Chapter 11: Network Entry and Quality of Service (QoS) Management
11.1 Ranging
The ranging process is defined in the 802.16 standard as the process that allows the SSs to:
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Acquire the correct timing offset of the network. The SS can then be aligned with the frame received from the BS (for OFDM and OFDMA PHYsical layers).
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Request power adjustments and/or downlink burst profile change. The SS can then be received within the appropriate reception thresholds
The SS realises ranging by transmitting the RNG-REQ MAC management message. The RNG-REQ message may be sent in the initial ranging uplink contention slots (see Chapter 10) or in data grant intervals allocated by the BS to the SS. Ranging is made by the SS at initialisation and periodically. This gives two types of ranging processes:
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the initial ranging;
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the periodic ranging (sometimes simply known as ranging).
In the OFDM PHYsical Layer, initial ranging uses the initial ranging uplink contention slots while the periodic ranging uses the regular (allocated uplink grant) bursts. The initial ranging procedure is a part of the network entry process described in Section 11.6.
11.1.1 Ranging Messages
Two MAC management messages are used for initial ranging: the Ranging Request (RNG-REQ) message and the Ranging Response (RNG-RSP) message. These messages are now detailed.
The Ranging Request (RNG-REQ) message is transmitted by the SS at initialisation. It can also be used at other periods to determine network delay and to request power and/or downlink burst profile change. The CID field value in the MAC header of the RNG-REQ message contains the following:
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Initial ranging CID (0x0000) if the SS is attempting to join the network or if the SS has not yet registered and is changing downlink (or both downlink and uplink) channels.
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In all other cases, the Basic CID is used (other cases assume that a Basic CID was assigned to the SS).
The parameters that may be included in the RNG-REQ message are shown in Table 11.1.
| RNG-REQ field | Description |
|---|---|
| Downlink channel ID | Identifier of the downlink channel on which the SS received the UCD describing the uplink channel on which this ranging req˜uest mess˜age is transmitted |
| Requested downlink burst profile | Indicated by the DIUe of the downlink burst profile requested by the the SS for downlink traffic |
| SS MAC address | Transmitted when the SS is attempting to join the network |
| MAC version | |
| Ranging anomalies | After the SS has received an RNG-RSP addressed to the SS, indicating that the SS is already at maximum power, minimum power, etc. |
| AAS broadcast capability |
Open table as spreadsheet The Ranging Response (RNG-RSP) message is transmitted by the BS in response to a received RNG-REQ. In addition, it may also be transmitted asynchronously to send corrections based on measurements that have been made on other received data or MAC messages. As a result, the SS is prepared to receive an RNG-RSP at any time, not just following an RNG-REQ transmission. Some of the parameters that may be included in the RNG-RSP message are shown in Table 11.2.
| RNG-RSP field | Description |
|---|---|
| Ranging status | Indicates whether uplink messages are received within acceptable limits by BS. This field is mandatory. Possible values are 1 = continue, 2 = abort, 3 = success, 4 = rerange |
| Timing adjust | The time required to advance the SS transmission to enable frames to arrive at the expected time instance at the BS. Units are PHYsical Layer specific. For the OFDM (WiMAX) PHYsical Layer, the unit is the symbol duration 1/fs). If this field is not included, no time adjustment is to be made |
| Power adjust | Transmitted power offset adjustment (signed 8-bit, in 0.25 dB units). This field specifies the relative change in the transmission power level that the SS must make in order for transmissions to arrive at the BS at the desired power. If this field is not included, no power adjustment is to be made |
| Offset frequency adjust | Transmitted frequency offset adjustment (signed 32-bit, Hz units). This field specifies the relative change in transmission frequency that the SS must make in order to better match the BS. This is a fine-frequency adjustment within a channel, not reassignment to a different channel |
| Basic CID | Assigned by the BS at initial access. In this case. the RNG GRSP message is being sent on the initial ranging CID in response to an RNG-REQ message that was itself sent on the initial ranging CID |
| Primary management CID | Assigned by the BS at initial access, in the same conditions as the Basic CID |
| MAC adress | SS MAC address (48-bit) |
| Downlink operational burst profile | Specifies the least robust DIUC that may he used by˜ {the BS for transmissions to the SS |
| Frame number | In which the corresponding RNG-REQ message (for OFDM/WiMAX) was received. When the frame number is included, the SS MAC address does not appear in the same message |
| Ranging subchannel | The OFDM ranging subchannel index that was used to transmit the initial ranging message |
| AAS broadcast permission |
Two other MAC management messages are used for periodic ranging: the Downlink Burst Profile Change Request (DBPC-REQ) Message and the Downlink Burst Profile Change Response (DBPC-RSP) Message.
