3G Evolution. HSPA and LTE for Mobile Broadband

multiplexing) of data to transmit. To each transport block, a CRC is attached and each such CRC-attached transport block is separately coded. The channel coding rate, including the rate matching necessary, is implicitly determined by the transport-block size, the modulation scheme, and the amount of resources assigned for transmission. All these quantities are selected by the downlink scheduler. The redundancy version to use is controlled by the hybrid-ARQ protocol and affects the rate-matching processing to generate the correct set of coded bits. Finally, in case of spatial multiplexing, the antenna mapping is also under control of the downlink scheduler.

The scheduled mobile terminal receives the transmitted signal and performs the reverse physical-layer processing. The physical layer at the mobile terminal also informs the hybrid-ARQ protocol whether the transmission was successfully decoded or not. This information is used by the MAC part of the hybrid-ARQ functionality in the mobile terminal to determine whether a retransmission shall be requested or not.

The physical-layer processing for the UL-SCH follows closely the processing for the DL-SCH. However, note that the MAC scheduler in the eNodeB is responsible for selecting the mobile terminal transport format and resources to be used for uplink transmission as described in Section 15.2.3. The UL-SCH physical layer processing is shown, in simplified form, in Figure 15.8.

1 transport blocks of dynamic size per TTI

Figure 15.8 Simplified physical-layer processing for UL-SCH.

The remaining downlink transport channels are based on the same general physical-layer processing as the DL-SCH, although with some restrictions in the set of features used. For the broadcast of system information on the BCH, a mobile terminal must be able to receive this information channel as one of the first steps prior to accessing the system. Consequently, the transmission format must be known to the terminals a priori and there is no dynamic control of any of the transmission parameters from the MAC layer in this case.

For transmission of paging messages on the PCH, dynamic adaptation of the transmission parameters can to some extent be used. In general, the processing in this case is similar to the generic DL-SCH processing. The MAC can control modulation, the amount of resources, and the antenna mapping. However, as an uplink has not yet been established when a mobile terminal is paged, hybrid ARQ cannot be used as there is no possibility for the mobile terminal to transmit an ACK/NAK.

The MCH is used for MBMS transmissions, typically with single-frequency network operation as described in Chapter 4 by transmitting from multiple cells on the same resources with the same format at the same time. Hence, the scheduling of MCH transmissions must be coordinated between the involved cells and dynamic selection of transmission parameters by the MAC is not possible.

15.4LTE states

In LTE, a mobile terminal can be in several different states as illustrated in Figure 15.9. At power-up, the mobile terminal enters the LTE_DETACHED state. In this state, the mobile terminal is not known to the network. Before any further communication can take place between the mobile terminal and the network, the mobile terminal need to register with the network using the random-access

Figure 15.9 LTE states.

16

LTE physical layer

In the previous chapter, the LTE radio-interface architecture was discussed with an overview of the functions and characteristics of the different protocol layers. This chapter will provide a more detailed discussion on the current state of the lowest of these protocol layers, the LTE physical layer.The next chapter will then go further into some specific LTE access procedures, including random access and cell search.

16.1Overall time-domain structure

Figure 16.1 illustrates the high-level time-domain structure for LTE transmission with each (radio) frame of length Tframe = 10 ms consisting of ten equally sized subframes of length Tsubframe = 1 ms.

To provide consistent and exact timing definitions, different time intervals within the LTE radio access specification can be expressed as multiples of a basic time unit Ts = 1/30720000.1 The time intervals outlined in Figure 16.1 can thus also be expressed as Tframe = 307200 · Ts and Tsubframe = 30720 · Ts.

One radio frame (Tframe = 10 ms)

One subframe (Tsubframe = 1 ms)

Figure 16.1 LTE time-domain structure.

1 The reason for this exact value of Ts will be clarified in the next section.

317

Within one carrier, the different subframes of a frame can either be used for downlink transmission or for uplink transmission. As illustrated in Figure 16.2a, in case of FDD, that is operation in paired spectrum, all subframes of a carrier are either used for downlink transmission (a downlink carrier) or uplink transmission (an uplink carrier). On the other hand, in case of operation with TDD in unpaired spectrum (Figure 16.2b), the first and sixth subframe of each frame (subframe 0 and 5) are always assigned for downlink transmission while the remaining subframes can be flexibly assigned to be used for either downlink or uplink transmission. The reason for the predefined assignment of the first and sixth subframe for downlink transmission is that these subframes include the LTE synchronization signals. The synchronization signals are transmitted on the downlink of each cell and are intended to be used for initial cell search as well as for neighbor-cell search. The principles of LTE cell search, including the structure of the synchronization signals, are described in more detail in Chapter 17.

