10.3 Uplink Access Grant-request Mechanisms

10.3 Uplink Access Grant-request Mechanisms

The 802.16 standard defines two main grant-request methods:

  • unicast polling (or polling);

  • contention-based polling.

By extension, the UGS class of QoS has unsolicited bandwidth grants, sometimes considered as an (implicit) grant-request mechanism although it is based on reserved slots dedicated for the concerned UGS class SSs. These grant-request mechanisms will now be described, starting with the simplest one, unsolicited bandwidth grants.

10.3.1 Unsolicited Bandwidth Grants

The unsolicited bandwidth grants technique consists of dedicated slots reserved for UGS class SSs. This type of bandwidth requests is useful for applications requiring a fixed rate data stream. Figure 10.3 illustrates the unsolicited bandwidth grant mechanism in the uplink and the downlink. This type of access grant is used only by the UGS class of QoS.

Image from book
Figure 10.3: Unsolicited bandwidth grants in the uplink

10.3.2 Unicast Polling

Polling is the process by which the BS allocates bandwidth to the SSs for the purpose of making bandwidth requests. These allocations may be to an individual SS or to a group of SSs. The use of polling simplifies the access operation and guarantees that applications can receive service on a deterministic basis if it is required. This allocation technique is used when bandwidth resource demand is not relevant enough to have unsolicited bandwidth grants for all users; the BS can then directly assign the request amount to the SS(s) as needed.

When an SS is polled individually, it is a unicast polling. In the case of unicast polling, no explicit message is transmitted to poll the SS. Rather, the SS is allocated, in the UL-MAP, sufficient bandwidth to respond with a Bandwidth (BW) request. The standard indicates that for any individual uplink allocation, the SS may optionally decide to use the allocation for data, requests or requests piggybacked in data transmission. Taking into account its (possibly) different pending uplink transmission requests, the SS scheduler decides if a bandwidth request must be made, standalone or piggybacked with data (see Section 10.2). If the SS do not have data to transmit and then no need for bandwidth, the allocation is padded, eventually using a padding CID (see Table 7.1). Figure 10.4 represents the unicast polling mechanism.

Image from book
Figure 10.4: Illustration of the unicast polling mechanism. If the SS has no needs, the allocated slots are padded

The standard states that unicast polling would normally be done on a per-SS basis by allocating a Data Grant IE (or Data Grant Burst Type IE) directed at its Basic CID. A Data Grant IE (or Data Grant Burst Type IE) is a UL-MAP_IE with the UIUC indicating the burst profile of the uplink access duration allocated to an SS.

The SSs with currently active UGS connections may set the PM bit in the grant management subheader (see Figure 10.1) in the MAC packet of their UGS connection to indicate to the BS that they need to be polled to the request bandwidth for one or more non-UGS connection(s). To reduce the individual polling bandwidth requirements in the downlink, SSs with active UGS connections need to be polled individually only if this PM bit is set. Once the BS detects this request for polling, it applies the individual polling process.

10.3.3 Contention-based Group (Multicast or Broadcast) Polling

The available bandwidth may not be sufficient to individually poll all inactive SSs. Contention-based grant-request mechanisms are allocated a small part of each uplink frame (in the FDD mode) or subframe (in the TDD mode), known as the bandwidth requests contention slot (see Chapter 9, Figure 9.7). The size of this contention slot, known in the standard as the Request IE, is indicated by the BS (see Section 10.3.7). With this contention slot, an SS can access the network by asking the BS for an uplink slot. If the BS receives the demand (which means that there was no collision), it evaluates the SS request in the context of its service-level agreement, the radio network state and the scheduling algorithm, and possibly allocates a slot in which the SS can transmit data. Some SSs, such as those inactive for a long period of time and/or with low access priority, may then be polled in multicast groups. In some cases, a broadcast poll may also be made. Thus, multicast polling saves the bandwidth with regard to the scheme where all SSs are polled individually. In the case where this polling is made to a group of SSs, the allocated bandwidth is specifically for the purpose of making bandwidth requests.

