Network Entry in IEEE 802.16-2009
The IEEE 802.16 standard distinguishes ten procedures in network initialization and entry. Of these ten procedures, shown in Figure 5.1, four procedures are implementation dependent and were hence identified as optional. These ten are (optional procedures are marked with an asterisk (*)):
- Scanning and synchronization;
- Obtaining downlink parameters;
- Initial ranging and automatic adjustments;
- Negotiating basic capabilities;
- Authorizing SS and performing key exchange*;
- Registering with BS;
- Establishing IP connectivity*;
- Establishing time of the day*;
- Transferring operational parameters*;
- Establish provisioned connections.
Upon initialization or powering up, an SS scans the band for a downlink channel. Once it recognizes one DL-MAP message and discerns the respective downlink Burst Profile information, synchronization is said to be achieved. An SS remains synchronized as long as it continues to successfully receive the DL-MAP and the DCD message for the channel.
Figure 5.1 Network Entry and Initialization flowchart for IEEE 802.16-2009. Reproduced by permission of © 2009 IEEE.
Once synchronization is established, the SS waits for an UCD to distinguish a possible uplink channel. A BS periodically transmits the UCD to the MAC broadcast address. If the SS cannot distinguish an uplink channel, it looks for another downlink channel. An SS distinguishes the operating mode (whether FDD or TDD) through differentiating between the center frequency in the DCD and the UCD. The absence of any frequency Type/Length/Value (TLV) in the channel descriptor indicates TDD. Once an SS finds an uplink channel that is most suitable for its purposes, it performs initial ranging.
Initial ranging is the procedure by which the BS recognizes an SS by its MAC address and transmission and reception capabilities. Through initial ranging, as well, the BS adjusts the SS's parameters such as transmission power, time offset, and frequency offset, in order to regulate interference and signal quality within the cell/sector. This procedure is expanded on in next section.
Once initial ranging is completed, the SS proceeds to negotiate basic capabilities with the BS. If authorization is enabled in the network, the SS will perform authorization and key exchange. Registration marks the final major procedure for a non-managed SS in the network entry process. A managed SS would indicate that it is so during initial ranging and, through registration, would obtain its secondary management CID and know which IP version is used. It would then proceed to establish IP connectivity through either Mobile IP or DHCP over the secondary management connection. The optional stages of establishing time of the day and transferring operational parameters (for managed SSs) are then pursued if needed.
The SS could now establish provisioned connections through Dynamic Service Addition (DSA) request and response messages (DSA-REQ and receiving DSA-RSP in response), and is considered operational. An SS's operational status thereafter is maintained through periodic ranging.
Initial Ranging
There are two ranging processes that an SS undergoes: initial and periodic. Initial ranging, made in the initial ranging contention-based interval, is made during two phases of operation: (re)registration or when synchronization is lost, or during transmission on a periodic basis. On the other hand, periodic ranging uses the regular uplink bursts granted by the BS. For a BS, the duration of the ranging slot for initial system access depends on the intended cell radius.
To contend, the SS scans the UL-MAP for initial ranging interval. A BS is required to afford transmission opportunities. For SC-FDMA as well as OFDM—based PHY, the size of each transmission opportunity (TxOP) is specified by the UCD TLV Ranging Request Opportunity Size. Moreover, the SS puts together a Ranging Request (RNG-REQ) in the initial ranging interval. On the other hand, for OFDMA PHY, the SS sends an initial ranging CDMA code on an uplink allocation dedicated to this objective.
There are a total of 256 CDMA ranging codes utilized in OFDMA, each having a length of 144 bits. Each BS is assigned a fraction of these codes, denoted S, and ranging between the values of S to ((S + 0 + N + M + L) mod(256)), where N codes are used for initial ranging, M for periodic ranging, L for bandwidth requests and O for handover ranging. In this manner, the BS can determine the purpose of the received code by identifying the subset to which it belongs.
When an initial ranging interval transmission opportunity occurs, the SS would send the RNG-REQ (or CDMA code in OFDMA). The SS then sends the messages as if it was collocated by the BS.
