4-1 Signaling Procedure for Network Layer Handoff
4-1.1 Data Path Set up
In the handoff signaling procedure proposed by WiMAX Forum [19], the access service network gateway (ASN-GW) performs the functionality of foreign agent (FA). If we switch the serving BS between the BSs controlled by the same ASN-GW, we only need to update the data path without performing the layer 3 handoff, which needs to change the care-of address if Mobile IP is used. If we change the serving BS between the BSs controlled by different ASN-GWs, the data path needs to be established between the new BS and ASN-GW first so that the user’s packets can be forwarded to the new BS correctly. Then, the Mobile IP handoff signaling might be initiated such that the correspondent can forward packets directly to the mobile’s care-of address. In so doing, the time to execute Mobile IP signaling which causes service disruption can be reduced. The Mobile IP signaling procedure is explored in next section.
Fig. 24 Signaling procedure for establishing data path
Fig. 24 is the signaling procedure for establishing data path. The user can decide to add the target BS into the diversity set based on the scanning results. The user transmits MOB_MSHO-REQ message to the serving BS for adding the target BS into the diversity set
and the target BS performs the signaling exchanges with the ASN-GW to set up the data path.
As the completion of establishing data path, the target BS will transmit handoff response to the serving BS, and the serving BS will inform the user that the data path has been established.
Then the user will transmit confirm message MOB_HO-IND to the serving BS.
4-1.2 Mobile IP
Before introducing the Mobile IP signaling procedure [24], define the following network elements:
z Home address: When the user is in its home network, the packet can be delivered to the user correctly through this IP address.
z Care-of Address (CoA): When the user is in the foreign networks, the user will obtain a Care-of Address. The user can use this CoA as its IP address such that the packets can be transmitted/received correctly.
z Home Agent (HA): A router located at the home network. When the user is in foreign networks, HA intercepts the packets belong to the user and forwards these packets to the current CoA of the user through IP-in-IP tunneling method.
z Foreign Agent (FA): A router located at the foreign network. FA receives the packets tunneled from the HA and de-tunnels these packets. According to the IP address of the IP header, the packets can be correctly sent to the user.
z Authentication, Authorization, Accounting (AAA) server: A server for authentication, authorization, and accounting. This server will verify if the user is a legal one. As performing the signal exchanges, this server is also used to verify the factuality of the signals.
z Mobile Node (MN): the user.
z Correspondent Node (CN): the user who is communicating with the MN currently.
Old
Fig. 25 Mobile IP signaling procedure
When the user receives the advertisement of the FA, he/she knows that he/she is in the foreign network. Therefore, the user needs to perform the Mobile IP signaling procedure to maintain the network connection such that packets can be transmitted/received correctly.
Fig. 25 is the detail signaling procedure for Mobile IP, and the signaling procedure is produced base on the WiMAX network reference model and Mobile IPv4 signaling procedure proposed by IETF. In WiMAX networks, the ASN-GW performs the functionality of the FA.
Message 1 represents that the user registers with the new ASN-GW and provides its home address and Network Access Identifier (NAI) to the ASN-GW. Next, the ASN-GW verifies if the user is a legal one through AAA servers using the message 2, 3, and 4 to transmit the user’s information to the HA for registration. If so, the HA will transmit messages back to the ASN-GW through AAA servers. Then the ASN-GW informs the old ASN-GW to let the old ASN_GW forward packets belong to the user to the new ASN-GW. Message 15 and 16 is to inform the correspondent node about the changing of the IP address such that the packets can be routed directly to the new ASN-GW without being routed to the old ASN-GW to reduce the consumption of network layer resources.
4-2 Signaling Procedure for Link Layer Handoff
4-2.1 Scanning and Association
The BS periodically broadcasts MOB_NBR-ADV message to all users in order to provide them with the information about neighboring cells so that users can synchronize with neighboring BSs quickly. Moreover, the users can utilize the information to determine the targets for handoff. Users can also decide if they need to perform scanning. If so, the users will be arranged to do initial ranging with the neighboring BSs to adjust timing, transmit power, and carrier frequency.
A MOB_SCN-REQ message may be transmitted by an MS to request scanning intervals for the purpose of seeking available BSs and determining their suitability as targets for handoff.
This information might also contain the base station id (BSID) and scanning types. In the message, the following items may also be included.
1. Scan duration: Duration (in units of frames) of the requested scanning period.
2. Interleaving interval: The period of MS’s normal operation which is interleaved between scanning durations.
3. Scan iteration: The requested number of iterating scanning interval by an MS.
A MOB_SCN-RSP message shall be transmitted by the BS either unsolicited or in response to an MOB_SCN-REQ message sent by an MS. MOB_SCN-RSP message consists of scanning durations (in units of frames), the report modes, the report periods, start frame, interleaving interval, scan iterations, BSID, and scanning types.
In the scanning procedure, if the users support association function, they would acquire and record the scanning results and service availability information which can become the basis of determining the targets for handoff and reducing its processing time. If the BSs can support coordination, the serving BS may coordinate the association procedure with the requested
neighboring BSs through the backbone network, arranging a contention-free initial rang code and ranging slots to the users. In so doing, the fast ranging can be achieved. The scanning procedure is shown in Fig. 26 [12]:
MS (Serving)BS1 (Target)BS2 (Target)BS3
MOB_SCN-REQ
Scan Duration = N frames Interleaving Interval =P frames Iteration = T times
MOB_SCN-RSP Start frame = M frames Duration = N frames
M fames
Synchronize with BS#2 Initial Ranging
RNG-RSP
Synchronize with BS#3 Initial Ranging
RNG-RSP Scanning
Interval
= N frames
Non-scanning Interval
= P frames Data Traffic
Scanning Interval
= N frames
MOB_NBR-ADV
Fig. 26 Signaling procedure for scanning
As shown in Fig. 26 in which N frames is meant for the scan duration, P frames for the interleaving interval, and T times for the iteration, the MS sends MOB_SCN-REQ message to the serving BS to request scanning the neighboring BS1 and BS2. When the BS receives the MS’s request, it will use the MOB_SCN-RSP message to reply back to MS with the same information receiving from it. Besides, the serving BS also informs the MS that the scanning will be start M frame later. If the serving BS requests the MS to report the scanning results, the MS can use MOB_SCN-REP message to report them back.
