supported, requiring a guard period of approximately 666.7 s. This is possible by choosing Format 0 in Table 3 for the special subframe. V. TDD SYSTEM DESIGN Due to the need to support numerous UL/DL allocations, TDD design must allow for greater flexibility and robustness. This section describes design challenges inherent to TDD operation and summarizes solutions employed in LTE. Note

Figure 5. DwPTS structure. In Figure 5, it is seen that the secondary synchronization signal (SSS) is transmitted on the last symbol of subframe 0. The PSS is transmitted on the third OFDM symbol in DwPTS to allow the same reference signal placement in the DwPTS as in other downlink subframes. This, however, will result in small degradation of cell search performance using coherence detection at very high vehicular speed due to channel variations.

that some concepts presented here are only preliminary as not all TDD system design aspects have been finalized. A. Downlink Control Design In LTE, downlink control signalling serves three main purposes ­ to indicate the size of the control region, provide downlink ACK/NACK, and provide signalling related to scheduling assignments and power control. For TDD, several issues were considered as described below. One important design criteria was to ensure that the maximum size of the downlink control region for FDD (3 OFDM symbols per subframe) was large enough to provide all the control signalling needed for the different downlink-uplink TDD allocations. For the uplink-asymmetric allocation, each control region must address several uplink subframes in addition to its own downlink subframe. However, by utilizing several control overhead reduction techniques, it is expected that the current design will be sufficient. For instance, scheduling grant addressing may span up to two uplink subframes using a 2-bit sub-frame indicator. In coverage-limited scenarios, uplink users can be scheduled aggressively and then rely on non-adaptive HARQ re-transmissions which do not require additional grants. Naturally, timing of the downlink acknowledgement is variable based on the UL/DL configuration. For PUSCH transmissions in subframe n, Node-B will transmit the acknowledgment in subframe n+k, where k is given in Table 4. This allows for at least 3ms processing time at the Node-B. Table 4. Downlink ACK/NACK timing index k for TDD

TDD UL/DL Configuration 0 1 2 3 4 5 6 subframe index n 0 1 2 4 4 6 6 6 6 4 3 7 6 6 6 6 4 6 6 6 5 6 7 4 4 6 4 8 7 6 7 9 6 -

For instance, with 2UL:2DL+DwPTS configuration, some uplink subframes must carry acknowledgements for two downlink subframes as shown in Figure 7. In this case, the UE must aggregate all the acknowledgements and transmit only on one uplink channel in order to preserve single-carrier property. Table 5. Uplink ACK/NACK timing index k for TDD

TDD UL/DL Configuration 0 1 2 3 4 5 6 subframe index n 0 4 7 7 4 12 12 7 1 6 6 6 11 11 11 7 2 3 4 9 4 4 8 8 8 5 4 7 7 7 7 7 7 6 6 6 6 6 7 6 7 7 6 6 5 8 4 5 5 4 9 4 8 5 4 13 5

For UEs with good coverage, there should be no issue in transmitting multiple acknowledgments in order to allow individual feedback for each H-ARQ process. However, details for multiple acknowledgments have not been finalized and this feature may be deferred to future LTE releases.

Figure 7. Example of uplink acknowledgement association. On the other hand, UEs in coverage-limited situation may encounter difficulties in transmitting multiple acknowledgements. As a result, acknowledgement bundling (AND of all acknowledgements) can be used [5]. This can significantly increase uplink coverage and UEs that are in poor coverage may be configured to operate in this mode. However, with bundling of acknowledgements, UE may transmit erroneous acknowledgement if some downlink assignment grants are missed. To solve this problem, information about the number of grants to be transmitted to a UE within the bundling window is included. The UE can then determine whether any grant was missed and, if so, will not transmit any acknowledgement (i.e. DTX). In the uplink, ACK/NACK resource indication will be implicitly tied to the downlink control channel used for scheduling assignment. For TDD, the UE may receive several assignments in different DL subframes within the same ACK/NACK response window (see Figure 7 for instance) and thus the implicit relationship should be clarified. The proposal here is to also include downlink subframe number when defining the implicit relationship. C. H-ARQ Process Number In LTE, N-channel stop and wait H-ARQ protocol is employed where the value of N depends on processing times at the Node-B and UE, as well as propagation time at the UE. Using processing time of 3ms at the Node-B and UE, the

Finally, an issue related to multi-subframe scheduling in the uplink is how to transmit multiple acknowledgements in one downlink subframe. Here, the solution is to associate a data transmission in an uplink subframe with a corresponding downlink subframe and PHICH group. This will allow a UE to implicitly determine where its acknowledgements will be transmitted. B. Uplink Control Design In the uplink, the main issue with TDD operation is the need to transmit several acknowledgements on the same subframe. This is because in most cases, the TDD split is asymmetrical in favor of the downlink. From a timing perspective, the UE will upon detection of a data transmission in subframe n, transmit the acknowledgement in uplink subframe n+k, where k is given in Table 5. This allows for at least 3ms processing time at the UE. Note that, in many cases, the UE will have to transmit multiple acknowledgments in one uplink subframe.

maximum number of H-ARQ processes for the supported UL/DL formats is given in Table 6. Table 6. Maximum number of H-ARQ processes.

