Chapter 6: Scheduling, Link Adaptation, and Hybrid ARQ We vant to efficiently handle the (inevitable) variations in instantaneous channel quality, due to, e.g., different kinds of fading, communication distance, interference from other cells, mobility, etc. Channel-dependent scheduling: Efficient resource allocation is desired. Link adaptation: Efficient use of radio links. Hybrid ARQ (Automatic Repeat-reQest): strategies for retransmissions of ”bad” data packets. Channel-dependent scheduling – the downlink: In LTE is downlink multiplexing performed in the OFDM time-frequency grid (combination of TDM and FDM). Each ms, the schedular assigns resource-blocks to users. Examples of some possible scheduling strategies: Max C/I (maximum rate scheduling): Assigns all resources to the user with the best instantaneous channel conditions. This gives a so called multi-user diversity gains. The max C/I gives a high system capacity, but in general it will not be fair, see Figures 6.2-6.3 on page 83. Round-robin scheduling: Pre-arranged (deterministic) assignment of resources to the users, independent of the instantaneous channel conditions. Can give a more fair allocation of resources. Proportional-fair scheduling: Resources are allocated to the user having the relatively best instantaneous channel conditions. Tends to ”follow the peaks”. Channel-dependent scheduling – the uplink: In LTE is uplink multiplexing performed in the OFDM time-frequency grid (combination of TDM and FDM). Each ms, the schedular assigns resource-blocks to users, such that the resource is efficienly utilized. The uplink resources, e.g., a frequency-time grid, should be allocated to the different users. Similar scheduling stratagies as in the downlink may be used. Users at the cell border will in general have significantly less resources (and low rate), while users close to the base station will have more resources (and higher rate). Channel-State Information (CSI): Known reference (or pilot) signals are sent from the transmitter. The receiver estimates the channel quality, and reports back to the transmitter. To be able to perform scheduling, channel state reports that are representative for the over-all time-frequency grid is needed. Retransmission schemes: As an important addition to conventional coding, ARQ can also be used. ARQ implies in principle that the receiver tries to find out if a data packet contains errors (error detection). If no errors are detected, the packet is considered correct (however, there can still be errors in the data packet) and a positive acknowledgement (ACK) is sent to the transmitter. If errors are detected a negative acknowledgement (NAK) is sent to the transmitter, and the same information is retransmitted. More or less advanced versions of ARQ-based solutions exists, and ARQ is widely used in practical systems. Typically, several retransmissions may some times be needed. As we will see later in this course ARQ can be implemented at different levels in the protocol structure. As long as the delay introduced by ARQ is small enough, the users will hardly notice it. Hybrid ARQ: Chase combining means that retransmissions are identical to the original transmission, i.e. repetition coding is obtained, see Figure 6.5 on page 91. Note that previous received packet(s) (with errors found) are saved and combined with the retransmitted packet. Incremental redundancy means that retransmissions typically are different than the original transmission, since they contain different coded bits. However, the carried information is identical with the original transmission. See Figure 6.6 ob page 92. Chapter 7: LTE Radio Access: An Overview Commercial network operation late 2009 (Release 8). LTE evolution is illustrated in Figure 7.1 on page 96. Chapters 8-18: LTE radio-access technology. An eNodeB is a concept approximatly equivalent to a base station. OFDM in the downlink, and DFT-spread OFDM in the uplink. The time-frequency grid is dynamically shared between users, combined with rate adaptation, see Figure 7.2 on page 98. Scheduling decisions can be made every ms, and covering blocks of 180 kHz (12 subcarriers). Channel-state information (CSI) contains channel quality information. Inter-cell interference coordination (ICIC) aims at improving the situation for users at the cell-edge. Fast hybrid ARQ with soft combining is used in LTE. Multiple antennas: Receive diversity, transmit diversity, beamforming, spatial multiplexing, multi-user MIMO. The base station selects the proper multiple-antenna scheme for each transmission and resource block. Release 8: Up to 4 layers can be spatially multiplexed. Flexible spectrum use is supported: different bandwidths, and different duplex modes (FDD, TDD). The basic bandwidth to use can be up to approximatly 20 MHz. LTE Release 9. Completed in late 2009. Support for Multicast/Broadcast Single-Frequency Network transmission (MBSFN). Note that the transmissions from the different cells should be time-synchronized. Support for determining the position of the terminal (not based on GPS), based on using special reference signals). Support for more advanced beamforming (dual-layer beamforming), i.e. more flexible use of multiple antennas. LTE Release 10 (LTE-Advanced), and IMT-Advanced Some ITU requirements for IMT-Advanced: At least 40 MHz bandwidth. A peak spectral efficiency of at least 15 bps/Hz in downlink (bitrate=600 Mbps). A peak spectral efficiency of at least 6.75 bps/Hz in uplink (bitrate=270 Mbps). User latency (delay) less than 10ms. Some of 3GPP targets/requirements are even more challenging. LTE Release 10 was completed in late 2010. Using carrier aggragation the bandwidth can be expanded up to 100 MHz (5x20MHz), see Figure 7.4 on page 104. Downlink spatial multiplexing is extended to up to eight layers. Supports downlink data rates up to 3 Gbps (spectral efficiency 30 bps/Hz). Supports uplink spatial multiplexing with up to four layers, and uplink data rates up to 1.5 Gbps (15bps/Hz). A relay node (a ”low-power base station”) is supported and to be used in the communication between the terminal and the base station, see Figure 7.5 on page 105. This technique can significantly improve the coverage. Introduce more advanced