Uplink Slot Structure

The UL slot structure is similar to the DL slot structure. Differences are mainly due to simplifications of reference symbols, robustness, and physical UL channel multiplexing. A radio frame lasts 10 ms and is divided into 10 subframes with two slots of 0.5 ms duration as a DL radio frame is split into subframes and slots. Figure 1 shows an UL radio frame with its subframe and slot structure. An UL RB has 12 subcarriers on the frequency axis and on the time domain it has, seven SC-FDMA symbols per slot when normal CP is used, and six SC-FDMA symbols when extended CP is used (see Figure 2).

Figure 1 Uplink radio frame and subframe with two slots including PUSCH, PUCCH, PRACH, DMRS, and SRS. Reproduced with permission from Nomor

Figure 2: Uplink resource grid showing on UL RB. Reproduced with permission from © 3GPP^™

UL synchronization signals are not used because all UL signals from UEs transmitting within the cell are time aligned at the eNB with an UL timing control procedure. The eNB signals a Timing Advance (TA) command to each UE to track the UL alignment. Timing varies due to the different special distribution of UEs within a cell region. Signals are delayed because of the propagation delay. Firstly, during the random access procedure a total timing offset to the UE is transmitted. After this there is a control loop just tracking and signaling differential timing offsets in steps of 0.52 μs (16 × T_s).

The PRACH always uses six consecutive RBs on the frequency axis and is time-wise one subframe wide. Which six RBs are used is variable and set in SIB type 2. The PRACH configuration index is signaled in SIB type 2 as well, which defines the subframes which carry the PRACH's six RBs. This subframe configuration applies for either even or any radio frame. There are 64 possible PRACH configuration index permutations.

Resources at the upper and lower edges of the system bandwidth are used to carry the PUCCH. The frequency resources in between the PUCCH bands are designated to PUSCH transmission.

UL reference signals are used for UL channel estimation. DL reference signals are spread over frequency and time as single REs, which leads to a two-dimensional spheric channel estimation. UL reference signals are similar to TDMA pilots in the middle of each time slot. LTE defines reference signals in the middle of each slot (on the fourth OFDM symbol assuming normal CP duration) and spanning via the complete allocated frequency range of each UE.

The eNB can instruct UEs to transmit special reference signals over the complete system bandwidth, or parts of it, independently of UL data transmission either on PUSCH or PUCCH. These reference signals are called Sounding Reference Signals (SRSs). SRSs are used to estimate UL channel quality of a wider frequency range in order to optimize frequency-selective UL scheduling.

Figure 2 depicts, in addition to the radio frame, a zoomed UL subframe with a PUCCH and PUSCH example configuration with UL demodulation reference symbols (DMRS) and SRS symbols.

Scheduling on LTE Upload

The LTE UL scheduling is very similar to the DL scheduling, although the UL scheduler is a distinct entity. This section describes the difference from the DL scheduling only.

UL scheduling grants are indicated to the UE by transmitting all relevant UL scheduling information within the PDCCH. This is done by using a dedicated DCI type, DCI 0. Each UE has to monitor the PDCCH in every subframe for DCI types 0 scrambled with their RNTI. This does not apply for the case when power saving mode DRX is enabled, which switches off the UE's receiver periodically. UL resources are allocated without a designated PDCCH UL grant in the case of SPS or for non-adaptive HARQ retransmissions. A non-adaptive HARQ retransmission is triggered by the transmission of a Negative Acknowledgment (NACK) by the UE.

 
Figure 1: Example PDCCH message of DCI format 0 (uplink scheduling grant)

In the UL only localized scheduling is allowed, which means that an integer number of consecutive RBs is allocated to one UE. Furthermore, there is only one scheduling process per UE, thus there is not a dedicated scheduling process per radio bearer.

The UE feeds the UL scheduler with CQI, Buffer Status Reports (BSRs), ACKs/NACKs, and Scheduling Requests (SRs). BSRs indicate the fill level status of the current transmit buffer. This buffer status is reported in bins, quantizing the fill level in bytes. Figure 2 shows an example of a MAC BSR message.

 
Figure 2: Example MAC Buffer Status Report (BSR) message

LTE trial studies show that there are various cases of scheduled empty UL grants. This happens when the eNB assigns UL resources to a UE and the designated UE does not use those UL resources for an UL transmission. Operators should minimize such empty scheduled UL resource in order to optimize the usage of UL radio resources.