Some CIDs are reserved for multicast groups and for broadcast messages (see Table 7.1). As for individual (unicast) polling, the poll is not an explicit message, but rather bandwidth allocated in the UL-MAP. The difference is that, rather than associating an allocated bandwidth with an SS's Basic CID, the allocation is to a multicast or broadcast CID. An example of a BS polling is provided in Section 10.3.7.

Group (multicast or broadcast) polling works as follows. When the poll is directed at a multicast CID or the broadcast CID, an SS belonging to the polled group may request a bandwidth during any request interval allocated to that CID in a UL-MAP. Figure 10.5 represents an illustration of the contention-based group polling mechanism. In order to reduce the likelihood of collision with multicast and broadcast polling, only SSs needing a bandwidth reply. These replying SSs apply a contention resolution algorithm, described in Section 10.3.5, to select the slot in which to transmit the initial bandwidth request. This mechanism allows a fair distribution of the bandwidth between different SSs without allocating a dedicated slot for each SS.

Image from book
Figure 10.5: Illustration of contention-based group polling. The three SSs shown are group (multicast or broadcast) polled. They all have a bandwidth request. SS 2 wins the contention and then receives a bandwidth allocation

A replying SSs assumes that the transmission has been unsuccessful if it does not receive a grant after a given number of subsequent UL-MAP messages. This parameter, called the contention-based reservation timeout, is given in the UCD MAC management message (see Chapter 9 for a UCD message). If necessary, an SS transmits during the total time of all of its uplink grants using a given padding mechanism.

10.3.4 Management of Multicast Polling Groups

The BS may add an SS to a multicast polling group, identified by a multicast polling CID value, by sending the MCA-REQ (Multicast Polling Assignment Request) MAC management message with the Join command. On the other hand, the BS can remove an SS from a multicast polling group by sending the MCA-REQ MAC management message with the Leave command. Upon receiving the MCA-REQ message, the SS will respond by sending the MCA-RSP (Multicast Polling Assignment Response) MAC management message. Among the MCA-REQ MAC management message TLV parameters are the following: multicast CID (that the SS must join or leave) and assignment (leave or join). Multicast groups may have a periodic polling allocation after a number of frames indicated (and TLV coded) in the MCA-REQ message. This type of periodic polling (REQ Region Full or REQ Region Focused, see Section 10.4) is also among the MCA-REQ TLV parameters.

The MCA-RSP is sent by the SS in response to an MCA-REQ and contains mainly the confirmation code equal to zero if the request was successful and to non-zero in case of failure. These two messages use the primary management connection; i.e. they are sent on the SS's primary management CID (in the generic MAC header CID field).

10.3.5 Contention Resolution for Group Polling Transmission Opportunity

A transmission opportunity in a contention-based procedure of 802.16 is defined as a contention space allocation provided in a UL-MAP for a group of SSs. In OFDM PHY, there are transmission opportunities dedicated to the transmission of bandwidth requests and others for the transmission of initial ranging. The initial ranging procedure is described in Chapter 11. This group of SSs may include either all SSs having an intention to join the cell or all registered SSs or some other multicast polling group.

The size of an individual transmission opportunity for each type of contention IE is indicated by the BS in the UCD MAC management message. This parameter, known as the Bandwidth request opportunity size or the Ranging request opportunity size (see Chapter 9), is the size in units of the PS (Physical Slot) of the PHY payload that an SS may use to format and transmit a bandwidth request message or an initial ranging request message in a contention request zone. The value includes all PHY overheads as well as allowance for the MAC data the message may hold. It should be remembered that for OFDM and OFDMA PHYsical layers, a PS is defined as the duration of four OFDM symbols.

The BS always allocates bandwidth for contention IEs in integer multiples of these published individual transmission opportunity values. The number of transmission opportunities associated with a particular UL-MAP_IE corresponding to an initial ranging or bandwidth request interval is then dependent on the total size of this contention space allocation as well as the size of an individual transmission. See the numerical example in Section 10.3.7. Contention Resolution Algorithm

Collisions may occur during initial ranging and bandwidth request intervals in the uplink (sub-) frame. The uplink transmission and contention resolution algorithm is the same for these two processes. Since an SS may have multiple active uplink service flows (and then, equivalently, multiple CIDs), it makes these ranging or request decisions on a per-CID or, equivalently, per-service QoS basis.

