Introduction

In WiMAX, HARQ scheme is the optional part of the MAC. HARQ and associated parameters shall be specified and negotiated using SBCREQ/RSP messages during network entry or re-entry procedure. The utilization of HARQ is on a per-connection basis, that is, it can be enabled on a per CID basis by using the DSA/DSC messages

Sub Packet Generation

Two main variants of HARQ are supported, Chase Combining or Incremental Redundancy (IR). SS may support IR. MS may support either Chase Combining or IR. For IR, the PHY layer will encode the HARQ packet generating several versions of encoded subpackets. Each subpacket shall be uniquely identified using a subpacket identifier (SPID). For Chase Combining, the PHY layer shall encode the HARQ packet generating only one version of the encoded packet. As a result, no SPID is required for Chase Combining
For downlink HARQ operation, the BS will send a version of the encoded HARQ packet. The SS will attempt to decode the encoded packet on this first HARQ attempt. If the decoding succeeds, the SS will send an ACK to the BS. If the decoding fails, the SS will send a NAK to the BS. In response, the BS will send another HARQ attempt. The BS may continue to send HARQ attempts until the SS successfully decodes the packet and sends an acknowledgement.
In order to specify the start of a new transmission, one-bit HARQ identifier sequence number (AI_SN) is toggled on every HARQ retransmission attempt on the same HARQ channel. If the AI_SN changes, the receiver treats the corresponding HARQ attempt as belonging to a new encoder packet, and discards previous HARQ attempt with the same ARQ identifier.
In this document the main focus is on chase combining variant.

Sub Packet Transmission Rule

The transmitter shall make at most four copy of sub packet labeled as ‘00’, ‘01’,’10’,’11’. The rule of subpacket transmission is as follows,

At the first transmission, BS shall send the subpacket labeled '00'.
BS may send one among subpackets labeled '00', ‘01’,’10’ or '11' in any order.
BS can send more than one copy of any subpacket, and can omit any subpacket except the subpacket labeled '00’.

In order to specify the start of a new transmission, one-bit HARQ identifier sequence number (AI_SN) is toggled on every successful transmission of an encoder packet on the same HARQ channel. If the AI_SN changes, the receiver treats the corresponding subpacket as a subpacket belongs to a new encoder packet, and discards ever-received subpackets with the same ARQ identifier.

Harq Channel ID	AI_SN in case of CC variant	HARQ Burst
0	0	New
1	0	New
2	0	New
1	0	Retransmitted
1	1	New
2	0	Retransmitted
0	1	New

Example of AI_SN, ACID and type of burst combination.

The HARQ scheme is basically a stop-and-wait protocol. The ACK is sent by the SS after a fixed delay (synchronous ACK) defined by HARQ DL ACK delay for DL burst, which is specified in UCD message. Timing of retransmission, however, is flexible and corresponds to the asynchronous part of the HARQ. The ACK/ NACK is sent by the BS using the HARQ Bitmap IE, and sent by an SS using the fast feedback UL subchannel.

Brown color shows DL MAP. DL MAP contains the HARQ DL MAP IE which indicates the HARQ Burst in the DL subframe. In figure rectangle A1B2A2B1 is the HARQ Burst and in the HARQ Burst there contains three sub burst which is indicates by the DL Harq chase sub burst IE.

There are following Sub Burst IE for different uses.

DL HARQ Chase sub burst IE: - This sub burst IE is used in case of normal chase combining.
DL HARQ IR CTC sub burst IE: - This Sub burst IE is used in case of Incremental Redundancy HARQ with CTC modulation.
DL HARQ IR CC sub burst IE: - This Sub burst IE is used in case of incremental redundancy HARQ with the CC modulation.
MIMO DL Chase HARQ sub burst IE: - This sub burst IE is used in case of chase combining HARQ with MIMO support.
MIMO DL IR HARQ for CTC Sub burst IE: - This sub burst IE is used in case of incremental redundancy HARQ with CTC modulation and with MIMO support.
MIMO DL IR HARQ for CC Sub Burst IE: - This sub burst IE is used in case of incremental redundancy HARQ with CC modulation and with MIMO support.
MIMO DL STC HARQ Sub Burst IE: - This sub burst IE is used in case when new zone is specified with MIMO and HARQ.

In the following, some of the key IEs are described.