SC-FDMA Principles and Modulation

The OFDM transmission scheme shows robustness against multipath fading and is especially useful for mobile communication systems due to various reasons. An example is the complexity of the receiver where fairly simple and channel estimation/equalization is done in the frequency domain. Why is another transmission scheme selected for the LTE UL? A major disadvantage of OFDM systems is that the time domain signal which is amplified and transmitted shows a large dynamic range after modulating symbols on subcarriers and transformation to a time signal. This leads to a high Peak-to-Average Power Ratio (PARP) of the signal, which should be avoided for battery-powered handsets, which underlies a limit on the budget to justify the business case. The linear transmission of such a signal needs a highly complex RF amplifier for the handsets in order not to run into the nonlinear region of the transmitter. Additionally, the power consumption is larger compared to a transmitter running a smaller linear dynamic range. The PAPR even increases with a wider OFDM bandwidth for a larger number of modulated subcarriers. This results in another transmission scheme for the UL: Single Carrier FDMA (SC-FDMA).

Actually SC-FDMA is very similar to OFDMA but shows a better PAPR, leading to a longer battery lifetime and a cost-effective RF amplifier design. Figure 1 illustrates the components of a SC-FDMA transmitter system with its block entities. Highlighted in the figure are the main differences from a regular OFDM system. The main difference is the Discrete Fourier Transform (DFT) in the transmitter and the inverse DFT (iDFT) in the receiver, respectively. Thus, SC-FDMA is sometimes called DFT-spread-OFDM. Due to this difference, we can picture the information to be transmitted as modulated (bits mapped to two-dimensional QAM symbols with an I and Q component) to a time domain signal instead of modulating subcarriers of a frequency domain signal. The output of the DFT can be interpreted as a spectrum of the previously modulated data symbols. This spectrum has the characteristic of consecutive modulated subcarriers; it has therefore no scattered spectral distribution. Thus, it has the inherent behavior of localized RE usage as described in Section 1.

 
Figure 1: Block diagram of SC-FDMA transmitter with localized mapping to frequency resources

This localized spectrum is now mapped to the consecutive frequency REs which are specified in the UL scheduling grant, as only localized frequency resource assignments are allowed with UL transmission, which refers to the intrinsic signal characteristic of DFT-spread-OFDM. The rest of the spectrum for the full system bandwidth is filled with zeros. This zero patched spectrum is fed to an iFFT unit transforming a full system-wide spectrum "back" to the time domain for transmission. Figure 1 shows this mapping of the DFT symbols to the full iFFT width by adding zeros to frequency positions at the block in the center. The zero patched frequency areas are not used by this user and could be assigned to other users transmitting in the same time slot.

As for the future, a current research item is to overcome the lack of a high PAPR of OFDM signals in such a way that a digital reverse distorted signal is added to the signal which is to be transmitted. The nonlinear distortion is known for a given RF amplifier. Therefore, it is possible to pre-calculate the distortion applied to the transmitted signal. This distortion is inverted and joined to the signal to such a degree that the RF amplifier distortion eliminates the inverted signal again.

OFDM Scheduling on LTE DL

The eNB advises each UE when and on which resources to transmit its data or informs a UE where it should listen to receive data. The resources are defined by frequency and time units. This procedure of assigning system resource is called scheduling. The system resources are divided into units of RBs. Only integer numbers of RBs can be assigned to one user. Localized and distributed RB allocations are possible. Localized allocations assign adjacent RBs to one UE, distributed allocations distribute the scheduled RBs over the spectrum with gaps, for example, in order to achieve frequency diversity.

A new scheduling assignment is transmitted for each subframe, thus the scheduling period on the time axis is 1 ms. The DL scheduling information is transmitted in the PDCCH. The assignments on the frequency scale vary between one RB (minimal scheduled transmission) and the maximum number of available RBs in respect of the system bandwidth.

Generally, the LTE scheduling algorithm is not defined by the standard; it is a matter for the eNB vendors. This enables the base station vendors to differentiate between each other and use different optimization goals. Various parameters can be used as input for the scheduling decisions: channel quality of different users (measured or reported by the UE with the Channel Quality Indicator, CQI), QoS, congestion/resource situation, fairness, charging policies, and so on. Most schedulers aim to maximize the cell throughput under consideration of fairness metrics between cell edge users and users with very good channel conditions. Figure 1 shows a screenshot of a typical scheduling and cell resource allocation analysis tool. It gives insight into the scheduling process of the eNB and is able to evaluate the scheduler performance and the utilization of cell resources.

