Related Work for Enhancing Inter-MBS HO

During inter-MBS zone HO, the MBS services may be disrupted. The service disruption may be resulted from the HO delay and the data discontinuity between source and target MBS zones. In the IEEE 802.16-2009 [1], it supports daisy-chaining of MBS MAP messages that can be applied to support the level-1 frame-offset coordination. In Chapter 4, the daisy-chaining scheme for inter-MBS zone HO is termed as the original WiMAX scheme [23]. In [24], the authors reduce the average inter-MBS zone HO delay by reducing the possibility of inter-MBS zone HO. An MBS zone is partitioned into multiple location management areas (LMAs). By tracking the locations of MBS users, the packets are only selectively transmitted to LMAs in which the MBS users reside. As a result of saving radio bandwidth in those LMAs in which no any MBS user resides, the LMA scheme [24] allows a large MBS zone configuration that can reduce the number of inter-MBS zone HOs. The LMA scheme focuses on inter-MBS zone HO avoidance instead of focusing on data continuity and packet loss issues during inter-MBS zone HO. The study in [25] focuses on reducing the inter-MBS zone HO delay by using an overlapping zone (OLZ) configuration. The OLZ scheme [25] configures the overlapped area between any two adjacent MBS zones in a cell. Therefore, a cell overlapped by three adjacent MBS zones uses triple bandwidth to send the same MBS service. When an MBS user moves to a cell with overlapped zones, the MBS user also joins all overlapped MBS zones belonging to the same MBS service. The OLZ scheme focuses on reducing the HO delay and avoiding inter-MBS zone HO. However, it results in large bandwidth overhead, especially in a cell overlapped by three adjacent MBS zones.

In Chapter 4, we propose a dynamic MBS zone (DMZ) framework to support the level-2 frame-offset coordination. Based on the proposed DMZ framework, a seamless dynamic inter-MBS zone handover (called DMZ HO) scheme is proposed to resolve the data discontinuity (or packet loss) problem during inter-MBS zone HO. In section 4.1, we will detail

the proposed DMZ HO. Table 2.2 shows a qualitative comparison of the existing inter-MBS zone HO schemes. Note that the proposed DMZ HO scheme is also included for comparison.

Note that the proposed DMZ HO scheme, using dynamic overlapping zones, is much more cost-effective than the OLZ scheme [25], which uses static overlapping zones and has large bandwidth overhead.

Chapter 3

A Network Assisted Fast Handover Scheme for IEEE 802.16e Networks

In this Chapter, we present a novel network architecture, which complies with the IEEE 802.16e standard, to support seamless frequent HO, especially for MSs with high mobility. Based on this architecture, a network assisted fast handover (NFHO) scheme is proposed to shorten service disruption time during the HO process. By resolving CIDs (connection identifiers) assignment and uplink timing adjustment issues, the proposed NFHO scheme can restart both the uplink (UL) and downlink (DL) packet transmissions before the MS proceeds to the HO ranging, which is a unique feature of our scheme. In addition, based on the NFHO scheme, an analytic model has been developed to investigate the expected number of buffered packets, packet loss probability, and service disruption time during HO. Performance evaluation results show that

the NFHO scheme reduces the DL service disruption time by 75% compared to the IEEE 802.16e hard HO scheme, and it also reduces the UL service disruption time by 55.6% and 75%

compared to Jiao et al. and the IEEE 802.16e hard HO scheme (Choi et al. as well), respectively.

In addition, the proposed NFHO scheme has the best performance in terms of expected number of buffered packets and packet loss probability among existing hard HO schemes for the IEEE 802.16e. Furthermore, our analytic model can be integrated to an admission control policy to guarantee proper QoS for ongoing HO MSs.

3.1 Proposed NFHO Scheme

To restart data transmission ahead of the Ranging and Network Re-entry stage, two issues need to be resolved. One is UL synchronization and the other is updating transport CIDs, which are used by active connections for data transmission in the target BS. We first propose a novel network architecture for IEEE 802.16e networks. Based on this architecture, a network-assisted fast HO (NFHO) scheme is proposed to reduce the execution time of the HO Execution procedure. Fig. 3.1 shows the proposed network architecture. A set of BSs are linked to a convergence sublayer switch center (CSSC) through the wired network. Here we design a CSSC that handles two original CS functions: the MAC function of the service-specific CS (either ATM CS or packet CS) and the backbone management messages exchange. In addition, the CSSC also handles control messages for bicasting data packets to both the serving BS and the selected target BS. A BS handles the functions of MAC CPS, security sublayer, and PHY layer.

