Potential Issue once the Subscriber Station Register via the Network Entry Procedure

It is an ample solution for a regenerated Subcriber Station (SS) to co-locate itself to the Base Station (BS) so that it able to join the network. A potential issue is exposed once the SS registers itself via the network entry process. The problem is that the SS keeps the same channel that is has scanned and found at initial.

The issue with this situation is that as every newly registering SS takes it action the distribution of the frequency band can be crowded in many section of the BS's operating frequency band. As in consequence, the effectiveness of those packed bands reduces relate to the number of SSs that are contending for that channel to transmit data.

In correlation, a reducing of the channel efficiency straightforwardly increases the probability that collision will occur.

The Importance of DCD and UCD

The DCD and UCD are important because they deal with the MAC of Subscriber Station (SS) by providing organizing information that useful when a SS is capable to send and receive data. The DCD message provides information when the SS is potentially to receive data. This is accomplished by recounting which frame the DL process will start on and the period of the process itself.

The UCD messages gives the SS with information of when it is capable to either re synchronize the channel that is currently in use or send data to the Base Station.

To obtain this channel descriptor messages the SS shall scan the whole downlink frequency band used for downlink channel. When a downlink channel is found then the MAC will attempt to set up synchronization with the BS through that one channel.

In an non-erroneous network, the SS should obtain UCD and DCD messages for the synchronized channel. From this messages the SS is capable to characterize the process and timing of the network that will let it to transmit data and receive data as well.

At the completion of the remaining core network entry procedures the SS is able to network with the BS and utilizing the channel that it has first found in the scanning step.

Subscriber Station Internal Nonvolatile Storage

At first a Subscriber Station (SS) has no resources or information of the network, provided that its internal nonvolatile storage does not hold any previous information of the surrounding network. With no any information of the network, the SS is incapable to get relevant data about the characteristic of the network and send information.

The SS has to find information regarding the system throughout channel descriptor messages in order to get or transmit data. These messages are distributed to the system from the Base Station (BS) at periodic interval.

Ranging and Power Adjustment in WiMAX

As explained in IEEE 802.16 standard, ranging is the process of obtaining the right timing offset such that the subscriber station transmission are associated to a symbol that marks the start of a minislot boundary.

Timing offset is dictated by the distance between subscriber station (SS) and to the base station (BS) and the equivalent signal propagation delay. The SS begin this process by scanning UL-MAP messages for an available maintenance interval.

Once an available maintenance interval has been determined, the SS will send a Ranging Request (RNG-REQ) message, within this contention based initial maintenance period, to the BS at minimum power level. If this transmission does not receive a response, the SS will increase the power level incrementally as necessary, but not to exceed the maximum specified transmission power.

The BS will reply with a Ranging Response (RNG-RSP) message, which specifies the appropriate timing advance and power adjustment for the SS, as well as the basic and primary managements CIDs.

Obtaining Transmit Parameter in Initial Scanning for Channel

Once a DL-MAP message has been detected, the MAC sublayer will listen for downlink and uplink transmission parameter. By listening for UCD messages from the Base Station (BS), the Subscriber Station (SS) is able to determine a usable uplink channel.

UCD messages are broadcast messages, send out periodically, providing pertinent parameters for all available uplink channel, and will attempt to establish communications on suitable channel.

If communications fail on one channel, the SS will move on to next suitable channel until a connection is establish or the list has been exhausted, in which case it will begin the scanning process again.

Scanning and Synchronization to the Downlink in Initial Scanning

Subscriber Station (SS) are designed to scan their frequency lists for active downlink channel right away upon installation or following any period of signal loss. In case of signal loss, the SS will store operational parameters of the last signal and will attempt to restore the connection.

After obtaining a channel with a valid downlink signal, the SS will try to synchronize the PHY by listening for DL-MAP management messages. The SS will continue to listen for DL-MAP management messages and in the case of missing DL-MAP messages the SS will repeat the scanning and synchronization process.

