Direct Communication Procedures

Based on the aforementioned management structures and admission control scheme, the direct communication procedures of the APC approach are explained in this subsection.

Considering a basic IEEE 802.16 PMP network that consists of a BS and two SSs, an intra-cell traffic flow is existed between the source 𝑆𝑆𝑠 and destination 𝑆𝑆_𝑑. Two transport connections are established for packet transmission, i.e., the UL transport connection (with CID = 𝐾₁) from 𝑆𝑆_𝑠 to the BS and the DL transport connection (with CID = 𝐾₂) from the BS to 𝑆𝑆_𝑑. It is noted that 𝐾₁ and 𝐾₂ are denoted as specific CID values.

The initialization of direct communication is achieved by conducting the procedure of link

Data packet on transport connection (CID = K1)

SSs BS SSd

Data + PDL subheader (Type = 000)

DL/UL-MAPs (excludes PDL_IEs)

Data + PDL subheader (Type = 001) SINR detection message

PDL subheader (Type = 011) Data packet

on transport connection (CID = K2) DL/UL-MAPs (excludes PDL_IEs)

(a)

SSs BS SSd

SINR detection message

PDL subheader (Type = 111)

PDL subheader (Type = 011) Data + PDL subheader (Type = 010)

Data packet

on transport connection (CID = K1)

Data packet on transport connection (CID = K2) DL/UL-MAPs (excludes PDL_IEs)

(b)

Figure 2.4: Schematic diagram of link request and information collection procedure for direct communication: (a) SS-initiated procedure and (b) BS-initiated procedure.

request and information collection. The pair of 𝑆𝑆_𝑠 and 𝑆𝑆_𝑑 that anticipates to establish a direct link are required to provide their location information and channel conditions to the BS. It is assumed that the SSs can acquire their location information by either using the GPS or performing network-based location estimation techniques [16, 17]. The collected information is utilized in the admission control scheme mentioned above. Fig. 2.4 illustrates an exemplified message flows for the initialization procedure of direct communication.

In the case that 𝑆𝑆_𝑠 intends to conduct direct communication with 𝑆𝑆_𝑑, as shown in Fig.

2.4(a), it attaches a PDL subheader to a data packet that will be delivered to the BS. The PDL subheader is utilized to request a direct link establishment with 𝑆𝑆_𝑑, where the Type field in the subheader is denoted as 000 and the 48-bit MAC address of 𝑆𝑆_𝑑 is filled in the SSID field (as shown in Tables 2.3 and 2.4). The LT field is assigned as 01 and the location information of 𝑆𝑆𝑠 will be filled into the corresponding LI IE. As the BS receives the request PDL subheader from 𝑆𝑆_𝑠, the BS will attach a PDL subheader (with Type field = 001) to the data packet and conduct the transmission to 𝑆𝑆_𝑑. The transport CID 𝐾₁ will be carried in the subheader for indicating that 𝑆𝑆_𝑠is the source of the direct communication, and it will send an SINR detection message via that connection. If the BS does not possess the location information of 𝑆𝑆_𝑑, it will set the LT field as 10 for requesting the location information of 𝑆𝑆_𝑑.

Furthermore, the BS will arrange a DL burst for 𝑆𝑆𝑠 with the assignment in the cor-responding DL-MAP message. The CID of the UL transport connection (i.e., CID = 𝐾1) will be recorded in the CID field of the DL-MAP IE; while the DIUC field is set with the value that corresponds to the BPSK 1/2 MCS. Specifically, the DL burst for the connection with CID 𝐾₁ is prepared for 𝑆𝑆_𝑠 to transmit an SINR detection message to 𝑆𝑆_𝑑 by using the BPSK 1/2 MCS. It is noticed that the UL transport CID will be assigned in the DL subframe, which will not happen by adopting the conventional mechanism. Therefore, the 𝑆𝑆_𝑠 is aware that the DL burst for the UL transport CID is utilized to transmit the SINR detection message. After receiving the PDL subheader and SINR detection message from the BS and 𝑆𝑆𝑠 respectively, 𝑆𝑆_𝑑 will transmit a response PDL subheader with Type 011. The

average SINR value calculated from the SINR detection message will also be recorded in the response PDL subheader. It is noted that the location information of 𝑆𝑆_𝑑 is carried in the response PDL subheader if it is required by the BS.

On the other hand, the BS-initiated direct communication procedure is shown in Fig.

2.4(b). Contrary to the SS-initiated procedure, the BS actively announces the link request along with the PDL subheader (with Type field = 111) to the specific SSs, i.e, 𝑆𝑆_𝑠 and 𝑆𝑆_𝑑. The Basic CIDs for both 𝑆𝑆_𝑠 and 𝑆𝑆_𝑑 are specified within the first two CID fields of the PDL subheader as shown in Table 2.3; while the corresponding UL transport CID 𝐾₁ is written in the third CID field. As the 𝑆𝑆_𝑠 receives the requesting PDL subheader with LT

= 01 or 11 from the BS, the 𝑆𝑆𝑠 will attach a PDL subheader (with Type field = 010) to a data packet that will be delivered to the BS. Correspondingly, the 𝑆𝑆𝑠will utilize the replying PDL subheader to provide its location information that is requested by the BS. The remaining procedures of the BS-initiated APC approach are similar to that of the SS-initiated case, such as the SINR detection procedure between 𝑆𝑆_𝑠 and 𝑆𝑆_𝑑, and the response PDL subheader from 𝑆𝑆_𝑑.

