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Localized and Distributed Resource Mapping • In some cases, downlink channel-dependent

scheduling, are not suitable to use or are not practically possible:

– For low-rate services (i.e., voice and the feedback signaling), they may lead to extensive relative overhead.

– At high mobility, it may be difficult to track the instantaneous channel conditions to the accuracy required for channel-dependent scheduling to be efficient.

• LTE allows for such distributed resource-block

allocation by resource allocation types 0 and 1

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Distributed Resource-Block Allocation • Drawbacks:

– For types 0 and 1, the minimum size of the allocated resource can be as large as four resource-block pairs and may thus not be suitable when resource allocations of smaller sizes are needed.

– Both these resource-allocation methods are associated with a relatively large PDCCH payload.

• Resource-allocation type 2 always allows for the allocation of a single resource-block pair and is also associated with a relatively small PDCCH payload size.

– Only allows for the allocation of resource blocks that are contiguous in the frequency domain.

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DCI Formats Used for Downlink Scheduling

10.4.4

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Virtual Resource Block (VRB) • A Virtual Resource Block (VRB) has used in

distributed resource-block allocation – In resource-allocation type 2 and in a single resource block

pair.

• The key to distributed transmission then lies in the mapping from VRB pairs to Physical Resource Block (PRB) pairs – to the actual physical resource used for transmission.

• Two types of VRBs: 1. Localized VRBs : there is a direct mapping from VRB pairs to

PRB pairs

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Distributed VRBs 2. Distributed VRBs : • Consecutive VRBs are not mapped to PRBs that are

consecutive in the frequency domain; – This provides frequency diversity between

consecutive VRB pairs. • Even a single VRB pair is distributed in the

frequency domain.

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Distributed VRBs A. The spreading in the frequency domain is done by

means of a block-based “interleaver” operating on resource-block pairs.

B. A split of each resource-block pair such that the two resource blocks of the resource-block pair are transmitted with a certain frequency gap in between. – This also provides frequency diversity for a single VRB

pair. – This step can be seen as the introduction of frequency

hopping on a slot basis.

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Distributed VRBs • Whether the VRBs are localized or distributed is

indicated on the associated PDCCH in type 2 resource allocation. – dynamically switch between distributed and localized

transmission – also mix distributed and localized transmission for different

terminals within the same subframe.

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Distributed VRBs • The exact size of the frequency gap depends on the

overall downlink cell bandwidth according to Table 10.1.

• Based on two criteria: 1.The gap should be of the order of half the downlink cell

bandwidth in order to provide good frequency diversity also in a single VRB pair.

2.The gap should be a multiple of P2, where P is the size of a resource-block group and used for resource allocation types 0 and 1.

» To ensure a smooth coexistence in the same subframe between distributed transmission as described above and transmissions based on downlink allocation types 0 and 1.

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Distributed VRBs

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Multi-antenna Transmission • A mapping from the output of the data modulation to

the different antennas ports.

1. The input to the antenna mapping thus consists of the modulation symbols (QPSK, 16QAM, 64QAM) corresponding to the one or two transport blocks.

2. The output of the antenna mapping is a set of symbols for each antenna port.

3. The symbols of each antenna port are subsequently applied to the OFDM modulator – mapped to the basic OFDM time–frequency grid corresponding to that antenna port.

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Multi-antenna Transmission • Nine different transmission modes, depend on

– the specific structure of the antenna – reference signals are assumed to be used for

demodulation – type of CSI (channel-state information) feedback.

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Multi-antenna Transmission • The different multi-antenna transmission schemes

– Mode 1: Single-antenna transmission. – Mode 2: Transmit diversity. – Mode 3: Open-loop codebook-based precoding in

the case of more than one layer, transmit diversity in the case of rank-one transmission.

– Mode 4: Closed-loop codebook-based precoding. – Mode 5: Multi-user-MIMO version of transmission

mode 4.

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Multi-antenna Transmission – Mode 6: Special case of closed-loop codebook-

based precoding limited to single layer transmission.

– Mode 7: Release-8 non-codebook-based precoding supporting only single-layer transmission.

– Mode 8: Release-9 non-codebook-based precoding supporting up to two layers.

– Mode 9: Release-10 non-codebook-based precoding supporting up to eight layers.

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Transmit Diversity • Two antenna ports,

– LTE transmit diversity is based on Space-Frequency Block Coding (SFBC).

• Two consecutive modulation symbols Si and Si+1 are mapped directly to frequency-adjacent resource elements on the first antenna port.

• On the second antenna port the frequency-swapped and transformed symbols Si+1 *and Si

* are mapped to the corresponding resource elements

– “*” denotes complex conjugate.

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Transmit Diversity for Two Antenna Ports • A transmit-diversity signal is being transmitted

correspond to the cell-specific reference signals (CRS), more specifically CRS 0 and CRS 1 in the case of two antenna ports.

