9.4 Maps of Multiple Access: DL-MAP and UL-MAP

The broadcasted DL-MAP and UL-MAP MAC management messages define the access to the downlink and uplink information respectively. The DL-MAP is a MAC management message that defines burst start times on the downlink. Equivalently, the UL-MAP is a set of information that defines the entire (uplink) access for all SSs during a scheduling interval. Then DL-MAP and UL-MAP are directories, broadcasted by the BS, of downlink and uplink frames. Figure 9.8 shows an example of DL-MAP and UL-MAP use in the FDD mode.

image from book
Figure 9.8: DL-MAP and UL-MAP indicate the use of downlink and uplink subframes (the FDD mode). (From IEEE Std 802.16-2004 ^[1]. Copyright IEEE 2004, IEEE. All rights reserved.)

For OFDM and OFDMA (or both WiMAX) PHY layers, access grants of DL-MAP and UL-MAP are in units of symbols and (for OFDMA) subchannels. Timing information in the DL-MAP and UL-MAP is relative. The following time instants are used as references for timing information for each of these two timings:

DL-MAP: the start of the first symbol (including the preamble if present) of the frame in which the message was transmitted;
UL-MAP: the start of the first symbol (including the preamble if present) of the frame in which the message was transmitted plus the value of the Allocation Start Time (whose value is given in the UL-MAP message, see below).

Information in the DL-MAP is about the current frame (the frame in which the DL-MAP message is sent). Information carried in the UL-MAP concerns a time interval starting at the Allocation Start Time measured from the beginning of the current frame and ending after the last specified allocation. Therefore, two possibilities exist concerning which frame is concerned with UL-MAP (differentiated by the Allocation Start Time field in the UL-MAP):

UL-MAP n serve the frame n + 1 (as in Figure 9.8), identified as the maximum time relevance of DL-MAP and UL-MAP in Reference ^[1].
UL-MAP n serves the frame n, identified as the minimum time relevance of DL-MAP and UL-MAP in Reference ^[1].

These two timings can be used for both the TDD and FDD variants of operation.

9.4.1 DL-MAP Message

The DL-MAP is a MAC management message that defines burst start time and profiles on the downlink. Each burst start time is indicated by a DL-MAP_IE (DL-MAP Information Elements). The DL-AMP_IE format is PHY layer-dependent. The BSs generate OFDM PHY DL-MAP messages in the format shown in Figure 9.9, including all of the following parameters:

MAC management message type (= 2 for DL-MAP).
PHY synchronisation. The PHY synchronisation field is dependent on the PHY specification used. This field is empty (zero bytes long) for the OFDM PHY Layer.
DCD count. The value of the Configuration Change Count (CCC field) of the DCD, which describes the downlink burst profiles concerned by this map.
Base Station ID. The Base Station ID is a 48-bit long field identifying the BS. The Base Station ID is programmable: the most significant 24 bits are used as the operator ID. This is a network management hook that can be combined with the Downlink Channel ID of the DCD message for handling edge-of-sector and edge-of-cell situations. Evidently, this is not the MAC address of the BS.

Management message type of DL-MAP (=2); (8 bits)	DCD count (8 bits)	Base Station ID (48 bits)	CID=j₁ DIUC=j₁ Start Time =t₁	CID=j₂ DIUC=j₂ Start Time=t₂	CID=j₃ DIUC=j₃ Start Time=t₃	– –
				DL–MAP IE₁	DL–MAP IE₂	DL–MAP IE₁

Open table as spreadsheet

Figure 9.9: DL-MAP MAC management message general from for OFDM PHY. Each DL-MAP IE indicates the start time of a downlink burst and the burst profile (channel details including physical attributes) of this burst.

The remaining part of a DL-MAP is the encoding of the DL-MAP IEs that are PHY-specification dependent. The DL-MAP IE of the OFDM PHY Layer has the format shown in Figure 9.10 and includes all of the following parameters:

Connection IDentifier (CID). This realises the assignment of the IE to a broadcast, multicast or unicast address. If the broadcast or multicast CID is used then it is possible to concatenate unicast MAC PDUs (with different CIDs) into a single downlink burst. During a broadcast or multicast downlink burst, it is the responsibility of the BS to ensure that any MAC PDUs sent to an H-FDD SS do not overlap any uplink allocations for that SS. An H-FDD SS for which a DL-MAP_IE and UL-MAP_IE overlap in time uses the uplink allocation and discards downlink traffic during the overlapping period.
DIUC. The 4-bit DIUC defines the burst type associated with that burst time interval. Burst profile descriptions are part of the DCD message for each DIUC used in the DL-MAP except those associated with Gap, End of Map and Extended (see DIUC in Section 9.5 below).
Preamble present. If set, the indicated burst will start with the short preamble (see above for preambles). In the downlink, a short preamble can be optionally inserted at the beginning of a downlink burst in addition to the long preamble that exists by default at the beginning of the frame.
Start Time. This indicates the start time, in units of OFDM symbol duration, relative to the start of the first symbol of the PHY PDU (including the preamble) where the DL-MAP message is transmitted. The time instant indicated by the Start Time value is the transmission times of the first symbol of the burst including the preamble (if present). The end of the last allocated burst is indicated by allocating an End of Map burst (DIUC = 14) with zero duration (see the DIUC part in Section 9.5.5).