As also illustrated in Figure 16.2, the flexible assignment of subframes in case of TDD allows for different asymmetries in terms of the amount of radio resources (subframes) assigned for downlink and uplink transmission, respectively. As the subframe assignment needs to be the same for neighbor cells in order to avoid severe interference between downlink and uplink transmissions between the cells, the downlink/uplink asymmetry cannot vary dynamically, on, for example, a

Downlink carrier

Uplink carrier

One subframe (Tsubframe = 1ms)

(a)

Time Division Duplex (TDD) Approximately

symmetric

Asymmetric (downlink focus)

Asymmetric (uplink focus)

First and sixth subframe always assigned for downlink transmission

(b)

Figure 16.2 Examples of downlink/uplink subframe assignment in case of TDD and comparison

with FDD.

frame-by-frame basis. However, it can be changed on a slower basis to, for example, match different traffic characteristics such as differences and variations in the downlink/uplink traffic asymmetry.

What is being illustrated in Figure 16.1 is sometimes referred to as the generic or Type 1 LTE frame structure. This frame structure is applicable for both FDD and TDD. In addition to the generic frame structure, for LTE operating with TDD there is also an alternative or Type 2 frame structure, specifically designed for coexistence with systems based on the current 3GPP TD-SCDMA-based standard. The remaining discussions within this and the following chapters will assume the Type 1 frame structure unless explicitly stated otherwise.

16.2Downlink transmission scheme

16.2.1 The downlink physical resource

As already mentioned in the overview of the LTE radio access provided in Chapter 14, LTE downlink transmission is based on Orthogonal Frequency Division Multiplex (OFDM). As described in Chapter 4, the basic LTE downlink physical resource can thus be seen as a time-frequency resource grid (see Figure 16.3), where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.2

For the LTE downlink, the OFDM subcarrier spacing has been chosen tof = 15 kHz. Assuming an FFT-based transmitter/receiver implementation, this corresponds to a sampling rate fs = 15000 · NFFT, where NFFT is the FFT size. The time unit Ts defined in the previous section can thus be seen as the sampling time of an FFT-based transmitter/receiver implementation with NFFT = 2048. It is important to understand though that the time unit Ts is introduced in the LTE radio-access

One resource element

Time

One OFDM symbol

Figure 16.3 The LTE downlink physical resource.

2 In case of multi-antenna transmission, there will be one resource grid per antenna.

coincide with the local-oscillator frequency at the base-station transmitter and/or mobile-terminal receiver. As a consequence, it may be subject to un-proportionally high interference, for example, due to local-oscillator leakage.

The total number of subcarriers on a downlink carrier, including the DC-subcarrier, thus equals Nsc = 12 · NRB + 1, where NRB is the number of resource blocks. The LTE physical-layer specification actually allows for a downlink carrier to consist of any number of resource blocks, ranging from 6 resource blocks up to more than 100 resource blocks. This corresponds to a nominal downlink transmission bandwidth ranging from around 1 MHz up to at least in the order of 20 MHz with a very fine granularity. As already touched upon in Chapter 14, this allows for a very high degree of LTE bandwidth/spectrum flexibility, at least from a physical- layer-specification point-of-view. However, as also touched upon in Chapter 14, LTE radio-frequency requirements are, at least initially, only specified for a limited set of transmission bandwidths, corresponding to a limited set of possible values for the number of resource blocks NRB.

Figure 16.5 outlines the more detailed time-domain structure for LTE downlink transmission. Each 1 ms subframe consists of two equally sized slots of length Tslot = 0.5 ms (15360 · Ts). Each slot then consists of a number of OFDM symbols including cyclic prefix.

One subframe two slots (Tsubframe 1 ms)

Tslot 0.5 ms

Normal CP

TCP : 160·Ts 5.2 s (first OFDM symbol), 144·Ts 4.7 s (remaining OFDM symbols)

TCP-e : 512·Ts 16.7 s

Figure 16.5 LTE downlink subframe and slot structure. One subframe consisting of two equally-sized slots. Each slot consisting of six or seven OFDM symbols in case of normal and extended cyclic prefix, respectively.