The Downlink Burst Profile Change Request (DBPC-REQ) message is sent by the SS to the BS on the SS Basic CID to request a change in the downlink burst profile used by the BS to transport data to the SS. The required burst profile for downlink traffic is indicated by its corresponding DIUC (as defined in the DCD message). The DBPC-REQ message is sent at the current (BS allocated) burst profile type. If the SS detects fading on the downlink, the SS uses this message to request transition to a more robust burst profile. The general format of the DBPC-REQ message is shown in Figure 11.1. A request for change of the downlink burst profile is made with the DBPC-REQ Message (instead of the RNG-REQ message as described above) if the SS has been granted an uplink bandwidth, i.e. a data grant allocation to the SS Basic CID. Otherwise, the SS uses the RNG-REQ message for a request for change of the downlink burst profile (see Section 11.1.3).
| Management message type (=23) | Reserved | DIUC | Configuration change count |
Figure 11.1: DBPC-REQ MAC management message format. The required burst profile for downlink traffic is indicated by its corresponding DIUC (as defined in the DCD message)
The Downlink Burst Profile Change Response (DBPC-RSP) message is transmitted by the BS on the SS Basic CID in response to a DBPC-REQ message from the SS. If the DIUC parameter is the same as requested in the DBPC-REQ message, the request was accepted. Otherwise, if the request is rejected, the DIUC parameter is the previous DIUC at which the SS was receiving downlink data. The general format is shown in Figure 11.2. The operation of DBPC Request and Response messages is illustrated in Figure 11.3.
| Management message type (=24) | Reserved | DIUC | Configuration change count |
Figure 11.2: DBPC-RSP MAC management message format. If the DIUC parameter is the same as requested in the DBPC-REQ message, the request was accepted
Figure 11.3: Illustration of DBPC Request and Response messages operation
11.1.2 Initial Ranging
Initial ranging allows an SS joining the network to acquire the correct transmission parameters, such as time offset, frequency and transmitted power level, so that the SS can communicate with the BS.
In the OFDM PHYsical Layer, initial ranging uses the initial ranging uplink contention slots. First, an SS synchronises to the downlink using the preamble and then learns the uplink channel characteristics through the UCD MAC management message. Then, the SS scans the UL-MAP message to find an initial ranging (contention slots) interval. As described in Chapter 10, the BS allocates an initial ranging interval made of one or more (initial ranging) transmission opportunities. In this interval, the SS sends an RNG-REQ MAC management message, with a CID value = 0 (see Table 7.1). For the OFDMA PHY, the initial ranging process is different. It uses initial ranging CDMA codes (see Section 10.5). In OFDM PHY, initial ranging transmissions use a long preamble (two OFDM symbols) and the most robust mandatory burst profile.
When the initial ranging transmission opportunity occurs, the SS sends the RNG-REQ message (using a CDMA code in the case of the OFDMA PHY). The SS sends the message as if it were colocated with the BS, as the propagation delay is taken into account in the initial ranging transmission opportunity.
11.1.2.1 Initial Ranging Message Initial Transmitted Power Value
The SS calculates the maximum transmitted power for initial ranging, denoted PTX_IR_MAX, as follow where
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BS_EIRP (Equivalent Isotropic Radiated Power) is the BS transmitted power value transmitted in the DCD message.
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EIRxPIR, max is the maximum equivalent isotropic received power at the BS. This value is obtained from the DCD message.
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RSS is the measured Received Signal Strength Indicator (RSSI) at the SS.
It can be verified that the above equation is the realisation of:
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Maximum transmitted power for initial ranging
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= intended maximal received power at BS + estimated path loss between the SS and the BS.
The SS antenna gains may be included in the above formula. In the case that EIRxPIR, max and BS_EIRP are not known, the SS starts from the minimum transmit power level defined by the BS.
11.1.2.2 Successful Initial Ranging
The CIDs for the basic and primary management connections (see Section 8.4) are assigned in the RNG-RSP and REG-RSP messages. This ranging process is now described.
