Presentación de resultados y prueba de hipótesis

PDCCH CCE 3 PDCCH CCE 4 Slot n Slot n+1

Figure 4

Downlink Scheduling

2.4 Multiplexing

The current versions of the 3GPP specifications hint at, but do not make clear, the methods by which individual air interface traffic flows will be identified and therefore how they could be successfully multiplexed together.

However, one interpretation of the apparently contradictory scheduling concepts outlined in the specifications is as follows.

Each separate traffic flow, carrying one or more upper-layer EPS bearer connections, is assigned an LCID at the physical layer.

The use of the LCID has not yet been fully described. It must be assumed that the LCID is used to differentiate between upper-layer data flows. However, as the LCID is only 5 bits long, it does not seem likely that the LCID will be significant on a global or even cell-wide basis. What seems most likely is that the LCID will be used to differentiate between logical data flows belonging to the same UE and will therefore be significant under the umbrella of one C-RNTI.

Scheduling of non-multicast/broadcast traffic takes place on a ‘per UE’, or more precisely, on a ‘per C-RNTI’ basis.

The allocation of uplink or downlink capacity to a UE is signalled on the PDCCH or on a CCE by the inclusion of the UE’s C-RNTI and the parameters of the allocated capacity.

A MAC PDU will be mapped into the capacity thus signalled.

The MAC PDU format makes it possible to multiplex a number of upper-layer SDUs and control messages together, the position of each within the PDU being flagged by a MAC header 3-tuple showing the flow’s LCID.

In this scenario, each MAC PDU will carry traffic for one UE only, but will be capable of multiplexing together traffic from several parallel services connected to that UE.

In the absence of clear guidance from 3GPP, the scenario outlined above must be regarded as informed speculation only.

Further Reading: 3GPP TS 36.321; 36.300

SAE LCID 10

LCID 12

Voice bearer

Data bearer C-RNTI – a12b

LCID 10 L E LCID

12 L E

PDCCH Scheduling Grant to C-RNTI a12b

Figure 5 Multiplexing

2.5 QoS and Priority Handling

QoS definition is the responsibility of the RRC layer, but as it has a bearing on the functionality of the MAC scheduler it will be discussed here.

2.5.1 Downlink QoS

On the downlink, SAE bearers are assigned a QoS level of either Guaranteed Bit Rate (GBR) or Aggregate Maximum Bit Rate (AMBR).

GBR is applied to individual bearers, whilst AMBR is applied to a group of aggregated bearers which have been given no individual rate guarantees but which must share bandwidth with other members of the group.

2.5.2 Uplink QoS

On the uplink, EPS bearers are first assigned a priority level, determined by the traffic type or some other differentiator.

Each priority group in a cell is assigned a Prioritized Bit Rate (PBR), which limits the bit rates available to members of each group. Additionally, each bearer will be assigned its own specific Maximum Bit Rate (MBR).

Scheduling for uplink allocations is handled on the basis of decreasing priority. For example, EPS bearers with the highest priority are scheduled first and then others in decreasing priority until capacity is exhausted.

Further Reading: 3GPP TS 36.321, 36.300

Downlink:

Guaranteed Bit Rate (GBR)

Aggregate Maximum Bit Rate (AMBR)

Uplink:

Prioritized Bit Rate (PBR) Maximum Bit Rate (MBR)

Figure 6 QoS

2.6 Automatic Retransmission Request (ARQ) and Hybrid ARQ (HARQ) The ARQ process is managed by the RLC layer, while HARQ is handled by the physical layer and the MAC. Both processes are employed to manage the transmission and retransmission of transport blocks.

ARQ performs retransmission of RLC PDUs carried by RLC acknowledged mode connections that fail to arrive and are therefore not acknowledged. Based on Ack and Nack responses from the peer RLC entity, ARQ will simply retransmit failed PDUs in sequence.

HARQ is designed to speed up the retransmission cycle and to reduce the effect of retransmission problems such as head-of-line blocking. It does this in two ways.

Firstly, HARQ employs complex TB coding methods which initially reduce the amount of data transferred per block. This is achieved using ‘puncturing’ techniques, where specific bits in each TB are ‘knocked out’ of the block following a puncturing algorithm. The reduced size of each punctured TB therefore increases the overall capacity of the air interface.

For punctured blocks that are received unerrored, the inverse of the puncturing process restores the missing data ready for the block to be processed.

HARQ has two methods of dealing with errored blocks – chase combining and incremental redundancy. Chase combining simply retransmits the errored block and the receiving station attempts to build a ‘good’ copy of the original data from the received versions. Incremental redundancy is more complex and uses three puncturing algorithms on the original transport block. If the first copy of the block, punctured with the first algorithm, arrives with errors, the retransmitted block will be created using an alternative puncturing method. Again, the receiving station attempts to build a ‘good’ copy of the data from the blocks received.

The other technique employed by HARQ is ‘N-process Stop and Wait’.

In this technique the sequential stream of TBs travelling over a bearer is divided into a number of logical HARQ subchannels – for example, the 1st, 4th, 7th, etc. blocks could form subchannel A, the 2nd, 5th, 8th, etc. subchannel B and so on.

Retransmission of an errored TB belonging to subchannel A only affects subsequent TBs belonging to that subchannel. TBs belonging to other subchannels will not be affected by the need to stop and wait for the missing TB to be retransmitted.

Further Reading: 3GPP TS 36.321; 36.300

Punctured Transmission

Pattern 1

Pattern 2

Pattern 3 Original data block

Chase Combining Incremental Redundancy

N-process stop and wait subchannels

Sequential transport blocks assigned to different HARQ

subchannels

A B C A B C A B C

Figure 7 HARQ

3.1 RLC Modes

The E-UTRA RLC layer is responsible for managing the flow of each EPS bearer across the air interface.

To deal with the range of EPS bearer types, the E-UTRA RLC layer is required to support three different transfer modes: Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM).

Peer RLC layers dealing with the transfer of one bearer are known as RLC entities.

3.1.1 Transparent Mode (TM)

TM bearers receive no service from the RLC layer, not even the addition of an RLC header to PDUs. Transport blocks are simply encapsulated and transferred.

Error correction, retransmission and segmentation services for TM connections are assumed to take place at higher layers, if at all.

3.1.2 Unacknowledged Mode (UM)

UM bearers receive a connectionless service from the RLC layer.

Segmentation and reassembly of RLC PDUs may occur if required; sequential transfer and reordering of PDUs will be performed, duplicate and errored PDUs will be discarded but retransmission is not supported.

3.1.3 Acknowledged Mode (AM)

AM performs all of the functions of UM but also supports retransmission of errored or missing PDUs.

3 RLC LAYER

Further Reading: 3GPP TS 36.322, 36.300