RFWiMAX Ranging Procedure: Initial, Periodic, Contention & Non-Contention
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This page describes the fixed WiMAX ranging procedure and mentions various ranging types as per IEEE 802.16 OFDM MAC layer specifications.
It covers RNG REQ and RNG RSP MAC messages used for ranging by the WiMAX SS (Subscriber Station). As soon as a WiMAX subscriber is switched on, it establishes an RF communication link by scanning available RF channels.
After this, it synchronizes itself with the Base Station (BS) using the received long preamble (2 symbols), performing time, frequency, and channel estimation and equalization.
After synchronization, it decodes the header (coded with BPSK1/2) information, which helps to know the modulation-code rate information of downlink burst #1 carrying the broadcast message.
This broadcast message transmits DLMAP, DCD, ULMAP, and UCD. Based on ULMAP and UCD, the SS knows when it has to transmit ranging messages in the uplink subframe. The process by which the SS and BS maintain the RF communication link between them is referred to as ranging.
As mentioned, RNG REQ and RNG-RSP MAC management messages, or a special CDMA code modulated message, are transmitted by the WiMAX SS for this purpose. There are various WiMAX ranging procedures, as mentioned below:
1. Initial Ranging
- Acquire correct transmission parameters (Power adjustment, timing offset estimation, and synchronization) to communicate with the BS.
2. Periodic Ranging
- To maintain uplink communication with the BS
- Adjust transmission parameters
3. Contention Ranging
- Required by the SS to access the system for the first time
- No dedicated connection resources assigned to the SS
4. Non-Contention Ranging
- Regulated by the BS to allow the SS to finish system access earlier when a dedicated channel is provided.
- Polled initial ranging
WiMAX RNG REQ
| RNG REQ field | Example Value |
|---|---|
| GEN HEADER(6 bytes) | 0x00, 0x40, 0x1a, 0x00, 0x00, 0xbe |
| Ranging req.type(1 byte) | 0x04 |
| DL Channel ID (1 byte) | 0x00 |
| Requested DL Burst Profile (TLV) | 0x01, 0x01, 0x11 |
| SS MAC Address(TLV) | 0x02, 0x06, 0x00, 0x0a, 0x1e, 0x0a, 0x1e, 0x01 |
| MAC Version(TLV format) | 0x94, 0x01, 0x04 |
| CRC(4 bytes) | 0x5d, 0x00, 0x9c, 0xfb |
| padding bytes if any | 0x00, 0x00 |
WiMAX RNG RSP
| RNG RSP field | Example Value |
|---|---|
| Genetic MAC Header(6 bytes) | 0x00, 0x40, 0x32, 0x00, 0x00, 0x00 |
| initial ranging msg type | 0x05 |
| Uplink channel id | 0x00 |
| SS mac address | 0x08, 0x06, 0x00, 0x0a, 0x1e, 0x0a, 0x1e, 0x01 |
| basic cid | 0x09, 0x02, 0x00, 0x08 |
| primary cid | 0x0a, 0x02, 0x10, 0x08 |
| ranging status | 0x04, 0x01, 0x01 |
| timing adjust | 0x01, 0x04, 0xff, 0xff, 0xff, 0x9e |
| power level adjust | 0x02, 0x01, 0xec |
| offset frequency adjust | 0x03, 0x04, 0xff, 0xff, 0xfd, 0xdb |
| downlink operational burst profile | 0x07, 0x02, 0x01, 0x01 |
| CRC | 0x00, 0x00, 0x00, 0x00 |
| Padding if required | 0x00, 0x00 |
RNG REQ is transmitted by SS to BS. After performing Synchronization (Time, Freq, Channel) RNG REQ Frame is decoded to bits at BS. The “Type” field just after GMH will describe the message type, which is ‘0x04’ for RNG REQ, which tells the BS what needs to be done after receiving the message.
RNG RSP decoded by SS will analyze the following:
- ‘Timing Adjust’ field (signed 32 bit) and will drive PHY to advance frame Transmission accordingly.
- ‘Power level Adjust’ field (signed 8-bit, 0.25dB) and will drive RF attenuators at RF layer accordingly.
- ‘Offset Frequency Adjust’ field (signed 32 bit, Hz units) and will drive RF synthesizer/Ref. OCXO accordingly.
- Ranging Status (if status is ‘re range’) will indicate whether SS needs to retransmit RNG REQ or (if status is ‘Success’) start BW REQ transmission for determining slot for SBC REQ.
