Downlink Data Transfer: Functionality Split Across Layers
HARQ Principles (within MAC Layer):
N-process Stop-And-Wait, Asynchronous adaptive HARQ
Uplink ACK/NAKs are sent on PUCCH or PUSCH.
PDCCH signals the HARQ process number and if it is a transmission or retransmission.
Retransmissions are always scheduled through PDCCH.
ARQ Principles (within RLC Layer):
ARQ retransmits RLC PDUs or RLC PDU segments.
ARQ retransmissions are based on RLC status reports and optionally ARQ/HARQ
interactions.
Polling for RLC status report is used when needed by RLC.
ARQ/HARQ Interaction:
Optional HARQ assisted ARQ operation
ARQ uses knowledge from the HARQ about transmission failure status and RLC
retransmission and re-segmentation can be initiated.
•DL Multiple Antenna Transmission: Generalized PDSCH TX
– 1 Code Word for rank 1 Transmission
– 2 Code Words for rank 2/3/4 Transmissions
• Layer Mapping: Fixed mapping between code words to layers
• Max Tx Antennas: 4
– Potentially up to 4 layers
• Pre-coding:
– Code book based precoding
• Channel Dependant precoding and Rank Indicator reported by the
UE for closed loop MIMO
A brief description of the generalized transmission blocks
scrambling of coded bits in each of the code words to be transmitted on a physical channel
modulation of scrambled bits to generate complex-valued modulation symbols
mapping of the complex-valued modulation symbols onto one or several transmission layers
precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports
mapping of complex-valued modulation symbols for each antenna port to resource elements
generation of complex-valued time-domain OFDM signal for each antenna port
Multiple Antenna Transmission/Reception (MIMO) schemes were inherent in the design of E-UTRA (R8)
MIMO classifications from the Number of Code Words Perspective:
Single Code Word (SCW) -> Use one Channel Coder per user
Multiple Code Word (MCW) -> Use Multiple Channel Coders per user
MCW offers better performance relative to SCW at the expense of additional complexity and signaling
LTE employs MCW (up to two Code Words) for MIMO transmissions.
Advantages and disadvantages of using either SCW or MCW
The SCW scheme has simple HARQ identical to SISO, simple ACK/NACK messaging identical to SISO, and low complexity MMSE receiver. The drawback of SCW is that it is not capacity achieving and suffers from a throughput loss in low rank channels, such as, spatially correlated channels or LOS channels with high Rician K-factor.
MCW allows Per Antenna Rate Control (PARC) and Successive Interference Cancellation (SIC). the MCW transmission with a successive interference cancellation (SIC) receiver is capacity achieving and hence optimal in performance. A successive interference cancellation (SIC) receiver is used to decouple the M layers providing higher throughput and more tolerance to spatial correlation. However, MCW with SIC comes at the cost of increased signaling overhead, receiver complexity and
memory requirements. The signaling overhead for the Reverse Link and Forward Link Control channels are larger than a SCW transmission since the channel quality CQI, coding and modulation, and acknowledgements have to be signaled for each MIMO layer. Furthermore, the SIC receiver memory requirements are high since the MIMO channel and received signals have to be stored for all HARQ transmissions. The receiver processing is more complicated and bursty since lower layers can not be
decoded until the upper layers are decoded.
M.M. Mao, et.al, “Multi-Antenna Techniques in Ultra Mobile Broadband Communication Systems,” IEEE Communications Networks and Services Conference, 2008
Mode 1: Single-antenna port; port 0
• Transmissions using a single Tx antenna at eNodeB (“conventional” approach)
Mode 2: Transmit diversity
• transmissions using Alamouti alike transmit diversity schemes
Mode 3: Open-loop spatial multiplexing
• transmissions using spatial multiplexing (up to two codewords), but no PMI feedback
• Also exploits Cyclic Delay Diversity (CDD) transmissions
Mode 4: Closed-loop spatial multiplexing
• transmissions in closed loop SU-MIMO configuration (PMI and RI feedback)
• Up to two codewords, 4 layers, 4 antennas
Mode 5: Multi-user MIMO
• Closed loop MU-MIMO or Space Division Multiple Access (SDMA) configuration
• Different users can use same time/frequency resources in different location within a cell
Mode 6: Closed-loop Rank=1 precoding
• Same as Mode 4 without the need of Rank reports
Mode 7: Single-antenna port; port 5
• Same as Mode 1 using UE-specific Reference Signals
PDSCH TX mode is signaled using different DCI Types over PDCCH
A UE not configured to receive PDSCH data transmissions based on one of the transmission modes may receive PDSCH data transmissions with DCI format 1A signalled by a PDCCH in its UE specific search spaces or the common search spaces.
-Multi-antenna transmission with 2 and 4 transmit antennas is supported.
-The maximum number of codeword is two irrespective to the number of antennas with fixed
mapping between code words to layers.
-Spatial division multiplexing (SDM) of multiple modulation symbol streams to a single UE using
the same time-frequency resource, also referred to as Single-User MIMO (SU-MIMO) is supported. When a MIMO channel is solely assigned to a single UE, it is known as SU-MIMO.
-Spatial division multiplexing of modulation symbol streams to different UEs using the same timefrequency
resource, also referred to as MU-MIMO, is also supported. There is semi-static switching between SU-MIMO and MU-MIMO per UE.