Fig. 3.1: Proposed network architecture for IEEE 802.16e networks.

Fig. 3.2: Protocol layering of the proposed network architecture.

Fig. 3.2 illustrates the protocol layering of the proposed network architecture. We place ATM CS and packet CS in the CSSC. The HO within a CSSC is termed as intra-CSSC HO, and the HO between CSSCs is termed as inter-CSSC HO. The CIDs need to be changed only when the MS handovers to a BS that belongs to another CSSC. That is, based on the proposed network architecture, the target BS does not need to reassign CIDs for intra-CSSC HO. To reassign CIDs for inter-CSSC HO, we propose control messages, exchanged between serving BS and target BS in the HO Decision and Initialization stage, to pre-assign CIDs for the MS. Therefore, the MS can obtain new CIDs before entering the HO Execution procedure. In this way, the CIDs reassignment issue can be resolved. The control messages exchange for CID pre-assignment during inter-CSSC HO will be detailed in section 3.1.2.

To resolve UL synchronization, we propose to redo association level 2 in the HO Decision and Initialization stage. After a target BS is selected, the MS may redo scanning and association level 2 for the selected target BS if the current UL synchronization parameters obtained from the Cell Reselection stage are considered not up-to-date. Therefore, the MS can obtain correct UL synchronization parameters before entering the HO Execution procedure. After synchronizing to

the DL in the Synchronization to Target BS Downlink stage, the MS can adjust the UL synchronization parameters to synchronize to the UL immediately. Besides, we also propose an open-loop fine-tuning method that can improve the accuracy of the UL timing adjustment offset.

The open-loop fine-tuning method is detailed in section 3.1.1.

We use the same CIDs and the pre-assigned CIDs for intra-CSSC HO and inter-CSSC HO, respectively. Furthermore, the proposed scheme enables the MS to synchronize to the UL immediately after the DL is synchronized. Since DL packets are bicast to the selected target BS as well as the serving BS at the HO Decision and Initialization stage, the transmission of both DL and UL packets can be immediately restarted after the MS completes the Synchronization to Target BS Downlink stage. Therefore, we can shorten the packet transmission delay resulted from waiting for the completion of the Ranging and Network Re-entry stage during the HO Execution procedure. The detailed intra-CSSC HO and inter-CSSC HO message sequence charts (MSCs) are described in the section 3.1.2.

Note that the proposed network architecture complies with the IEEE 802.16e standard.

Since the CSSC still handles its original CS functions, it can follow MAC SAP specifications to access the MAC CPS. As to the backbone management messages, exchanging between CSSC and MAC CPS, can be handled by an add-on software component for supporting the proposed NFHO scheme. That is, only an add-on software component with a small cost is required to support the proposed NFHO scheme. Note that even without the add-on software component, the CSSC and BS can still function through the MAC SAP for supporting the original IEEE 802.16e hard HO scheme. Therefore, the proposed network architecture complies with the IEEE 802.16e standard. Note that in the proposed network architecture, the CSSC is logically separated as an individual component; however, from an implementation perspective, the CSSC can be still located in one of the BSs, and the other BSs have wired links to the CSSC.

Fig. 3.3: Re-association to renew UL parameters.

3.1.1 Acquiring UL Synchronization Parameters

Association in the Cell Reselection stage is used to acquire ranging parameters and service availability information that could expedite the HO Execution procedure. In the IEEE 802.16e, association level 2 provides better efficiency than the other two association levels (0 and 1).

Here we assume association level 2 is adopted. As mentioned above, after an HO target BS is selected, we redo association to acquire fresh UL synchronization parameters in the HO

Decision and Initiation stage. Fig. 3.3 shows the flow chart of re-association to the selected target BS for acquiring UL synchronization parameters. If the UL parameters of the selected target BS are considered to be out-of-date, the MS should renew the UL parameters by doing association to the selected target BS. The decision to renew UL parameters depends on some factors, such as the difference of DL arrival times since last association, the decay of mean CINR (carrier to interference and noise ratio) since last association, and a refresh timer. Here we simply assume that the MS decides to renew the UL parameters when a refresh timer has expired.