 
Figure 1: Scheduling and cell resource allocation analysis

The CQI is reported in the UL direction and can be derived periodically or upon request by the eNB. It gives reception quality feedback to the scheduling algorithm in the eNB in order to schedule data on those frequency regions with the best possible reception characteristics. 

The scheduling information is encoded as Downlink Control Information (DCI). The DCI is then mapped to REGs of the PDCCH. The length of the PDCCH can vary between one and three OFDM symbols depending on the load to be transmitted on the PDCCH. The number of used OFDM symbols is indicated in the PCFICH.

The DCI does not just transmit RB assignments and its assignment type, but also other information needed for the transmission or reception of data. This information is, for example, the Modulation and Coding Scheme (MCS), HARQ feedback information, or power control commands for UL transmission of the Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH) (see below).

The following DCI formats are defined and used for scheduling and UL Transmit Power Control (TPC) commands:

• DCI format 0: UL scheduling grant.
• DCI format 1: Single transport block (code word) scheduling assignment, for example, used for assigning resources to system information, paging, or random access. Further information: MCS, HARQ (New Data Indicator (NDI), redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1A: Compact single transport block scheduling assignment. Further information: MCS, HARQ (NDI, redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1B: A special DCI format for transmission mode 6 (MIMO closed loop rank 1 pre-coding). Further information: precoding vector, MCS, HARQ (NDI, redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1C: Even more compact scheduling format as DCI 1A. For example, used for assigning resources to SIBs, paging, or RARs. This DCI is always transmitted using frequency diversity via distributed virtual resource block assignments using resource allocation type 2 (see below). This is done because channel feedback cannot be derived for such common information, as it is received by multiple users. The modulation is fixed to QPSK. Further information: MCS, HARQ (NDI, redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1D: A special DCI format for transmission mode 5 (multi-user MIMO). Further information: MCS, HARQ (NDI, redundancy version, HARQ process number), power offset indicator if two UEs share power resources, and TPC for PUCCH.
• DCI format 2: Scheduling for transmission mode 4 (closed loop MIMO), multiple antenna port transmission operation, addressing multiple transport blocks (code words) to be transmitted on different antenna ports (layers). Further information: MCS for each transport block, HARQ (NDI, redundancy version, HARQ process number), number of transmission layers, precoding, and TPC for PUCCH.
• DCI format 2A: Used with transmission mode 3 (open loop MIMO using Cyclic Delay Diversity (CDD)), multiple antenna port transmission operation, addressing multiple transport blocks (code words) to be transmitted on different antenna ports (layers). Further information: MCS for each transport block, HARQ (NDI, redundancy version, HARQ process number), number of transmission layers, precoding, and TPC for PUCCH.
• DCI format 3: A 2-bit UL TPC command applying for PUSCH and PUCCH. Multiple users are addressed.
• DCI format 3A: A 1-bit UL TPC command applying for PUSCH and PUCCH. Multiple users are addressed.

The resource allocation assignments of the above DCI formats can use different resource allocation types. Table 1 maps the DCI formats to the allowed resource allocation types:

• Resource allocation type 0: With resource allocation type 0 a bit map is transmitted describing Resource Block Groups (RBGs). A RBG is a number of consecutive physical resource blocks (RBs). The number depends on the system bandwidth and has a range between one and four physical RBs. Table 1 maps the size of a RBG to the system bandwidth. The allocated RBGs do not have to be adjacent.

  Table 1: Resource allocation types and the applying DCI formats TS36.213. Reproduced with permission from © 3GPP^™ 

  Resource allocation type	Applying DCI formats
  Type 0	1, 2, 2A, and 2B
  Type 1	1, 2, 2A, and 2B
  Type 2	1A, 1B, 1C, and 1D