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Transmit Diversity for Four Antenna Ports • LTE transmit diversity is based on a combination of SFBC and

Frequency-Switched Transmit Diversity (FSTD). – It implies that pairs of modulation symbols are transmitted by

means of SFBC with transmission alternating between pairs of antenna ports (antenna ports 0 and 2 and antenna ports 1 and 3 respectively).

• Thus, combined SFBC/FSTD operates on groups of four modulation symbols and corresponding groups of four frequency-consecutive resource elements on each antenna port.

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Codebook-Based Precoding • Codebook-based precoding is the modulation

symbols corresponding to one or two transport blocks that are first mapped to NL layers.

– The number of layers may range from a minimum of one layer up to a maximum number of layers equal to the number of antenna ports.

• The layers are then mapped to the antenna ports by means of the precoder functionality.

The basic structure of LTE codebook-based antenna precoding.

cell-specific reference signals (CRS)

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Codebook-Based Precoding • As codebook-based precoding relies on the cell-

specific reference signals for channel estimation, and there are at most four cell-specific reference signals in a cell.

• Codebook-based precoding allows for a maximum of four antenna ports and, as a consequence, a maximum of four layers.

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One and two transport blocks • One transport block is a single layer (NL =1). • Two transport blocks is two or more layers (NL > 1). • A hybrid-ARQ retransmission, if only one of two

transport blocks needs to be retransmitted and that transport block was mapped to two layers for the initial transmission, the retransmission may also be carried out on two layers.

– A single transport block may also be transmitted using two layers.

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Codebook-Based Precoding • The mapping to layers is such that the number of

modulation symbols on each layer is the same and equal to the number of symbols to be transmitted on each antenna port.

• In three layers, – there should be twice as many modulation symbols

corresponding to the second transport block (mapped to the second and third layers) compared to the first transport block (mapped to the first layer).

• In four layers, – the first transport block is mapped to the first and

second layers while the second transport block is mapped to the third and fourth layers.

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Non-Codebook-Based Precoding • Non-codebook-based precoding is only applicable to

DL-SCH transmission. • The layer mapping also follows the same principles

as that of codebook-based precoding but is extended to support up to eight layers.

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Non-Codebook-Based Precoding • In particular, at least for an initial transmission, there

are two transport blocks per TTI. – except for the case of a single layer, in which case

there is only one transport block within the TTI.

• Similar to codebook-based precoding, for hybrid-ARQ retransmissions there may in some cases be a single transport block also in the case of multi-layer transmission.

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Carrier Aggregation and Cross-Carrier Scheduling

• Carrier aggregation, a terminal receives or transmits on multiple component carriers.

– LTE from release 10 onwards – The terminal needs to know to which component carrier a

certain DCI relations.

• Enabling cross-carrier scheduling is done individually via RRC (Radio Resource Control) signaling on a per-terminal and per-component-carrier basis.

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Carrier Aggregation and Cross-Carrier • Without cross-carrier scheduling: • Downlink scheduling assignments are valid for the

component carrier upon which they are transmitted. • For uplink grants, there is an association between

downlink and uplink component carriers such that each uplink component carrier has an associated downlink component carrier.

– The association is provided as part of the system information.

• From the uplink–downlink association, the terminal will know to which uplink component carrier the downlink control information relates to.

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Carrier Aggregation and Cross-Carrier • With cross-carrier scheduling: • where downlink PDSCH or uplink PUSCH is

transmitted on an (associated) component carrier, – the carrier indicator in the PDCCH provides

information about the component carrier used for the PDSCH or PUSCH.

• If cross-carrier scheduling is not configured then no carrier indication field is included in the DCI.

• Most of the DCI formats come in two “flavors”, with and without the carrier indication field.

– “flavor” is determined by enabling/ disabling support for cross-carrier scheduling.

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Carrier Aggregation and Cross-Carrier • To signal which component carrier a grant relates to,

the component carriers are numbered.

• The primary component carrier is always given the number zero,

• The secondary component carriers are assigned a unique number each through UE-specific RRC signaling.

• Even if the terminal and the eNodeB may have different understandings of the component carrier numbering during a brief period of reconfiguration, at least transmissions on the primary component carrier can be scheduled.

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Carrier Aggregation and Cross-Carrier • Irrespective of whether cross-carrier scheduling is

used or not, PDSCH/PUSCH on a component carrier can only be scheduled from one component.

• Where PDSCH/ PUSCH transmissions on component carrier 1 are scheduled using PDCCHs transmitted on component carrier 1.

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Carrier Aggregation and Cross-Carrier • PDSCH/PUSCH transmissions on component carrier

2 are crosscarrier scheduled from PDCCHs transmitted on component carrier 1.

– Hence, the DCI formats in the UE-specific search space for component carrier 2 include the carrier indicator.

• As transmissions on a component carrier can be

scheduled by PDCCHs on one component carrier only, component carrier 4 cannot be scheduled by PDCCHs on component carrier 5.