The BS executes the admission control procedure after it received the response PDL subheader transmitted from 𝑆𝑆_𝑑. Based on the collected information, the aforementioned two-tiered control scheme is exploited by the BS to either confirm or deny the direct communication request between 𝑆𝑆𝑠 and 𝑆𝑆_𝑑. It is noted that the constraints 𝒞1 and 𝒞2 can be exploited either jointly or separately. The performance of the separately implemented mechanisms, i.e., the APC-TO and the APC-EO approaches, will be evaluated via the simulation results in Subsection 2.5.2. The confirming results will be broadcasted along with the PDL subheader (with Type field = 110) by the BS. The Basic CIDs for both 𝑆𝑆_𝑠 and 𝑆𝑆_𝑑 are written within the the first two CID fields of the PDL subheader as shown in Table 2.3; while the corresponding confirmed connections are recorded in the remaining CID fields. In the case that all the connections belonging to the indirect transmission from 𝑆𝑆_𝑠to 𝑆𝑆_𝑑are confirmed by the BS, the individual CIDs will be replaced by the Basic CID of 𝑆𝑆_𝑠. In other words, the Basic CID of 𝑆𝑆𝑠will be filled into the third CID field in order to reduce the excessive control

SSs BS SSd DL/UL-MAPs (includes PDL_IEs)

(provides bandwidth request opportunity for polling-based service)

Bandwidth request (polling-based service)

DL/UL-MAPs (includes PDL_IEs) Data packet

(non-polling-based service)

Data packet (non-polling-based service)

Data packet (polling-based service)

Figure 2.5: Schematic diagram of bandwidth request procedure in APC approach.

overhead caused by the individually confirmed CIDs. Consequently, the BS will arrange the corresponding PDL bursts for conducting direct communication in subsequent frames.

After receiving the confirmation announcement, the considered SSs will activate the pro-cedure of direct communication. According to the received MAPs associated with the PDL IEs, 𝑆𝑆_𝑠 will conduct packet transmission directly to 𝑆𝑆_𝑑 within the PDL bursts. Moreover, 𝑆𝑆_𝑑 will continuously observe and evaluate the cannel condition for the direct link with the adaptation to an appropriate MCS. The calculated SINR is compared with the receiver SNR range of the current MCS (as listed in Table 2.7) by 𝑆𝑆_𝑑. If the existing MCS is observed to be improper for the current channel condition, 𝑆𝑆_𝑑 will initiate a PBPC-REQ message to the BS for suggesting an appropriate MCS via the PIUC value. Consequently, the BS will respond with a PBPC-REP message with a recommended PIUC value. If the PIUC values from both the PDPB-REQ and the PBPC-REP messages are perceived to be the same, the request for the change of MCS will be accepted. If the condition is not satisfied, the PIUC value will remain unaltered.

It is worthwhile to mention that the bandwidth requests are conducted by an SS based on individual transport connection. On the other hand, the bandwidth grant from the BS is executed according to the accumulated requests from the SS. In other words, the bandwidth grant is addressed to the Basic CID of the corresponding SS, not to the individual transport CIDs. As a result, the CID specified for the PDL burst becomes the Basic CID of 𝑆𝑆_𝑠. Furthermore, in order to fully compatible with the existing specification, the procedures for the

bandwidth request and allocation as specified in the IEEE 802.16 standard are implemented within the proposed APC approach. Fig. 2.5 illustrates the bandwidth request procedure while the APC approach is adopted. It can be observed that the BS preserves PDL bursts for non-polling based service periodically. Furthermore, the BS will continue to provide unicast bandwidth request opportunity for polling-based service based on the original transport CIDs of 𝑆𝑆_𝑠. The unicast bandwidth grant of the polling-based service will consequently be assigned to PDL bursts based on the Basic CID of 𝑆𝑆_𝑠.

The procedure for the link termination occurs as one of the following two conditions is satisfied: (a) the channel condition of the direct link is becoming worse than that from the indirect channels (i.e., via the BS); (b) the direct communication is determined to be ceased.

It is noted that the link termination can be initiate by either the BS or the SSs. In the SS-initiated termination procedure, the SS will transmit a termination PDL subheader (with Type field = 100). As the message is received by the BS, it will broadcast an announce-ment along with a PDL subheader (with Type field = 101) to both 𝑆𝑆_𝑠 and 𝑆𝑆_𝑑 regarding the termination of the direct communication link. On the other hand, for the BS-initiated termination procedure, the termination information is actively announced by the BS. As a result, the BS and the associated SSs will return to adopt the original packet transmission mechanism as defined in the IEEE 802.16 standard.

Fig. 2.6 depicts the flowchart of switching process for packet transmission in the proposed APC approach. The entire process is constructed by the aforementioned procedures. It can be observed that the communication operation switched between direct and indirection manners is dominated by the constraint 𝒞₂. In other words, the APC approach always selects the most efficient transmission manner for intra-cell traffic based on the channel conditions among the BS and SSs. It is noted that the channel conditions can be obtained via calculating the SINR value periodically. Consequently, it can be expected that the network throughput is enhanced while the proposed APC approach is exploited.

Begin of APC approach

The 1st time to execute APC

approach

Information collection:

Location of SSs

Information collection:

SINR of direct link

Admission control:

satisfy C₁ and/or C₂ constraint

Indirect communication (IEEE 802.16)

MCS change for the direct link Direct communication

(proposed) Satisfy C₂ constraint

Terminate APC approach

No Yes

No

Yes Yes

Yes

No

No

No

End of APC approach

Yes

Figure 2.6: Flow diagram of APC approach.