  Table 2: Type 0 resource allocation RBG size vs. DL system bandwidth 

  System bandwidth NRD BL	RBG size (P)
  ≤10	1
  11–26	2
  27–63	3
  64–110	4

• Resource allocation type 1: The bit map transmitted with resource allocation type 1 makes use also of RBGs but can address single physical RBs by introducing additional flags. The number of RBGs is smaller than the ones used with resource allocation type 0, thus not reaching the complete bandwidth. The bit map addresses not whole RBGs, but a subset within each RBG which is pointed to by the bit map. A selection flag indicates the position within the RBG regions and a shift flag shows the position of the numbered RBGs within the system bandwidth as the number of RBGs does not address the complete system bandwidth: shift flag = 0 indicates that the RBGs start at the beginning of the system bandwidth leaving an unaddressable region at the end of the system bandwidth; shift flag = 1 indicates that the RBGs are shifted to the end of the system bandwidth leaving the unaddressable region at the beginning of the system bandwidth.
• Resource allocation type 2: This resource allocation type uses virtual RBs as scheduling units. Two types of virtual RB scheduling assignments are used:
  • – A localized type, where the allocated virtual RBs equal a number of consecutive physical RBs addressed with a starting RB and a number of adjacently assigned RBs. This information is encoded into an 11-bit Resource Indication Value (RIV).
  • – A distributed type, where the addressed virtual RBs are distributed over the frequency with one or two gaps (depending on the system bandwidth) hopping at slot boundaries. Virtually distributed RB assignments are always used with DCI format 1C. There is a 1-bit flag indicating whether virtual distributed or virtual localized RB assignment is used in the case of DCI formats 1A, 1B, and 1D.

Instead of addressing a UE with a PDCCH scheduling assignment (DCI) directly by adding a UE ID (e.g., a RNTI) to the DCI, the 16-bit CRC of the PDCCH message is scrambled with the RNTI, introducing common and UE-specific search spaces. This CRC scrambling saves additional resources in DCIs, but increases slightly the chance of decoding a DCI for a different UE which is not intended to be addressed.

Figure 2 depicts an example PDCCH message of DCI format type 1 with all the transmitted information.

 
Figure 2: Example PDCCH message of DCI format 1 (downlink scheduling assignment)

Some special applications require the transmission of small data chunks in equidistant periods of time. An example is a VoIP application. In order to minimize the signaling overhead in such cases, a mode Semi Persistent Scheduling (SRS) is introduced. SPS parameters are configured by the RRC layer, enabling the transmission of data on defined RBs in frequency and time without further scheduling on the PDCCH.

Battery energy saving is always an important topic with mobile handset systems. A potential scheduling assignment could be sent in each PDCCH which occurs every millisecond. Therefore, each attached UE would need to monitor the PDCCH each millisecond for scheduling information. The DRX mode enables the UE just to listen to defined subframes for scheduling assignments and turn off its receiver in between, in order to save battery consumption. Short and long DRX cycle periods are defined. The DRX parameters are set by MAC and RRC. Figure 3 depicts a DRX cycle.

 
Figure 3: DRX cycle.

Downlink Slot Structure | Radio Interface Basics

This section introduces the LTE DL structure with respect to time. In the time domain, the DL is divided into slots and frames. A radio frame is the largest unit and lasts 10 ms. As both duplex modes, FDD and TDD, use similar timings, this makes a UE starting to synchronize to a LTE cell unaware of the duplex method used. Thus, TDD and FDD both introduce radio frames of 10 ms timing.

Both duplex methods define different types of radio frames:

• Frame type 1 is the frame type used with FDD.
• Frame type 2 is the frame type used with TDD.

One radio frame is split into 10 subframes of 1 ms duration. A subframe is also the most important unit of scheduling and for physical control channel durations the PDCCH is found in the first OFDM symbols of each subframe. Furthermore, subframes are divided into two slots of 0.5 ms. The smallest unit is an OFDM symbol. Depending on the CP length (normal or extended) used, the length will be seven or six OFDM symbols transmitted within one slot, resulting in 14 or 12 OFDM symbols within one subframe, respectively. Figure 1 shows one radio frame of type 1 (FDD) with 20 slots and 10 subframes. The length of one radio frame is defined as Tf = 307 200 × T_s whereT_s is the sampling period. Therefore, the base sampling frequency of 30.72 MHz (T_s = 1/30.72 MHz) is used.

 
Figure 1: Frame structure type 1 used with FDD TS36.211. Reproduced with permission from © 3GPP^™

Figure 2 shows a complete radio frame of type 1 as used with FDD. It illustrates all 10 subframes with areas for the PDSCH. The first OFDM symbols are allocated by the PDCCH. In the area of the PDCCH, additional physical channels for control information are embedded. These channels are the Physical HARQ Indicator Channel (PHICH) and the Physical Control Format Indicator Channel (PCFICH).