γ

γ

τ

Fig. 3.4: The difference of DL signal delays between synchronization at the association and synchronization at the Synchronization to Target BS DL stage.

Based on the difference of DL signal delays between the synchronization at the association and the synchronization at the Synchronization to Target BS DL stage, we propose an open-loop

target BS at location ‘a’, and a frame was synchronized at γa, which was the clock time measured by the MS. During the Synchronization to Target BS DL stage, the MS moves to location ‘b’, and a frame is synchronized at γb, which is the clock time measured by the MS. Therefore, we have

f f

a

b

− γ = n ⋅ T + τ τ << T

γ ,

where Tf is the frame duration, n is the number of frames in the time period between ‘a’ and ‘b’, and τ is the difference of DL signal delays between locations ‘a’ and ‘b’. Since the UL has also the same difference of the UL signal delays between ‘a’ and ‘b’ as that of the DL, we add the difference, τ, to fine-tune the UL timing adjustment offset. Thus, we have

τ η

η _b = _a +

where ηa is the UL timing adjustment offset obtained from the last association, and ηb is the fine-tuned UL timing adjustment offset that will be used at the Synchronization to Target BS Downlink stage. By acquiring a correct UL timing adjustment offset, the MS can also synchronize to the UL immediately after synchronizing to the DL of the target BS. As a result, the MS can restart the DL and UL data transmissions before the Ranging and Network Re-entry stage.

3.1.2

To reduce the packet transmission delay due to the HO Execution procedure, based on our proposed network architecture, we bicast DL traffic to both the serving BS and the selected target BS to avoid the latency caused by data forwarding. Note that bicasting DL traffic will incur extra bandwidth overhead between CSSC and BS. However, since the link between CSSC and BS is a wired link, the extra bandwidth overhead is not a concern. In the HO Execution procedure of the IEEE 802.16e, the UL/DL data packets can only be transmitted after the Ranging and Network Re-entry stage. In the Ranging and Network Re-entry stage, the IEEE

802.16e provides HO optimization options that could omit authorization by sharing the MS’s context between the serving BS and target BS and grouping the Basic Capabilities Negotiation and Registration into the HO ranging. Note that the configuration of the HO optimization options in each BS shall be announced in the MOB_NBR-ADV message. Therefore, by configuring the HO optimization options, data transmission could be restarted immediately after the HO ranging. In the HO ranging, the target BS provides the MS necessary parameters, including UL timing adjustment offset, frequency corrections, transmission power level corrections, and basic and primary management CIDs. The proposed NFHO scheme shortens the data transmission delay by restarting the UL and DL data transmissions ahead of the HO ranging. Remind that these parameters, except basic and primary management CIDs, can be obtained in the last association, and the UL timing adjustment offset can be further fine-tuned for UL synchronization in the Synchronization to Target BS Downlink stage. In addition, based on the proposed network architecture, the intra-CSSC HO does not need to update transport CIDs. Therefore, normal data packet transmission can be restarted before the HO ranging, and the service disruption time resulted from the HO ranging could be eliminated.

Fig. 3.5: Intra-CSSC HO MSC.

Fig. 3.5 shows the MSC of the proposed intra-CSSC NFHO scheme. The bold lines show packet flows and the others are management messages. The management messages with dashed lines exchanged between BSs are relayed via a CSSC. The management messages with solid lines are direct messages from source to destination. The management message terms with all capital letters are the MAC management messages defined in the IEEE 802.16e and the other management messages terms with leading capital letters and followed by low case letters are added messages for the NFHO scheme. The proposed intra-CSSC NFHO scheme that shortens service disruption time is detailed as follows:

1) After the HO target is selected in the HO Decision and Initialization stage, the MS starts a re-association to renew UL parameters for the target BS if the MS considers the current UL parameters of the target BS to be out-of-date. The renewed UL parameters will be used for UL synchronization in the Synchronization to Target BS DL stage.

2) When an MS issues MOB_MSHO-REQ to the serving BS, the serving BS should negotiate the QoS requirement of the MS with the target BS via the added backbone messages:

HO_notification_req and HO_notification_rsp.

3) The serving BS issues an HO_bicast_enable message to enable bicasting DL packets to the target BS. After that, each DL packet bicast to both the serving and target BSs is tagged with a sequence number. In Fig. 3.5, we denote a tagged packet flow as Packets(tag).

