RFWiMAX DLMAP vs ULMAP: Key Differences Explained
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This article explains the difference between DLMAP and ULMAP messages in a fixed WiMAX system, as defined by the IEEE 802.16 OFDM MAC specifications.
Figure: WiMAX frame structure
As shown in the figure above, a WiMAX frame consists of a downlink subframe (DL SF) and an uplink subframe (UL SF). The DLMAP and ULMAP messages are transmitted within the downlink subframe, specifically in the broadcast burst, as they contain critical system information.
The DLMAP message carries location information for downlink bursts within the frame, while the ULMAP message carries location information for uplink bursts. This location data is provided in terms of the number of OFDM symbols relative to the start of the frame.
Let’s delve into the details of each message:
WiMAX DLMAP
The DLMAP message contains DLMAP Information Elements (IEs). The number of IEs corresponds to the number of downlink bursts. For example, if there are four downlink bursts, there will be four DLMAP IEs.
The following table outlines the fields within the DLMAP message:
| WiMAX DLMAP Field | Example Value |
|---|---|
| Generic MAC Header (6 bytes) | 0x00, 0x40, 0x16, 0xff, 0xff, 0x00 |
| DLMAP type (1 byte - 8 bits) | 0x02 |
| DCD count (8 bits) | 0x01 |
| BS ID (48 bits) | 0x00, 0x09, 0x02, 0x00, 0x9f, 0x45 |
| CID (2) (16 bits) | 0x00, 0x00 |
| DIUC (2) (4bits) | 0xe (Used for scrambler seed value) (END OF MAP) |
| Preamble (2) (1 bit) | 0 |
| Start time (#Burst2) (11 bits) | Gives the start time of DL Burst#2. In units of symbol duration, relative to the start of the first symbol of the PHY PDU. For instance, |
| CRC | 0x00, 0x00, 0x00, 0x00 |
The DLMAP IE structure is as follows:
DL MAP IE = { CID, DIUC, preamble present, Start time }
WiMAX ULMAP
Similar to the DLMAP, the ULMAP message consists of ULMAP Information Elements (IEs). The number of ULMAP IEs corresponds to the number of uplink bursts in the frame.
The following table outlines the fields within the ULMAP message:
| WiMAX ULMAP Field | Example Value |
|---|---|
| Generic header (6 bytes) | 0x00, 0x40, 0x1d, 0xff , 0xff, 0x00 |
| UL MAP type (8 bits) | 0x03 |
| Uplink channel ID (8 bits) | 0x00 |
| UCD count (8 bits) | 0x01 |
| Allocation start time - 32 bits (in units of PSs) | Starting from after how many PSs uplink allocation will start. For example, (Long Preamble+FCH+DLB#1+silent period+ TTG=>=>5600 PSs) { 0x00, 0x00, 0x15, 0xE0} |
| CID (16 bits) | 0xFFFF for broadcast case |
| Start time (11 bits) | 0x000 |
| Sub channel Index (5 bits) | (10000) no subchannelization |
| UIUC (4 bits) | 0x2 |
| Duration (10 bits) | for example 0x008 |
| Midamble rep. interval (8 bits) | 0x00 |
| CRC - 4 bytes |
The UL MAP IE structure is as follows:
UL MAP IE = { CID, start time, sub channel index, UIUC, duration }
Summary
In essence, the DLMAP and ULMAP messages are crucial for the operation of a WiMAX system. They provide the necessary information for the Subscriber Station (SS) to locate and access both downlink and uplink bursts within the WiMAX frame. The difference lies in the direction of the bursts they describe: DLMAP for downlink and ULMAP for uplink.