The method of contention resolution required by the 802.16 standard is based on a truncated binary exponential backoff, with the initial backoff window and the maximum backoff window values selected by the BS. These two parameters are specified in the UCD message. They are given as a power-of-two value -1 (minus 1). For example, a value of 4 indicates a backoff window between 0 and 15; a value of 10 indicates a backoff window between 0 and 1023. For these four windows, the range of values of n is 0–15; i.e. the possible sizes are between 0 and 65535.

The contention resolution algorithm works as follows. When an SS has information to send and wants to enter the contention resolution process, it sets its internal backoff window equal to the request (or ranging for initial ranging) initial backoff window defined in the UCD message. This UCD message is itself referenced by the UCD count in the UL-MAP message currently in effect.

The SS randomly selects a number within this backoff window. The obtained random value indicates the number of contention transmission opportunities that the SS will defer before transmitting. An SS considers only the contention transmission opportunities for which this transmission would have been eligible. The contention zones are defined in the standard as Request IEs (or Initial Ranging IEs for initial ranging) in the UL-MAP messages, identified by appropriate UL-MAP_IEs (see Section 10.3.7). Note that each IE may consist of more than one contention transmission opportunity. Using bandwidth requests as an example, consider an SS whose initial backoff window is 0–15 and assume it randomly selects the number 11. The SS must defer a total of 11 contention transmission opportunities. If the first available Request IE is for six requests, the SS will not use this Request IE and has five more opportunities to defer. If the next Request IE is for two requests, the SS has three more to defer. If the third Request IE is for eight requests, the SS transmits on the fourth opportunity, after deferring for three more opportunities (see Figure 10.6).

Image from book
Figure 10.6: Example of a backoff mechanism. The SS has to wait 11 transmission opportunities (a randomly selected number between 0 and the internal backoff window). In this figure, only the Request IE (contention slot) is represented and not the rest of the uplink (sub-) frame

After a contention transmission, the SS waits for a Data Grant Burst Type IE in a subsequent map (or for a Ranging Response (RNG-RSP), message for initial ranging). Once received, the contention resolution is complete. For bandwidth requests, if the SS receives a unicast Request IE or Data Grant Burst Type IE at any time while deferring for this CID, it stops the contention resolution process and uses the explicit transmission opportunity. The SS considers the contention transmission lost if no data grant has been given within a given duration (or no ranging response within another given duration for initial ranging). In this case, the SS increases its backoff window by a factor of two, as long as it is less than the maximum backoff window. The SS then randomly selects a new number within its new backoff window and repeats the deferring process described above. This retry process continues until the maximum number of retries (i.e. Request Retries for bandwidth requests and Contention Ranging Retries for initial ranging) has been reached. At this time, for bandwidth requests, the SS discards the pending transmission. The minimum value for Request Retries is 16. Due to the possibility of collisions, bandwidth requests transmitted after a broadcast or multicast polling must be aggregate requests.

The choices of the Request (or Ranging) Backoff Start and the Request (or Ranging) Backoff End by the BS gives it much flexibility in controlling the contention resolution. These choices can be changed as frequently as the UCD message frequency if needed.

It is pointed out that this contention resolution algorithm is the same used for WiFi IEEE 802.11 WLAN for a contention-based distributed access function, which is the only mode effectively used, until now, for 802.11 WLANs.

10.3.6 Bandwidth Stealing

A bandwidth is always requested on a CID basis and allocated on an SS basis (to the SS basic CID). The process of bandwidth stealing is defined in the standard as the use, by a subscriber station (SS), of a portion of the bandwidth allocated in response to a Bandwidth Request for a connection to send another Bandwidth Request rather than sending data (see Figure 10.7). This process is allowed for some classes of QoS (see Chapter 11).

Image from book
Figure 10.7: Illustration of the bandwidth stealing principle

10.3.7 Example of Uplink Access

The information sequence for unicast, multicast and broadcast polling is now illustrated in an example. The OFDM Layer is considered as well as the following numerical hypothesis:

  • Bandwidth = 3.5 MHz;

  • n = 8/7 (sampling factor);

  • G (guard time factor) = 1/8;

  • Frame duration = 5 ms;

  • Duplexing mode is FDD.