4) Because the HO process optimization options are enabled to omit authorization, the serving and target BSs must use MS_context_transfer_req and MS_context_transfer_rsp to transfer the context of the MS. After that, the serving BS sends MOB_BSHO-RSP to the MS.

5) The MS sends MOB_HO-IND to inform the serving BS that the HO is started. Then, the MS enters the Synchronization to Target BS DL stage and starts synchronizing to the target BS.

Meanwhile, the serving BS sends HO_nextframe_indication to notify the target BS of the tagged sequence number of the next expected DL packet. After synchronizing to the DL of the target BS, the MS also synchronizes to the UL by the acquired UL parameters described in section 3.1.1. As mentioned above, there is no need to update transport CIDs for the active connections of the MS during intra-CSSC HO. Therefore, the target BS can start to allocate DL_MAP_IE and UL_MAP_IE for DL and UL data packets after receiving HO_link_up.

The MS sends HO_link_up to inform the target BS of UL synchronization. Note that the HO_link_up could be a bandwidth request packet instead. After that, the target BS issues HO_bicast_disable to disable bicasting and let the DL data path switch to the target BS only.

packet transmission. Since the HO optimization options are enabled, the unsolicited SBC-RSP (Basic Capabilities Negotiation) and REG-RSP (Registration) are appended to the RNG-RSP.

Fig. 3.6 shows the MSC for the proposed inter-CSSC NFHO scheme. The MSC is similar to that of the intra-CSSC HO except that the target CSSC updates transport CIDs for the active connections of the MS, and these updated transport CIDs are sent back to the MS through MOB_BSHO-RSP. The message followed by “(CIDs)” denotes that these updated transport CIDs are attached. The following details the inter-CSSC HO MSC.

1) The MS performs association level 2 to the selected target BS and then obtains UL parameters from MOB_ASC-REP.

2) The MS starts an HO by issuing MOB_MSHO-REQ to the serving BS. The serving BS uses HO_notification_req to negotiate MS’s QoS requirements with the target BS. Since this HO is an inter-CSSC HO, the target CSSC needs to update transport CIDs for the active connections of the MS. The target BS requests a connection setup to the target CSSC by sending HO_connection_setup_req. Then the target CSSC assigns transport CIDs and initializes a classifier which will classify upcoming bicasting packets by the assigned transport CIDs. The target CSSC sends HO_connection_setup_complete with assigned transport CIDs to the target BS. Finally, the assigned transport CIDs are delivered to the serving BS through HO_notification_rsp.

3) The HO_bicast_enable is issued by the serving BS to enable bicasting DL data packets to the target CSSC. After that, each DL packet tagged with a sequence number is bicast to both the serving and target BSs. The target CSSC and target BS will use the assigned transport CIDs to transmit packets.

4) MS_context_transfer_req and MS_context_transfer_rsp are used to transfer the context of the

MS from the serving BS to the target BS. The serving BS sends an MOB_BSHO-RSP message with the assigned transport CIDs appended to respond to the received MOB_MSHO-REQ.

5) After sending MOB_HO-IND, the MS enters the Synchronization to the target BS DL stage.

After synchronizing to the DL, the MS also immediately synchronizes to the UL by the acquired UL parameters described in section 3.1.1. The serving BS sends the target BS HO_nextframe_indication to notify the next expected sequence number of the tagged DL packet. Since the connection setup between the target BS and the target CSSC have been done and the bicast DL packets in the target BS have been constructed with assigned transport CIDs, the target BS can go ahead to schedule for UL/DL data transmission without waiting for the completion of the HO ranging. The HO_link_up issued by the MS is used to indicate that the MS has synchronized to the UL of the target BS; therefore, the HO_link_up could be

After synchronizing to the DL, the MS also immediately synchronizes to the UL by the acquired UL parameters described in section 3.1.1. The serving BS sends the target BS HO_nextframe_indication to notify the next expected sequence number of the tagged DL packet. Since the connection setup between the target BS and the target CSSC have been done and the bicast DL packets in the target BS have been constructed with assigned transport CIDs, the target BS can go ahead to schedule for UL/DL data transmission without waiting for the completion of the HO ranging. The HO_link_up issued by the MS is used to indicate that the MS has synchronized to the UL of the target BS; therefore, the HO_link_up could be