TDD UL/DL Configuration 0 1 2 3 4 5 6 Process Number DL 4 7 10 9 12 15 6 UL 7 4 2 3 2 1 6

For full-buffer traffic, the spectral efficiency for TDD systems is similar to that of FDD except for a slight loss resulting from the use of the special subframe. VII. CONCLUSIONS This paper presents an overview of TDD design for LTE and highlights key differences with FDD. Design challenges unique to TDD are presented together with technical solutions adopted in LTE. Finally, simulation results are provided to demonstrate typical TDD system performance with data applications. REFERENCES

[1] [2] [3] [4] [5] [6] [7] Classon, B. et al, "Overview of UMTS Air-Interface Evolution," IEEE 64th Vehicular Technology Conference, September 2006. 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation, v.8.1.0, November 2007. Liu, G.. et al, "Evolution Map from TD-SCDMA to FuTURE B3G TDD," IEEE Communications Magazine, March 2006. Nory, R. et al, "Uplink VoIP Support for 3GPP EUTRA," IEEE 65th Vehicular Technology Conference, March 2007. R1-080602, "Way Forward on Special Sub-frame Patterns for FS2," CATT, RAN1#51-Bis, Seville, Spain, Jan 2008. Ghosh, A. et al, "Uplink Control Channel Design for 3GPP LTE," IEEE 18th International Symposium on Personal, Indoor and Mobile Radio Communications, September 2007. R4-081262, "Ideal Simulation Results for PRACH Format 4," Motorola, RAN4#47-Bis, Munich, Germany, Jun 2008.

Note that synchronous H-ARQ is used in the uplink while asynchronous H-ARQ is used in the downlink. VI. PERFORMANCE RESULTS Simulations were performed to analyze system performance using parameters outlined in Table 10. Table 7 presents the system simulation cases of interest. In our analysis, DwPTS/GP/UpPTS lengths as given by Format 3 of Table 3 are used. In this format, eleven OFDM symbols are available in DwPTS. Thus, the DwPTS field may be considered as a slightly shortened downlink subframe. The results shown assume the maximum amount of control overhead (three OFDM symbols) in each downlink subframe, although the PDCCH was not explicitly modeled. Table 7. System simulation scenario.

Simulation Case 1 Inter-Site Distance (m) 500 Penetration Loss (dB) 20 Vehicular Speed (km/h) 3

Note ­ 3GPP documents may be downloaded from ftp://ftp.3gpp.org

Table 10. System simulation parameters

Parameter Cellular Layout Distance-dependent path loss Lognormal Shadowing Shadowing standard deviation Correlation distance of Shadowing Between cells Shadowing correlation Between sectors Penetration Loss Carrier Frequency Channel model UE speed of interest Total BS TX power UE power class Inter-cell Interference modeling Min distance between UE and cell Assumption Hexagonal grid, 19 cell sites, 3 sectors per site L=I + 37.6log10(.R), R in kilometers I=128.1 ­ 2GHz Similar to UMTS 30.03, B 1.41.4 8 dB 50 m 0.5 1.0 10, 20dB 2.0GHz Typical Urban (TU) 3 km/h 43dBm 24dBm UL: Explicit modeling (all cells occupied by UEs) >= 35 meters

System data throughput is analyzed using full buffer file transfer model. The system bandwidth is 10 MHz shared between downlink and uplink transmission. The chosen UL/DL allocation was 2UL:2DL+DwPTS. Table 8 illustrates the sector and user spectral efficiency of LTE TDD in the downlink for Simulation Case 1. Table 8. DL spectral efficiency (10 MHz, Case 1).

Antenna Configuration 2x2, MIMO 4x2, MIMO Sector Throughput (bps/Hz) 1.59 1.85 Edge Throughput (bps/Hz) 0.043 0.044

Likewise, Table 9 illustrates the sector and user spectral efficiency of LTE TDD in the uplink. Table 9. UL spectral efficiency (10 MHz, Case 1).

Antenna Configuration 1x2 1x4 1x4, MU-MIMO Sector Throughput (bps/Hz) 0.76 1.00 1.10 Edge Throughput (bps/Hz) 0.020 0.044 0.046

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TDD Design for UMTS Long-Term Evolution

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