4-2.2 Initial Ranging and Fast Handover Ranging
In the IEEE 802.16e-2005 system [12], the uplink sub-frame allocates the ranging sub-channels under the OFDMA mode to let users perform ranging which can adjust timing, transmit power, and carrier frequency. For different purposes, ranging processes can be divided as follows:
1. Initial ranging 2. periodic ranging 3. bandwidth request 4. handover ranging
There are 256 pseudo-noise ranging codes (0~255) divided into four groups. The first N codes produced are for initial ranging, the next M codes produced are for periodic ranging, the next L codes produced are for bandwidth request, and the next O codes produced are for handover ranging. As shown in Fig. 27, MS can randomly choose a code from its group according to what the MS would like to perform. When the BS receives the message, it can recognize which operation MS would like to execute through the code.
N
(Initial Ranging)
M
(Periodic Ranging)
L
(Bandwidth Request)
O
(Handover Ranging)
Fig. 27 Ranging Codes
Fig. 28 is the initial ranging procedure. The brief description of ranging procedure is as follows:
1. When the user receives the UL_MAP, it will find out the location of ranging channels (slots). The user transmits randomly selected code in a randomly selected ranging slot to the BS.
2. The BS will reply the power, timing, and carrier frequency corrections with ranging code and ranging slot to the user using RNG-RSP.
3. Receiving RNG-RSP, the user will adjust its power, timing and carrier frequency.
4. The process is performed iteratively to adjust parameters. Until the signal quality of the
uplink is satisfied, the BS will inform the user that the state is success and the connection is established. The user can transmit/receive data at this moment.
When the connection has been set up and the data have been transmitted, the user needs to perform ranging periodically to maintain the connection quality owing to the movement of the user which causes the change of the connection quality. The periodic ranging procedure is similar with the initial ranging one, and the difference is that it is a contention-free ranging.
MS UL-MAP
Send map containing CDMA Initial Ranging IE
with a broadcast Connection ID
[ Time to send the CDMA Initial Ranging opportunity ]
Ranging Code
RNG-RSP
Transmit randomly selected Initial Ranging Code in a randomly selected Ranging Slot
from available Ranging Region
[ Receive Ranging Code ]
Send RNG-RSP with Time and Power Corrections and original
Ranging Code and Ranging Slot
Receive RNG-RSP message with Ranging Code and Ranging Slot matching sent values. Adjust Time and Power parameters.
Status = Continue
Send map containing CDMA Initial Ranging IE
with a broadcast Connection ID
[ Time to send the CDMA Initial Ranging opportunity ]
[ Receive Ranging Code ]
Send RNG-RSP with Initial Ranging Code in a randomly selected Ranging Slot
from available Ranging Region
Receive RNG-RSP message with Ranging Code and Ranging Slot matching sent values. Adjust Time and Power parameters.
Fig. 28 Initial ranging signaling procedure
We explain how to use the scanning results to expedite the link layer handoff such that the service disruption time can be reduced. In IEEE 802.16e-2005, it proposed the association functionality which records the scanning results for fast handoff. We assume that as performing the association, MS has performed the following actions:
1. For the neighbor BSs, the user’s timing and carrier frequency have been adjusted correctly through association. So, as performing handover ranging, only the power needs to be adjusted.
2. The possible handoff targets have already reserved a dedicated ranging slot and a dedicated ranging code to the user, so the user doesn’t have to perform the contention ranging.
For the power correction, our assumption is presented as follows:
1. The user transmits the dedicated ranging code using the maximum transmit power.
2. When the BS receives the ranging code and estimates it, the margin users need to adjust would be replied back by using the defined “TLV” (type/length/value) with 8 bites appended to UL_MAP_Fast_tracking_IE to complete adjusting only once instead of
“step size” adjustment.
3. The operation speed of power amplifier is very fast, and the power can be corrected about 5 microseconds.
We will transmit the information by further merging 1-(c) into 2-(A), so that we can shorten the processing time required for ranging.
The modified ranging procedure is shown in Fig. 29. Because uplink power can be oriented by the power amplifier in one time, the strength receiving from the users in 2-(b) can meet the basic communication needs. After handover ranging (2 frame time), the MS can return to the normal operation with the new BS.
Owing to the establishment of data path, the MS’s packets have been forwarded to the new BSs. Therefore, the time required for performing link layer ranging becomes the main factor which causes the service disruption. In IEEE 802.16e-2005, the frame durations are from 2 to 20 milliseconds; in the handoff process, thus, the maximum time of service disruption caused
by handover ranging is 40 milliseconds which is counted by 20 milliseconds for 2 frames. In so doing, the requirement for service disruption can satisfy real time services.
MS UL-MAP
Send map containing CDMA Fast Ranging IE
with a broadcast Connection ID
[ Time to send the CDMA Fast Ranging opportunity ]
Ranging Code
RNG-RSP
Transmit a unique Fast Ranging Code within the
dedicated Ranging Region
with MaximumTransmit Power
[ Receive Ranging Code ]
Status = Continue
Send map containing CDMA Initial Ranging IE
with a broadcast Connection ID.
[ Time to send the CDMA Initial Ranging opportunity ]
[ Receive Ranging Code ]