Designing a Broadband Wireless Network: Overview and Channel Structure of LTE

This chapter is from the book

Fundamentals of LTE

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This chapter is from the book

This chapter is from the book 

Fundamentals of LTE

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6.3 Downlink OFDMA Radio Resources

In LTE, the downlink and uplink use different transmission schemes due to different considerations. In this and the next section, we describe downlink and uplink radio transmission schemes, respectively. In the downlink, a scalable OFDM transmission/multiaccess technique is used that allows for high spectrum efficiency by utilizing multiuser diversity in a frequency selective channel. On the other hand, a scalable SC-FDMA transmission/ multiaccess technique is used in the uplink since this reduces the peak-to-average power ratio (PAPR) of the transmitted signal.

The downlink transmission is based on OFDM with a cyclic prefix (CP), which was described in Chapter 3 along with the associated multiple access scheme described in Chapter 4. We summarize some key advantages of OFDM that motivate using it in the LTE downlink:

• As shown in Chapter 3, OFDM is efficient in combating the frequency-selective fading channel with a simple frequency-domain equalizer, which makes it a suitable technique for wireless broadband systems such as LTE.
• As shown in Chapter 4, it is possible to exploit frequency-selective scheduling with OFDM-based multiple access (OFDMA), while HSPA only schedules in the time domain. This can make a big difference especially in slow time-varying channels.
• The transceiver structure of OFDM with FFT/IFFT enables scalable bandwidth operation with a low complexity, which is one of the major objectives of LTE.
• As each subcarrier becomes a flat fading channel, compared to single-carrier transmission OFDM makes it much easier to support multiantenna transmission, which is a key technique to enhance the spectrum efficiency.
• OFDM enables multicast/broadcast services on a synchronized single frequency network, that is, MBSFN, as it treats signals from different base stations as propagating through a multipath channel and can efficiently combine them.

The multiple access in the downlink is based on OFDMA. In each TTI, a scheduling decision is made where each scheduled UE is assigned a certain amount of radio resources in the time and frequency domain. The radio resources allocated to different UEs are orthogonal to each other, which means there is no intra-cell interference. In the remaining part of this section, we describe the frame structure and the radio resource block structure in the downlink, as well as the basic principles of resource allocation and the supported MIMO modes.

6.3.1 Frame Structure

Before going into details about the resource block structure for the downlink, we first describe the frame structure in the time domain, which is a common element shared by both downlink and uplink.

In LTE specifications, the size of elements in the time domain is expressed as a number of time units T_s = 1/(15000 x 2048) seconds. As the normal subcarrier spacing is defined to be Δf = 15kHz, T_s can be regarded as the sampling time of an FFT-based OFDM transmitter/receiver implementation with FFT size N_FFT = 2048. Note that this is just for notation purpose, as different FFT sizes are supported depending on the transmission bandwidths. A set of parameters for typical transmission bandwidths for LTE in the downlink is shown in Table 6.2, where the subcarrier spacing is Δf = 15kHz. The FFT size increases with the transmission bandwidth, ranging from 128 to 2048. With Δf = 15kHz, the sampling frequency, which equals Δf x N_FFT, is a multiple or sub-multiple of the UTRA/HSPA chip rate of 3.84MHz. In this way, multimode UTRA/HSPA/LTE terminals can be implemented with a single clock circuitry. In addition to the 15kHz subcarrier spacing, a reduced subcarrier spacing of 7.5kHz is defined for MBSFN cells, which provides a larger OFDM symbol duration that is able to combat the large delay spread associated with the MBSFN transmission. Unless otherwise stated, we will assume Δf = 15kHz in the following discussion.

Table 6.2. Typical Parameters for Downlink Transmission

In the time domain, the downlink and uplink multiple TTIs are organized into radio frames with duration T_f = 307200 · T_s = 10 ms. For flexibility, LTE supports both FDD and TDD modes.⁵ Most of the design parameters are common to FDD and TDD in order to reduce the terminal complexity and maximize reuse between the designs of FDD and TDD systems. Accordingly, LTE supports two kinds of frame structures: frame structure type 1 for the FDD mode and frame structure type 2 for the TDD mode.

Frame Structure Type 1

Frame structure type 1 is applicable to both full duplex and half duplex FDD. There are three different kinds of units specified for this frame structure, illustrated in Figure 6.8. The smallest one is called a slot, which is of length T_slot = 15360 · T_s = 0.5 ms. Two consecutive slots are defined as a subframe of length 1 ms, and 20 slots, numbered from 0 to 19, constitute a radio frame of 10 ms. Channel-dependent scheduling and link adaptation operate on a subframe level. Therefore, the subframe duration corresponds to the minimum downlink TTI, which is of 1 ms duration, compared to a 2 ms TTI for the HSPA and a minimum 10 ms TTI for the UMTS. A shorter TTI is for fast link adaptation and is able to reduce delay and better exploit the time-varying channel through channel-dependent scheduling.