CID	16 bits
DIUC	4 bits
Preamble present	1 bit (0=not present, 1=present)
Start Time	11 bits

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Figure 9.10: DL-MAP IE fields for the OFDM (WiMAX) PHY Layer.

If the length of the DL-MAP message is a nonintegral number of bytes, the LEN field in the MAC header is rounded up to the next integral number of bytes. The message is padded to match this length, but the SS disregards the pad bits.

9.4.2 UL-MAP Message

The UL-MAP message allocates access to the uplink channel. The general format of the UL-MAP message is almost identical to DL-MAP and is shown in Figure 9.11. There is only one new field: the Allocation Start Time, which is the start time of the uplink allocation. The unit of the Allocation Start Time is the PS starting from the beginning of the downlink frame in which the UL-MAP message is placed. For the OFDM PHY, the minimum value specified for this parameter is defined as the point in the frame 1 ms after the last symbol of the UL-MAP. The Start Time field is in units of OFDM symbol duration (as for DL-MAP_IEs).

Management message type of UL–MAP (=3); (8 bits)	UCD count (8 bits)	Base Station ID (48 bits)	Allocation Start time (32 bits)	CID=j₁ UIUC=j₁ Start Time=t₁ Duration=D₁	CID=j₂ UIUC=j₂ Start Time=t₂ Duration=D₂	–––
					UL–MAP IE₁	UL-MAP IE₂

Open table as spreadsheet

Figure 9.11: UL-MAP MAC management message general form. For the sake of simplicity, not all the fields are shown in this figure. Each UL-MAP IE indicates the start time of an uplink burst and the burst profile (channel details including physical attributes) of this burst.

UL-MAP IE has some new elements with regard to DL-MAP:

Duration indicates the duration, in units of OFDM symbols, of the allocation. The duration is inclusive of the preamble, the midambles and the postamble, contained in the allocation.
Subchannel Index corresponds to the frequency offset indices of the subcarriers of the allocated subchannel (see Chapter 5 for OFDM subchannellisation).
Midamble Repetition Interval indicates whether there is a midamble and, if there is one, the midamble repetition interval in OFDM symbols (8, 16 or 32 data symbols). When the last section of the symbol after the last midamble is longer than half the midamble repetition interval, a postamble must be added at the end of the allocation.

When specifically addressed to allocate a bandwidth grant to an SS, the CID is the Basic CID of the SS. A detailed example of a UL-MAP message with numerical values is given in Chapter 10. Contentions slots at the beginning of an uplink subframe are included in this example.

Within a frame, the switch from non-AAS to AAS-enabled traffic is marked by using UIUC = 15 with the AAS_IE to indicate that the subsequent allocation until the end of the frame is for AAS traffic ^[1]. Stations not supporting the AAS functionality ignore the portion of the frame marked for AAS traffic.

9.4.3 OFDMA PHY UL-MAP and DL-MAP Messages

The DL-MAP IEs and UL-MAP IE are PHY-specification dependent. The OFDMA DL-MAP IE defines a two-dimensional allocation pattern instead of one for OFDM DL-MAP IE. The OFDMA DL-MAP IE parameters are shown in Table 9.2.

Table 9.2: OFDMA DL-MAP IE main parameters
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DIUC	Used for the burst
OFDMA symbol offset	The offset of the OFDMA symbol in which the burst starts, measured in OFDMA symbols from the beginning of the downlink frame in which the DL-MAP is transmitted
Subchannel offset	The lowest index OFDMA subchannel used for carrying the burst, starting from subchannel 0
Boosting	Indication of whether the subcarriers for this allocation are power boosted
Number of OFDMA triple symbols	The number of OFDMA symbols that are used (fully or partially) to carry the downlink PHY burst. The value of the field is given in multiples of three symbols
Number of subchannels	The number of subchannels with subsequent indexes used to carry the burst
Repetition coding indication	Indicates the repetition code used inside the allocated burst

The OFDMA UL-MAP IE has almost the same parameters as the OFDMA DL-MAP IE.

A parameter proper to OFDMA UL-MAP IE is the Ranging Method parameter, which indicates one of two possible ranging bursts:

Initial Ranging/Handover Ranging;
BW Request/Periodic Ranging.