The SS sends its RNG-REQ at a power level below the maximum allowed ranging transmission power (PTX_IR_MAX). If the SS does not receive any response, it adjusts its power level until success is indicated by an RNG-RSP with the SS's MAC address. A BS that is unable to decode an RNG-REQ would send an RNG-RSP with only the request's transmission parameters and indicating frame opportunity. For OFDMA, the SS sends the CDMA code at a power level below PTX_IR_MAX and would increase the power level if no response is received. An unsuccessful attempt would be indicated by the BS via an RNG-RSP with code parameters and a Continue status. The SS would then implement corrections and randomly select the next ranging slot. An UL-MAP with a CDMA allocation IE containing the SS's code parameters is considered an RNG-RSP indicating success. When received, the SS sends an RNG-REQ in the indicated bandwidth slot.
An RNG-RSP would be identified by the SS's initial ranging CID. It would also contain the Basic and Primary Management CIDs. If needed, the RNG-RSP would also include adjustments on the SS's RF power level and offset frequency, in addition to corrections in the timing offset. An RNG-RSP with a Success status indicates the end of the initial ranging process. An RNG-RSP with Continue status will make the SS wait for an individual initial ranging interval assigned to its Basic CID for its next RNG-REQ. Once an SS is “ranged” it joins the BS's normal data traffic.
It is possible during network entry that a BS redirects a ranging SS to another channel with an offset frequency adjustment. If the adjustment is less than half the channel's bandwidth, it would be considered as a fine tuning. However, if the adjustment is greater than half the channel's bandwidth, the SS would understand that this is effectively a channel reassignment and would have to restart the ranging process on the new channel.
Any adjustment made by the BS must be within the standard's defined operating ranges. An SS response to an RNG-RSP, including any required adjustments, is mandatory. An SS will not make any transmission until adjustments indicated in an RNG-RSP are made.
Periodic Ranging
To maintain connection quality for an SS, the BS and SS are engaged in a periodic ranging. Distinct ranging processes are used for managing the downlink and the uplink. At the same time, certain PHY modes support ranging mechanisms unique to their properties. In what follows, periodic ranging in OFDM and OFDMA PHY is explained.
Periodic Ranging in OFDM
The signal quality at an SS determines the selection of the burst profile at the BS. To reduce uplink traffic volume, an SS monitors the Carrier-to-Interference-Noise-Ratio (CINR) it perceives and compares the average value against the allowed range. If the SS finds the CINR out of preset bounds, the SS requests a change of burst profile by either using the allocated data to send a downlink Burst Profile Change Request (DBPC-REQ) or starting an initial ranging. The latter option is used only when the SS is interested in a more robust profile. A BS receiving a DBPC-REQ shall respond with a DBPC-RSP indicating whether a change in the SS's burst profile is possible.
For uplink ranging, the BS maintains a timer (T27) for each SS that resets whenever a unicast grant is made. Upon expiry, a BS grants bandwidth to the SS for an uplink transmission in the form of a data grant or an invited ranging opportunity. The SS maintains another timer (T4) indicating how long has it been since the SS was given an opportunity to transmit. Once this timer expires, the SS restarts all its MAC operations. In turn, the BS monitors an SS's use of its unicast grants and terminates the link if it has not been utilized for certain duration. For each utilized uplink grant, the BS adjusts the SS's power level through the use of an RNG-RSP to which the SS must adhere. If the SS does not make the required adjustments, the BS would terminate the connection through the use of an aborting RNG-RSP.
An SS interested in maintaining its connection would always utilize its uplink data grant with either data, an RNG-REQ, a padding PDU or stuff bytes.
Periodic Ranging in OFDMA
Periodic ranging in OFDMA utilizes a regular uplink burst. The ranging channel is composed of one or more groups of six adjacent subchannels. Groups are defined starting from the first subchannel, and channels are considered adjacent if they have successive logical numbers. The indices are specified in the UL-MAP and users are allowed to simultaneously contend and collide.
For OFDMA, the standard specifies both the ranging subchannels and special pseudonoise ranging code. As explained above, different subsets of the code are used for different objectives. For each objective, an SS would select (with equal probability) one of the codes from the respective subset, modulate it onto the ranging subchannel choosing (with equal probability) a slot from the available ranging subslots. When needed, backoffs with random duration are used to mitigate contention. As the BS cannot identify SSs through code alone, a BS broadcasts ranging response with code, ranging slot that had the code (OFDMA symbol number, subchannel), and required adjustments (time, power, frequency).