Figure 6.8 Frame structure type 1. For the normal CP,T_cp= 160·T_s 5.2μs for the first OFDM symbol, and T_cp = 144· 4.7μs for the remaining OFDM symbols, which together fill the entire slot of 0.5 ms. For the extended CP, T_ecp = 512·T_s 16.7us.

Each slot consists of a number of OFDM symbols including CPs. As shown in Chapter 3, CP is a kind of guard interval to combat inter-OFDM-symbol interference, which should be larger than the channel delay spread. Therefore, the length of CP depends on the environment where the network operates, and it should not be too large as it brings a bandwidth and power penalty. With a subcarrier spacing Δf = 15kHz, the OFDM symbol time is 1/Δf 66.7μs. As shown in Figure 6.8, LTE defines two different CP lengths: a normal CP and an extended CP, corresponding to seven and six OFDM symbols per slot, respectively. The extended CP is for multicell multicast/broadcast and very-large-cell scenarios with large delay spread at a price of bandwidth efficiency, with length T_eCP = 512 · T_s 16.7μs. The normal CP is suitable for urban environment and high data rate applications. Note that the normal CP lengths are different for the first (T_CP = 160 · T_s 5.2μs) and subsequent OFDM symbols (T_CP = 144 · T_s 4.7μs), which is to fill the entire slot of 0.5 ms. The numbers of CP samples for different bandwidths are shown in Table 6.2. For example, with 10MHz bandwidth, the sampling time is 1/(15000 x 1024) sec and the number of CP samples for the extended CP is 256, which provides the required CP length of 256/(15000x1024) 1.67μs. In case of 7.5kHz subcarrier spacing, there is only a single CP length, corresponding to 3 OFDM symbols per slot.

For FDD, uplink and downlink transmissions are separated in the frequency domain, each with 10 subframes. In half-duplex FDD operation, the UE cannot transmit and receive at the same time while there are no such restrictions in full-duplex FDD. However, full-duplex FDD terminals need high quality and expensive RF duplex-filters to separate uplink and downlink channels, while half-duplex FDD allows hardware sharing between the uplink and downlink, which offers a cost saving at the expense of reducing data rates by half. Half-duplex FDD UEs are also considered a good solution if the duplex separation between the uplink and downlink transmissions is relatively small. In such cases, the half-duplex FDD is the preferable approach to mitigate the cross-interference between the transmit and receive chains.

Frame Structure Type 2

Frame structure type 2 is applicable to the TDD mode. It is designed for coexistence with legacy systems such as the 3GPP TD-SCDMA-based standard. As shown in Figure 6.9, each radio frame of frame structure type 2 is of length T_f = 30720 · T_s = 10 ms, which consists of two half-frames of length 5 ms each. Each half-frame is divided into five subframes with 1 ms duration. There are special subframes, which consist of three fields: Downlink Pilot TimeSlot (DwPTS), Guard Period (GP), and Uplink Pilot TimeSlot (UpPTS). These fields are already defined in TD-SCDMA and are maintained in the LTE TDD mode to provide sufficiently large guard periods for the equipment to switch between transmission and reception.

Figure 6.9 Frame structure type 2.

• The DwPTS field: This is the downlink part of the special subframe, and can be regarded as an ordinary but shorter downlink subframe for downlink data transmission. Its length can be varied from three up to twelve OFDM symbols.
• The UpPTS field: This is the uplink part of the special subframe, and has a short duration with one or two OFDM symbols. It can be used for transmission of uplink sounding reference signals and random access preambles.
• The GP field: The remaining symbols in the special subframe that have not been allocated to DwPTS or UpPTS are allocated to the GP field, which is used to provide the guard period for the downlink-to-uplink and the uplink-to-downlink switch.

The total length of these three special fields has a constraint of 1 ms. With the DwPTS and UpPTS durations mentioned above, LTE supports a guard period ranging from two to ten OFDM symbols, sufficient for cell size up to and beyond 100 km. All other subframes are defined as two slots, each with length T_slot = 0.5 ms.

Figure 6.9 only shows the detail structure of the first half-frame. The second half-frame has the similar structure, which depends on the uplink-downlink configuration. Seven uplink-downlink configurations with either 5 ms or 10 ms downlink-to-uplink switch-point periodicity are supported, as illustrated in Table 6.3, where "D" and "U" denote subframes reserved for downlink and uplink, respectively, and "S" denotes the special subframe. In the case of 5 ms switch-point periodicity, the special subframe exists in both half-frames, and the structure of the second half-frame is the same as the first one depicted in Figure 6.9. In the case of 10 ms switch-point periodicity, the special subframe exists in the first half-frame only. Subframes 0, 5, and the field DwPTS are always reserved for downlink transmission, while UpPTS and the subframe immediately following the special subframe are always reserved for uplink transmission.

