10.4 Contention-based Focused Bandwidth Request in OFDM PHY

The focused bandwidth or focused contention request in the OFDM PHYsical Layer is described in this section. This bandwidth request uses only part of all the OFDM subcarriers (instead of all of them as in the so-called full contention request) in association with contention codes. For the OFDM PHYsical Layer, two contention-based Bandwidth Request mechanisms, each referring to a REQ (Request) Region in the uplink frame, are defined in the standard:

The full contention transmission corresponding to an uplink contention space called in the standard REQ Region Full, is indicated by a UIUC = 2 (see the UIUC table, Table 9.9), and is the contention mechanism used in Section 10.3.7 if subchannelisation is not activated.
The focused contention transmission corresponding to an uplink contention space called in the standard REQ Region Focused, is indicated by a UIUC = 3 (see the UIUC table, Table 9.9). This transmission consists of a Contention Code modulated on a Contention Channel consisting of four OFDM subcarriers, both being randomly chosen by the candidate SS. The backoff procedure is always the one described in Section 10.3.5. The REQ Region Focused contention method has evidently a smaller collision probability than REQ Region Full.

The full contention transmission is mandatory. Capability of the focused contention transmission is optional. If the two types of request are possible, the SS may choose either of them.

10.4.1 Full Contention (REQ Region Full)

In a REQ Region Full:

When subchannelisation is not active, the bandwidth request is the mechanism seen until this point in this book: each Transmission Opportunity (TO) consists of a short preamble (one OFDM symbol) and one OFDM symbol using the most robust mandatory burst profile (BPSK, coding rate of 1/2).
When subchannelisation is active, the allocation is partitioned into TOs, both in frequency and in time. The width (in subchannels) and length (in OFDM symbols) of each TO are defined in the UCD message, along with the description of the burst profile corresponding to UIUC = 2. The number of subchannels used by each transmission opportunity may be 1, 2, 4, 8 or 16. The number of OFDM symbols that must be used by each transmission opportunity is also given in the UCD message. The transmission of an SS must contain a subchannelised preamble corresponding to the TO chosen, followed by data OFDM symbols using the most robust mandatory burst profile.

10.4.2 Focused Contention (REQ Region Focused)

In a REQ Region Focused, a transmission opportunity (sometimes called a transmit opportunity) consists of four subcarriers on two OFDM symbols (see figure 10.10). Each transmission opportunity is indexed by consecutive transmission opportunity indices, the first occurring transmission Opportunity being indexed 0. A candidate SS (requesting uplink bandwidth) sends a short code over a transmission opportunity as described below. This transmission is made in a REQ Region Focused defined by the BS using a UL-MAP_IE UIUC = 3 (see the UIUC table, Table 9.9).

image from book
Figure 10.10: Example of the subcarriers of a focused contention transmission opportunity (contention channel index = 20). The SS transmits zero amplitude on all other subcarriers

The focused transmission consists of a contention code modulated on a contention channel consisting of four OFDM subcarriers. The SS transmits zero amplitude on all other subcarriers. The selection of the contention code is done with equal probability among eight possible codes of four bits each. The selection of the contention channel is done with equal probability among the time/frequency transmission opportunities that the concerned SS can use. There is no MAC message here; the BS only needs to detect a contention code on a contention channel. Upon detection, the BS provides an uplink allocation for the SS to transmit a Bandwidth Request MAC PDU and optionally additional data, but instead of indicating a Basic CID, a DL-MAP_IE is sent in combination with an OFDM Focused_Contention_IE (UIUC = 4), which specifies the contention channel, contention code and transmission opportunity that were used by the SS. This allows the SS to determine whether it has been given an allocation by matching these parameters with the parameters it used. The SS then can send a Bandwidth Request MAC PDU on the allocated uplink grant. This procedure is summarised in Table 10.2.

Table 10.2: The steps of the focused contention (REQ Region Focused) procedure
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Step		Action
0	The BS broadcasts	The BS sends an UL-MAP _IE with UIUC = 3, indicating a REQ Region Focused (Focused Contention Request region in the uplink frame). Other REQ-focused contention parameters are broadcasted on the UCD message
1	SS→BS	A candidate SS (an SS requesting an uplink bandwidth) sends a short code (a contention code, set of 4 bits, modulated on a contention channel, a set of four OFDM subcarriers) over a TO randomly chosen among the TOs (four OFDM subcarriers on two OFDM symbols) that this SS has the right to use.
2	BS→SS	If the BS detects the REQ-focused code, the BS provides an uplink allocation for the SS to transmit a Bandwidth Request in a (DL-MAP) OFDM Focused_Contention_IE (UIUC = 4), which specifies the contention channel, contention code and transmission opportunity that were used by the SS
3	SS→BS	The SS sends a Bandwidth Request MAC PDU on the allocated uplink grant

During a transmission opportunity, the amplitude of each of the four subcarriers must be boosted above its normal amplitude, i.e. the amplitude used during a noncontention OFDM symbol, including the current power-control correction. The boost in dB is equal to the value of the Focused Contention Power Boost parameter indicated by the BS in the UCD message.

10.4.2.1 Focused Contention (REQ Region Focused) with Subchannelisation

The number of contention codes that can be used by a Subchannelisation-enabled SS to request a subchannelised allocation is denoted C_SE in the standard. This value is given by the BS in the UCD message. The default value is 0 (typically, for BSs not supporting subchannelisation) and the allowed value range is 0–8.

The contention code is selected randomly with equal probability from the appropriate subset of contention according to the value of If the C_SE BS supports subchannelisation, only the last C_SE contention codes (among the eight available contention codes) may be used by subchannelisation-enabled SSs that wish to receive a subchannelised allocation. In response, the BS can:

provide the requested allocation as a subchannelised allocation, where the UL-MAP IE for allocation of bandwidth in response to a subchannelised network entry signal, in the subchannelised section of the UL-MAP, is identified by UIUC = 13;
provide the requested allocation as a full allocation (default);
provide no allocation.

10.4.3 Summary of Contention-based Uplink Grant-request Methods

To end this section, Table 10.3 provides a summary of OFDM PHY contention-based uplink grant-request methods.

Table 10.3: Summary of OFDM PHY contention-based uplink grant-request methods
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Full contention		Focused contention
Transmitted in a REQ Region full in the uplink frame		Transmitted in a REQ Region Focused in the uplink frame
Without subchannelisation	With subchannelisation	Without subchannelisation	With subchennelisation
The basic method of contention-based uplink grant request (see the example in Section 10.3.7)	Allocation is partitioned into transmission opportunities (TOs) both in frequency and in time The width (in subchannels) and length (in OFDM symbols) of each transmission opportunity (TO) are defined in the UCD message	See Table 10.2	The contention code sent by a candidate SS is selected randomly with equal probability from the appropriate subset of all contention codes according to the value of C_SE indicated in UCD In response, the BS may provide the requested allocation as a subchannelised allocation: provide a full allocation or provide no allocation