inter-cell handling for situations like, e.g., a pico-cell within a macro-cell. Terminals are classified in different UE categories depending on which capabilities they support, see Table 7.1 on page 106. Hence, all Release X terminals will not support all X-capabilites, only the high-end terminals will. Chapter 8: Radio-Interface Architecture The Radio-Access Network (RAN) handles all radio-related issues for the entire network. The connection between the Internet and RAN is handled by the Evolved Packet Core (EPC). RAN and EPC consists of several logical nodes, see Figures 8.1-2 on page 110. The connection between the internet and the EPC is handled by the logical node referred to as the Packet Data Network Gateway (P-GW). The user-plane connection between the EPC and the RAN is handled by the logical node referred to as the Serving Gateway (S-GW). The logical node eNodeB (may handle several cells) belongs to the RAN. The eNodeB uses two interfaces (user-plane and control-plane) to connect to the EPC. The user-plane RAN protocol structure consists of: PDCP (Packet Data Convergence Protocol). RLC (Radio-Link Control). MAC (Medium-Access Control). PHY (Physical Layer). Much more information is found in Figure 8.4 on page 112. There are several types so-called logical channels, and they are classified as control channels or traffic channels. As an example, the DTCH (Dedicated Traffic Channel) concerns user data transmission to/from a terminal. There are also several transport channels, and these are organized in transport blocks . As an example, downlink transport channel for user data is referred to as the DL-SCH (Downlink Shared Channel). Scheduling decisions are made every ms and scheduling info are sent to respective terminals. A physical channel is a subset of time-frequency resources. Some examples of physical channeld are: PDSCH (Physical Downlink Shared Channel) PDCCH (Physical Downlink Control Channel) PUSCH (Physical Uplink Shared Channel) PUCCH (Physical Uplink Control Channel) See also Figures 8.7 and 8.8 on pages 117-118. A resource block = 12 consequtive subcarriers over a time slot (0.5ms)= 84 (72) resource elements with a normal (extended) cyclic prefix. The minimum scheduling unit is a subframe (1 ms), i.e. a resource block pair. Each downlink subframe may carry both control data and user data. LTE Release 10: Carrier aggregation. A bandwidth of up to 100 MHz (5x20 MHz) can be used, both in downlink and in uplink. Intra-band aggregation Interband aggregation FDD: full-duplex, half-duplex (terminal). TDD: special subframes, guard period. Chapter 10: Downlink Physical-Layer Transmission DL-SCH transport-channel physical layer processing A transport block can be sent each TTI (= 1ms, Transmission Time Interval) to a user (in case of no spatial multiplexing). For each transport block: CRC (24 parity bits) Segmentation + CRC Coding (interleaver size at most 6144 bits, Turbo encoder), see Figure 10.3 on page 146. RM+HARQ (the schedular selects the redundancy version), see Figure 10.5 on page 147. Scramling (bit-level) Modulation (QPSK, 16-QAM, 64-QAM) Antenna mapping (assignment to OFDM time-frequency grid for each antenna) Note that some elements in a resource-block are reserved for control signals and pilots. Features to obtain frequency diversity is included, e.g., by splitting of resource-block pairs and transmitting the two resource-blocks with quite large frequency-separation. Consider OFDM and a 2x3 MIMO. At sub-carrier level: How can the channel matrix be obtained in the receiver? Downlink cell-specific reference signals (CRS) May be used for: channel estimation, CSI, cell-selection. Four known cell-specific reference symbols in each resource block. The period of the cell-specific sequence equals a 10 ms frame. See Figure 10.8 on page 154. CRS structure for several antennas is illustrated in Figure 10.10 on page 155. Note that CRS symbols are orthogonal to each other. Downlink demodulation reference signals (DM-RS) Aimed at a specific terminal. Special purpose channel estimation, and up to eight antennas. ”Similar” distribution within resource blocks. Downlink channel state information reference signals (CSI-RS) LTE Release 10. Used when DM-RS are used and motivated by Release 10 extensions to eight transmit antennas. Downlink demodulation reference signals (DM-RS) Aimed at a specific terminal. Special purpose channel estimation (transmission modes 7, 8 , 9), and up to eight antennas. 12 DM-RS symbols per resource block pair. Note in Figure 10.11 that for two transmit antennas the same 12 resource elements are used. So, called cover codes are used, and they are especially good assuming that the channel parameters change slowly with time. In the same way,, in Figure 10.12, with four transmit antennas, the same 12 resourceelements are used, but now the cover codes are 4 symbols long to be able to identify the four antennas. Downlink channel state information reference signals (CSI-RS) LTE Release 10. Used when DM-RS are used and motivated by Release 10 extensions to eight transmit antennas. Se Figure 10.13 on page 159. Multicast-Broadcast SFN reference signals (MBSFN-RS) These signals are used for channel estimation . Positioning reference signals To make it possible for the terminal to make measurements such that its geographical position can be determined. Multi-antenna transmission Data modulation, antenna mapping of QAM symbols, OFDM for each antenna. Transmission mode 1,….., Transmission mode 9. Transmit Diversity (Transmission mode 2) From Chapter 5: Space-Frequency Block Coding Channel estimation is assumed to use CRS. Closed-loop operation means that the network informs the terminal which precoding matrix W is chosen (from codebook), see also Table 10.3 on page 168. Open-loop operation: W is chosen in a deterministic way known by the terminal, e.g., in high mobility situations. Non-codebook based precoding Release 9 (max two layers) and 10 (max eight layers) Channel estimation is assumed to use DM-RS applied before precoding, see Figure 10.19. The receiver do not need knowledge about the precoding matrix W! However, the choice of W is based on terminal measurements.
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