For these hypothesis, it can be verified that the number of OFDM symbols per frame is 69 (see Section 5.2.4). The standard states that an uplink subframe consists of a contention interval scheduled for initial ranging, contention interval(s) scheduled for Bandwidth Request purposes and one or multiple uplink PHY PDUs, each transmitted from a different SS.

Table 10.1 shows an example of UL-MAP MAC management message contents. The character of each IE, defined by an UL-MAP IE, changes depending on the type of CID used in this IE (see CID defined values in Table 7.1). When broadcast and multicast defined CID values are used, this is an invitation for all (or some of) the SSs to contend for requests. If a basic CID (then an SS's CID) is used, this is an invitation for a particular SS to transmit data and/or to request a bandwidth (see Section 10.3.2). In this table, two UL-MAP_IE fields, the subchannel index and the midamble repetition interval, are not shown in order to simplify the table. For OFDM (fixed WiMAX) PHYsical Layer parameters, i.e. UL-MAP_IE formats, UIUC values (see Table 9.9), etc., Start Time and Duration fields are in units of OFDM symbol duration (as for DL-MAP_IEs).

Table 10.1: Example of UL-MAP message contents. Two UL-MAP_IE fields, subchannel index and midamble repetition interval are not shown in this table
Open table as spreadsheet

UL-MAP message field(s)



Management message type of UL-MAP (=3) Uplink channel ID (8 bits)

Identifier of the uplink channel to which this message refers (not to be confused with CID). Arbitrarily chosen by the BS, this ID acts as a local identifier for some transactions


UCD count (8 bits)

Configuration change count of the UCD (the same as for the DL-MAP)


Base Station ID (48 bits)

48-bit long field identifier of the BS (same as for the DL-MAP)


Allocation start time (32 bits)

Effective start time of the uplink allocations defined by the UL-MAP, starting from the beginning of the downlink frame in which this UL-MAP message is placed


CID = 0x0000 UIUC = 1 Start time = 0 Duration = 16

Defines the (Initial) Ranging IE


CID = 0xFFFF UIUC = 2 Start time = 16 Duration = 12

Defines a (Bandwidth) Request IE associated with the broadcast CID. This is then a broadcast polling


CID = 0xFF10 UIUC = 2 Start time = 28 Duration = 8

Defines a (Bandwidth) Request IE associated with CID = 0xFF10 (multicast CID, see the CID table, Table 7.1). This is then a multicast polling


CID = 0xFF20 UIUC = 2 Start time = 36 Duration = 4

Defines a (Bandwidth) Request IE associated with CID = 0xFF20 (multicast CID, see the CID table, Table 7.1). This is then a multicast polling


CID = 0x0023 UIUC = 5 Start time = 40 Duration = 10

Uplink grant (allocation) to CID = 0x0023 (the Basic CID of a specific SS). This corresponds to one uplink burst (or uplink PHY PDU), possibly containing more than one MAC message, transmitted in modulation/coding (in addition to other burst profile parameters) corresponding to UIUC = 5 (see the UIUC table, Table 9.9)


CID = 0x00121 UIUC = 7 Start time = 50 Duration = 7

Uplink grant (allocation) to CID = 0x00012 (the Basic CID of a specific SS). UIUC = 7


CID = 0x000A UIUC = 7 Start time = 57 Duration = 7

Uplink grant (allocation) to CID = 0x000A (the Basic CID of a specific SS). UIUC = 7


CID = 0x0005 UIUC = 9 Start time = 64 Duration = 5

Uplink grant (allocation) to CID = 0x0005 (the Basic CID of a specific SS). UIUC = 9


CID = 0 UIUC = 14 Start time = 69 Duration = 0

The end of the last allocated burst is indicated by allocating an End of Map burst (UIUC table, 9.9) with Duration field = 0 and CID = 0

Initial ranging transmissions use a long preamble (two consecutive OFDM symbols) and the most robust mandatory burst profile. The most robust is BPSK with a Channel Coding rate of 1/2. It is estimated that the Ranging Request MAC management message is two OFDM symbols long. Then, the Initial Ranging Request PPDU is four OFDM symbols long. In the case of initial ranging, the maximum SS/BS round-trip propagation delay must also be taken into account. Four OFDM symbols are added (this is for a large cell).