HARQ ACK Region Allocation IE

This IE may be used by the BS to define a UL region to include one or more ACK channel for HARQ supporting MS. The slot in the ACKCH region is divided into two half slots. The first half slot is composed of tiles 0, 2, 4 and the second half slot is composed of tiles 1, 3, 5. In the ACKCH region, ACK channel 2n is the first half of slot n; ACK Channel (2n+1) is the second half of slot n. The slot number n is increased first along the subchannel axis until the end of ACKCH region and then along the time axis. The HARQ enabled MS that receives the HARQ DL burst at frame “i" should transmit the ACK signal through ACK channel in the ACKCH region at frame “i+j”. The frame offset “j” is defined by the “HARQ ACK Delay for the DL Burst” field in the UCD message.

The half-subchannel offset in the ACKCH Region is determined by the order of HARQ enabled DL burst in the DL MAP. For example, when a MS receives a HARQ enabled burst at frame i, and the burst is the nth HARQ enabled burst among the HARQ related IEs, the MS should transmit HARQ ACK at nth half-subchannel in ACKCH Region that is allocated by the BS at frame (i+j). When the “ACK disable” bit is set (in DL HARQ IR CTC Sub-Burst IE format), no ACK channel is allocated for the sub-burst.

HARQ ACK IE

This IE is used by BS to send HARQ acknowledgment to UL HARQ enabled traffic. The bit position in the bitmap is determined by the order of the HARQ enabled UL bursts in the UL-MAP. The frame offset “j” between the UL burst and the HARQ ACK-BITMAP is specified by “HARQ_ACK_Delay_for UL Burst” field in the DCD message. For example, when a MS transmits a HARQ enabled burst at frame i and the burst is the nth HARQ enabled burst in the MAP, the MS should receive HARQ ACK at nth bit of the BITMAP which is sent by the BS at frame (i+j). The existence of this IE shall be optional.

HARQ Buffer Capability

Downlink/Uplink HARQ buffering capability indicates the maximal number of data bits the SS is able to store for downlink/uplink HARQ. The buffering capability is separately indicated for NEP/NSCH based incremental redundancy used for CTC, and for DIUC/duration based HARQ methods (Chase combining and CC-IR), and separately for uplink and downlink transmissions. The buffering capability is indicated by two parameters:

Number of bits per channel - this is the total number of data bits that the SS may buffer per HARQ channel.
Aggregation flag - when this flag is clear, the number of bits is counted separately for each channel.

When the flag is set, buffering capability may be shared between channels, as explained below. The number of bits per channel is indicated as follows:

For incremental redundancy CTC (NEP based): Number of bits is indicated by NEP code,
For Chase combining and CC-IR (DIUC based): Number of bits is indicated by a value K=0..63 according to the following equation: Number of bits = floor(512 2K/4) Bits.

When aggregation flag is clear, the number of bits that were allocated in each HARQ channel in the last transmission must not exceed Number of bits per channel. When aggregation flag is set, the sum over all HARQ channels, of the number of bits that were allocated in the HARQ channel in the last transmission, must not exceed the Number of bits per channel multiplied by the maximum number channels supported by the SS. Note that sum total of the data bits supported is the same in both cases is the same. The IR-CTC HARQ buffer capability shall also be applied to bursts for which ACK channel is not allocated (ACK disable is set).

PDU SN extended subheader for HARQ reordering in Service Flow message.

This TLV is valid only in HARQ enabled connection. It specifies whether PDU SN extended subheader should be applied by the transmitter on every PDU on this connection. This SN may be used by the receiver to ensure PDU ordering. This counter should start at 0 and should be reset after HHO/FBSS operations The relevance connections of this parameter when appears in REG-REQ/RSP messages are Basic, Primary and Secondary CIDs (each should have its own PDU numbering) Value of 0 in either of the messages means the endpoint does not support the PDU SN number for the specific connection. If both end points support PDU SN for the connection the larger SN number should be chosen.

HARQ service Flows

The HARQ Service Flows field specifies whether the connection uses HARQ or not. The relevance connections of this parameter when appears in REG-REQ/RSP messages are Basic, Primary and Secondary CIDs. Transport CIDs which have HARQ Connection enabled indication must only be transmitted inside HARQ PHY burst type. Basic, Primary and secondary CIDs which has HARQ Connection enabled indication can be either transmitted inside HARQ or non-HARQ PHY burst type

HARQ Channel Mapping

This TLV is valid only in HARQ enabled connection. It specifies a HARQ channel number that may be used to carry data from this connection. This TLV may specify more then one channel per connection. HARQ channels may share more then one connection. An absent of this TLV means all HARQ channels can be used by this connection. The absence of this TLV in any of the REQ or RSP messages of the connection creation means all HARQ channels can be used by this connection. The relevant connections of this parameter when appears in REG-REQ/RSP messages are Basic, Primary and Secondary CIDs.
HARQ channel Index (1 byte each)