Once the BS has successfully received the RNG-REQ message, the BS returns a RNG-RSP message using the initial ranging CID. This RNG-RSP contains the MAC address of this new SS. Within the RNG-RSP message, the BS also puts the basic and primary management CIDs assigned to this SS. The same CID value is assigned to both members of each connection pair (uplink and downlink). The RNG-RSP message also contains information on the transmitted power level adjustment and offset frequency adjustment as well as any timing offset corrections. At this point the BS starts using individually allocated initial ranging intervals addressed to the SS Basic CID to complete the ranging process, unless the status of the RNG-RSP message is ‘success’, in which case the initial ranging procedure is finished. The RNG-REQ and RNG-RSP messages dialogues can also provide the CID value for the secondary management connection.
If the status of the RNG-RSP message is ‘continue’, the SS waits for an individual initial ranging interval assigned to its Basic CID. Using this interval, the SS transmits another RNG-REQ message using the Basic CID along with any power level and timing offset corrections. The BS sends another RNG-RSP message to the SS with any additional fine tuning required. The ranging request/response steps are repeated until the ranging response contains a ranging successful notification or the BS aborts ranging. Once successfully ranged (RNG-REQ is within tolerance of the BS), the SS joins normal data traffic in the uplink. This process is illustrated in Figure 11.4.
Figure 11.4: Illustration of the initial ranging process
11.1.2.3 Unsuccessful Initial Ranging
If, after having sent the RNG-REQ message, the SS does not receive a response, it sends again the RNG-REQ message at another initial ranging transmission opportunity at one power level step higher. This step value is not fixed in the standard, although it indicates it cannot be greater than 1 dB. If the SS receives an RNG-RSP message containing the frame number in which its RNG-REQ message was transmitted, the SS considers that the transmission attempt was unsuccessful. This RNG-RSP message indicates that the BS has detected a transmission in the ranging slot that it is unable to decode. However, the SS implements the corrections specified in the RNG-RSP message and issues another RNG-REQ message after the appropriate backoff delay.
11.1.3 Ranging (or Periodic Ranging)
After the initial ranging where physical parameters are adjusted, the periodic ranging allows the SSs to adjust transmission parameters so that the SSs can maintain communication quality with the BS. Distinct processes are used for managing the uplink and downlink. Some PHY modes support ranging mechanisms unique to their capabilities.
11.1.3.1 Downlink Ranging
In the downlink, if the received CINR goes outside an allowed operating region, according to the link adaptation mechanism, the SS requests a change to a new burst profile using one of two methods: the RNG-REQ message or the DBPC-REQ message. With both methods, the message is sent using the Basic CID of the SS:
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DBPC-REQ message. If the SS has been granted an uplink bandwidth, i.e. a data grant allocation to the SS Basic CID, the SS sends a DBPC-REQ message using that allocation. The BS responds with a DBPC-RSP message.
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RNG-REQ message. If a grant is not available and the SS requires a new burst profile on the downlink, the SS sends an RNG-REQ message in an initial ranging (contention slot) interval, using the same procedure as for initial ranging.
Link adaptation of the downlink is described in Section 11.2.
11.1.3.2 Uplink Ranging
In the uplink, periodic ranging is realised as follows. For each (unicast) uplink burst grant in which a signal is detected, the BS determines the quality of the uplink signal. If the signal is not within acceptable limits, the BS issues the RNG-RSP message including the appropriate correction data (see the RNG-RSP format above) and a status of ‘continue’. If a sufficient number of correction messages are issued without the SS signal quality becoming acceptable. the BS sends the RNG-RSP message with a status of ‘abort’ and then terminates the link management of the SS. Accordingly, the SS processes the RNG-RSP messages it receives, implementing any PHYsical layer corrections that are specified (when the status is ‘continue’) or initiating a restart of MAC activities (when the status is ‘abort’).
The SS responds to each uplink bandwidth grant the BS addresses to it. When the status of the last RNG-RSP message received by the SS is ‘continue’, the SS includes the RNG-REQ message in the allocated transmitted burst. When the status of the last RNG-RSP message received is ‘success’ (due to the fact that the BS considers that the signal is now within acceptable limits), the SS uses the grant to service its pending uplink data queues. If no data is pending, the SS responds to the grant by transmitting a block of padded data.
For each (unicast) uplink burst grant, the BS determines whether or not a transmitted signal is present. If no signal is detected in a specified number of successive grants, the BS terminates the link management for the associated SS.
The possibility to change the burst profiles is the basis of the link adaptation mechanism, allowing a very efficient use of the radio resource. Link adaptation in 802.16/WiMAX is described in the following section.
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