Table 6.3. Uplink-Downlink Configurations for the LTE TDD Mode

6.3.2 Physical Resource Blocks for OFDMA

The physical resource in the downlink in each slot is described by a time-frequency grid, called a resource grid, as illustrated in Figure 6.10. Such a time-frequency plane representation is a common practice for OFDM systems, which makes it intuitive for radio resource allocation. Each column and each row of the resource grid correspond to one OFDM symbol and one OFDM subcarrier, respectively. The duration of the resource grid in the time domain corresponds to one slot in a radio frame. The smallest time-frequency unit in a resource grid is denoted as a resource element. Each resource grid consists of a number of resource blocks, which describe the mapping of certain physical channels to resource elements. The detail of these resource units is described as follows.

Figure 6.10 The structure of the downlink resource grid.

Resource Grid

The structure of each resource grid is characterized by the following three parameters:

• The number of downlink resource blocks : It depends on the transmission bandwidth and shall fulfill , where and are for the smallest and largest downlink channel bandwidth, respectively. The values of for several current specified bandwidths are listed in Table 6.2.
• The number of subcarriers in each resource block : It depends on the subcarrier spacing Df, satisfying , that is, each resource block is of 180kHz wide in the frequency domain. The values of for different subcarrier spacings are shown in Table 6.4. There are a total of subcarriers in each resource grid. For downlink transmission, the DC subcarrier is not used as it may be subject to a too high level of interference.

  Table 6.4. Physical Resource Block Parameters for the Downlink

  Configuration
  Normal CP	Df = 15kHz	12	7
  Extended CP	Df = 15kHz	12	6
  	Df = 7.5kHz	24	3

• The number of OFDM symbols in each block : It depends on both the CP length and the subcarrier spacing, specified in Table 6.4.

Therefore, each downlink resource grid has resource elements. For example, with 10MHz bandwidth, Df = 15kHz, and normal CP, we get from Table 6.2, and from Table 6.4, so there are 50 x 12 x 7 = 4200 resource elements in the downlink resource grid.

In case of multiantenna transmission, there is one resource grid defined per antenna port. An antenna port is defined by its associated reference signal, which may not correspond to a physical antenna. The set of antenna ports supported depends on the reference signal configuration in the cell. As discussed in Section 6.2.3, there are three different reference signals defined in the downlink, and the associated antenna ports are as follows:

• Cell-specific reference signals support a configuration of 1, 2, or 4 antenna ports and the antenna port number p shall fulfill p = 0, p {0, 1}, and p {0, 1, 2, 3}, respectively.
• MBSFN reference signals are transmitted on antenna port p = 4.
• UE-specific reference signals are transmitted on antenna port p = 5.

We will talk more about antenna ports when discussing MIMO transmission in the downlink in Section 7.2.2.

Resource Element

Each resource element in the resource grid is uniquely identified by the index pair (k, l) in a slot, where k = 0, 1,..., and l = 0, 1,..., are indices in the frequency and time domains, respectively. The size of each resource element depends on the subcarrier spacing Df and the CP length.

Resource Block

The resource block is the basic element for radio resource allocation. The minimum size of radio resource that can be allocated is the minimum TTI in the time domain, that is, one subframe of 1 ms, corresponding to two resource blocks. The size of each resource block is the same for all bandwidths, which is 180kHz in the frequency domain. There are two kinds of resource blocks defined for LTE: physical and virtual resource blocks, which are defined for different resource allocation schemes and are specified in the following section.

6.3.3 Resource Allocation

Resource allocation's role is to dynamically assign available time-frequency resource blocks to different UEs in an efficient way to provide good system performance. In LTE, channel-dependent scheduling is supported, and transmission is based on the shared channel structure where the radio resource is shared among different UEs. Therefore, with resource allocation techniques described in Chapter 4, multiuser diversity can be exploited by assigning resource blocks to the UEs with favorable channel qualities. Moreover, resource allocation in LTE is able to exploit the channel variations in both the time and frequency domain, which provides higher multiuser diversity gain than HSPA that can only exploit the time-domain variation. Given a wide bandwidth in LTE, this property is beneficial especially for slow-time varying channels, such as in the scenario with low mobility, where taking advantage of channel selectivity in the time domain is difficult.