 
Figure 2: Downlink FDD radio frame (normal cyclic prefix) with PDCCH, PDSCH, PBCH, reference signals, and synchronization signals. Reproduced with permission from Nomor

The PCFICH indicates the number of OFDM symbols allocated for the complete PDCCH in its current subframe. Thus, the PDCCH is transmitted on a variable number of OFDM symbols depending on how much control or scheduling information has to be transmitted, and the resulting number of OFDM symbols which are used for the PDSCH is variable as well. The PDCCH allocates between 1 and 3 OFDM symbols and the PDSCH between 13 and 11 for normal CP and between 11 and 9 OFDM symbols for extended CP, respectively.

In the middle (around the DC subcarrier) of the bandwidth are six RBs used for some common signals and channels in some subframes. These locations are used for initial cell search and cell synchronization. This central location enables a bandwidth-independent cell and frame synchronization as well as initial cell access. Two step hierarchical synchronization signals are defined and located in the first and sixth subframes. The Primary Synchronization Signal (PSS) is transmitted on the seventh OFDM symbol and the Secondary Synchronization Signal (SSS) is transmitted on the sixth OFDM symbol of subframes described above. PSS and SSS can be seen in Figure 2 shows the bandwidth of the synchronization signals. Only 62 subcarriers of the 72 provided by six allocated RBs are used for synchronization signals. 

Resources for the PBCH are allocated apart from the SSS. The PBCH is transmitted in each first subframe of all radio frames. Other than the synchronization signals, the PBCH uses all 72 subcarriers of the six central RBs 

Reference signals are needed by the channel estimation process in order to correct the wireless channel distortion in the signal at the receiver. Four sets of reference signals are specified for each transmit antenna as LTE defines multi-antenna transmissions (DL MIMO). The receiver needs to know the propagation conditions of each transmit antenna for using the complete MIMO gain. Therefore, a defined signal (reference signal) is transmitted from each individual antenna completely independently and all other transmit antennas do not transmit any signals on those specific frequencies and time resources (REs). Figure 3 illustrates the frequency and time RE used for transmitting reference signals depending on the number of antennas configured.

 
Figure 3: Mapping of downlink reference signals (normal cyclic prefix) (TS36.211). Reproduced with permission from © 3GPP^™

Frame type 2 is used with TDD. The general slot structure of a TDD frame is similar to a FDD frame, since a mobile is not aware of the duplex method before synchronizing to the cell. A radio frame of type 2 has a duration of 10 ms as its FDD equivalent. The TDD radio frame is also divided into 10 transmission time intervals of 1 ms duration called subframes. Furthermore, TDD subframes are split as well into two slots of 0.5 ms period.

TDD systems switch on one frequency between DL and UL transmission. Therefore, TDD needs defined switch points and guard intervals between UL and DL transmission.

LTE defines two basic switch point interval durations of 5 and 10 ms. A switch point is a designated subframe divided into three zones which are already known from basic UMTS TDD: Downlink Pilot Time Slot (DwPTS), GP, and Uplink Pilot Time Slot (UpPTS). Other subframes are used for either UL or DL transmission. Operators can decide from UL–DL configurations which subframes are used as DL and UL depending on the UL and DL traffic mixture of a network. Seven different UL and DL structures are defined. The possible UL–DL configurations with the switch points and the subframe used for UL and DL transmission are given in Table 1.

Table 1: Uplink–downlink configurations for TDD. Reproduced with permission from © 3GPP^™ 

Uplink-downlink configuration	Downlink-uplink switch-point periodicity (ms)
		Subframe number
		0	1	2	3	4	5	6	7	8	9
0	5	D	S	U	U	U	D	S	U	U	U
1	5	D	S	U	U	D	D	S	U	U	D
2	5	D	S	U	D	D	D	S	U	D	D
3	10	D	S	U	U	U	D	D	D	D	D
4	10	D	S	U	U	D	D	D	D	D	D
5	10	D	S	U	D	D	D	D	D	D	D
6	5	D	S	U	U	U	D	S	U	U	D
S = Swithching point.

Figure 4 shows a radio frame with frame structure type 2 used with TDD. This is an example with a 5 ms switch-point periodicity.

 
Figure 4: Frame structure type 2 used with TDD (for 5 ms switch-point periodicity) (TS36.211).