The Request IE burst profile depends of the bandwidth request type as there are three possible uplink requests (see the UIUC table, Table 9.9). In this example, REQ Region Full is considered, which is the ‘classical’ uplink request. Thus, subchannelisation is not active. For these conditions, each transmit opportunity consists of a short preamble, i.e. one OFDM symbol and one OFDM symbol transmitting the bandwidth request, using the most robust mandatory burst profile. This symbol (96 uncoded bits) is enough to transmit the 48 bits of the MAC bandwidth request frame. The most robust burst profile is BPSK with a Reed–Solomon convolutional channel coding rate of 1/2. In fact, The Reed–Solomon convolutional coding rate of 1/2 is always used as the coding mode when requesting access to the network, except in subchannelisation modes, which use only convolutional coding of 1/2. Then, the Bandwidth Request PPDU is two OFDM symbols long.

The size of an individual transmission opportunity for each type of contention IE is indicated by the BS in the UCD MAC management message. This parameter, known as the bandwidth request opportunity size or the ranging request opportunity size, is the size in units of PS (Physical Slot) of the PHY payload that an SS may use to transmit a bandwidth request message or initial ranging request message in a contention request opportunity.

The PS is the basic unit of time. A PS corresponds to four (modulation) symbols used on the transmission channel (for OFDM and OFDMA PHY layers). The individual transmission opportunity (the bandwidth request opportunity size or the ranging request opportunity size) includes all PHY overheads as well as allowance for the MAC data the message may hold. It is assumed for this example that:

  • The initial ranging request opportunity size, indicated by the UCD MAC management message, is equal to eight OFDM symbols. The initial ranging IE (contention slots), indicated by the UL-MAP MAC management message is 16 symbols long (see Table 10.1).

  • The bandwidth request opportunity size, indicated by the UCD MAC management message, is equal to two OFDM symbols (see Figure 10.8). The Bandwidth Request IE (contention slots), indicated by the UL-MAP MAC management message, is 12 symbols long (see Table 10.1). These numerical values are used for the uplink frame figure.

Image from book
Figure 10.8: Example of Bandwidth Request IE (bandwidth request contention slots). The BS must allocate a bandwidth for Bandwidth Request IE in integer multiples of individual transmission opportunity values (indicated in the UCD message)

It can be verified that the length of a bandwidth request opportunity size or the ranging request opportunity size (in PS, as indicated in the UCD message) is 144 PS (two OFDM symbols).

The duration field in UL-MAP_IE indicates the duration, in units of OFDM symbols, of the allocation. This value includes the preamble, the (possible) midambles and the postamble contained in the allocation. In the example given in this section, it is assumed that there is no midamble; i.e. the midamble repetition interval field (2 bits) in the beginning of the UL_MAP message (see Chapter 9 for the UL-MAP) is equal to 0b00. The standard indicates that all SSs must acquire and adjust their timing such that all uplink OFDM symbols arrive time coincident at the BS with an accuracy of ±50% of the minimum guard interval or better. Figure 10.9 shows the uplink bursts in the uplink subframe described by the UL-MAP in Table 10.1.

Image from book
Figure 10.9: Uplink bursts in the uplink subframe described in Table 10.1

Assuming that the Modulation and Coding Scheme (MCS) corresponding to UIUC = 7 is 16-QAM, 3/4, estimate the uplink data rate of the SS with Basic CID = 0x000A (on the duration of the considered frame)? Consider that this allocation is made once every four frames (e.g. for an UGS class), what is the uplink data rate of the SS with Basic CID = 0 × 000A?

  • Data rate: UIUC = 716-QAM, 3/4;

  • 7 symbols in 5 ms: 7 × 192 × 4 × 3/4 = 4032 bits in 5 ms 806.5 kb/s;

  • One frame over four 201.6kb/s.