With OFDMA, the downlink resource allocation is characterized by the fact that each scheduled UE occupies a number of resource blocks while each resource block is assigned exclusively to one UE at any time. Physical resource blocks (PRBs) and virtual resource blocks (VRBs) are defined to support different kinds of resource allocation types. The VRB is introduced to support both block-wise transmission (localized) and transmission on non-consecutive subcarriers (distributed) as a means to maximize frequency diversity. The LTE downlink supports three resource allocation types: type 0, 1, and 2 [8]. The downlink scheduling is performed at the eNode-B based on the channel quality information fed back from UEs, and then the downlink resource assignment information is sent to UEs on the PDCCH channel.

A PRB is defined as consecutive OFDM symbols in the time domain and consecutive subcarriers in the frequency domain, as demonstrated in Figure 6.10. Therefore, each PRB corresponds to one slot in the time domain (0.5 ms) and 180kHz in the frequency domain. PRBs are numbered from 0 to in the frequency domain. The PRB number n_PRB of a resource element (k, l) in a slot is given by:

The PRB is to support resource allocations of type 0 and type 1, which are defined for the DCI format 1, 2, and 2A.

• In type 0 resource allocations, several consecutive PRBs constitute a resource block group (RBG), and the resource allocation is done in units of RBGs. Therefore, a bitmap indicating the RBG is sufficient to carry the resource assignment. The allocated RBGs to a certain UE do not need to be adjacent to each other, which provides frequency diversity. The RBG size P, that is, the number of PRBs in each RBG, depends on the bandwidth and is specified in Table 6.5. An example of type 0 resource allocation is shown in Figure 6.11, where P = 4 and RBGs 0, 3, 4, ..., are allocated to a particular UE.

  Table 6.5. Resource Allocation RBG Size vs. Downlink System Bandwidth

  Downlink Resource Blocks	RBG Size (P)
  10	1
  11 – 26	2
  27 – 63	3
  64 – 110	4

  

  Figure 6.11 Examples of resource allocation type 0 and type 1, where the RBG size P = 4.

• In type 1 resource allocations, all the RBGs are grouped into a number of RBG subsets, and certain PRBs inside a selected RBG subset are allocated to the UE. There are a total of P RBG subsets, where P is the RBG size. An RBG subset p, where 0 p < P, consists of every P -th RBG starting from RBG p. Therefore, the resource assignment information consists of three fields: the first field indicates the selected RBG subset, the second field indicates whether an offset is applied, and the third field contains the bitmap indicating PRBs inside the selected RBG subset. This type of resource allocation is more flexible and is able to provide higher frequency diversity, but it also requires a larger overhead. An example of type 1 resource allocation is shown in Figure 6.11, where P = 4 and the RBG subset 0 is selected for the given UE.

In type 2 resource allocations that are defined for the DCI format 1A, 1B, 1C, and 1D, PRBs are not directly allocated. Instead, VRBs are allocated, which are then mapped onto PRBs. A VRB is of the same size as a PRB. There are two types of VRBs: VRBs of the localized type and VRBs of the distributed type.

For each type of VRB, a pair of VRBs over two slots in a subframe are assigned together with a single VRB number, n_VRB . VRBs of the localized type are mapped directly to physical resource blocks such that the VRB number n_VRB corresponds to the PRB number n_PRB = n_VRB . For VRBs of the distributed type, the VRB numbers are mapped to PRB numbers according to the rule specified in [6].

For resource allocations of type 2, the resource assignment information indicates a set of contiguously allocated localized VRBs or distributed VRBs. A one-bit flag indicates whether localized VRBs or distributed VRBs are assigned.

Details about the downlink resource allocation can be found in [8]. The feedback for channel quality information and the related signaling is discussed in Chapter 9.

6.3.4 Supported MIMO Modes

Multiantenna transmission and reception (MIMO), as described in Chapter 5, is a physical layer technique that can improve both the reliability and throughput of the communications over wireless channels. It is considered a key component of the LTE physical layer from the start. The baseline antenna configuration in LTE is two transmit antennas at the cell site and two receive antennas at the UE. The higher-order downlink MIMO is also supported with up to four transmit and four receive antennas.

The downlink transmission supports both single-user MIMO (SU-MIMO) and multiuser MIMO (MU-MIMO). For SU-MIMO, one or multiple data streams are transmitted to a single UE through space-time processing; for MU-MIMO, modulation data streams are transmitted to different UEs using the same time-frequency resource. The supported SU-MIMO modes are listed as follows:

• Transmit diversity with space frequency block codes (SFBC)
• Open-loop spatial multiplexing supporting four data streams
• Closed-loop spatial multiplexing, with closed-loop precoding as a special case when channel rank = 1
• Conventional direction of arrival (DOA)-based beamforming

The supported MIMO mode is restricted by the UE capability. The PDSCH physical channel supports all the MIMO modes, while other physical channels support transmit diversity except PMCH, which only supports single-antenna--port transmission. The details about MIMO transmission on each downlink physical channel are provided in Chapter 7, while the feedback to assist MIMO transmission is discussed in Chapter 9.