Initial Scanning for Channel in WiMAX point-to-multipoint (PMP)

When Subscriber Station (SS) wants to join a WiMAX in point-to-multipoint (PMP) network, it must follow a set of procedure as IEEE 802.16 Standard. The set of essential routine steps is as outlined below:

1. Scan for downlink channel and establish synchronization with the BS.
2. Acquire transmit parameters.
3. Perform ranging.
4. Negotiate basic capabilities.
5. Authorize subscriber station and perform key exchange.
6. Perform registration.
7. Establish IP connectivity.

Network Entry and Initialization, part 3.

Upon the network entry of a client into the network, the BS scans through its frequency list attempting to synchronize with a BS. The BS performs the admission control algorithm to make a decision whether to admit the SS, based on the QoS requirements requested by the SS and the existing resource availability of the BS. If admitted, the BS generates a set of connection and Connection Identifier (CIDs), together with the management and transport connections, to connect with the SS. Three pairs of management connections are established for control packets, and the transport connection is used for data transmission.

Along with the new CIDs, the BS also assigns new service flow identifiers (SFIDs) to the new data flows linked with the station. A service flow (SF) is a unidirectional flow of MAC SDUs between a pair of BS and SS, of which is provided with a particular set of QoS parameters, and a SFID uniquely identifies the service flow.

WiMAX Network Entry and Initialization, part 2.

The network entry process can be separated into a few phases. Within each phase, particular MAC management messages  are exchanged among Base Station (BS) and Subscriber Station. On the establishment of process, the SS scans individual DL channels searching in favor of the DL-MAP and DCD messages broadcasted by the BS.

Using these messages, the SS get synchronize with the BS and obtains DL paremeters. As successfully synchronized, the SS wait for the UL-MAP and UCD messages to acquire UL parameters.

WiMAX Network Entry and Initialization, part 1.

Once a SS senses a BS, for a network entry, it will first scans for downlink channel (DL) and trying synchronizes itself with the BS. After that, SS shall obtain transmit parameters, which are integrated in the Uplink Channel Descriptor (UCD), Uplink (UL)-MAP, and Downlink (DL)-MAP. The final step would be to complete initial ranging.

The BS is required to discover different received ranging codes, approximate the timing offset and the power for every user that bears and initial ranging code. The BS then broadcast the detected ranging codes with some change instruction for the proper timing and power level. Other than that, the status notifications of either ranging successful or retransmission are also broadcasted.

WiMAX Point-to-Multipoint and Mesh Network

In Point-to-Multipoint (PMP) mode, a single-hop network contains a central unit as a Base Station (BS) to makes decision to the entire nodes such as Subscriber Station (SS) within its cell that is a central geographic entity of a cellular structure.

In a mesh network, a number of SS are not in direct contact through the BS where a relay system is necessary on intermediary SS that resolve relay information toward other SS that are not in direct contact with the BS.

WiMAX MAC Layer

One of the important aspects of WiMAX is the MAC layer supports more than one physical layer (PHY) mode. This characteristic not only enable companies to differentiate their product from each other, but also makes WiMAX to be an adaptive technology that can suit special requirements depending on application.

The WiMAX MAC layer performs the standard function of providing a medium independent interface to the PHY layer. The major focal point of the MAC layer is handling resource of air-link proficiently.

MAC layer is responsible on the whole connection to arrange packet transmission among multiple Point-to-Multipoint (PMP) and mesh network models.

The MAC layer is separated into three sub layers. There are called specific convergence sub layer (CS), MAC common part sub layer (CPS), and the security sub layer.

The MAC CS support asynchronous transfer mode (ATM) service and packet service used for packet-switched networks. The MAC CPS provides utilities to facilitate both types of service in the CS.

The security sub layer is the third sub layer within the MAC layer. It is designed for privacy, authentication, and confidentiality used for the SSs in the network.

The MAC is capable to grant differentiated service to users on the equivalent medium throughout a range of method. MAC design plays an essential role in WiMAX networks. A MAC protocol defining rules for orderly access to the physical shared medium is vital when it comes to efficient utilization and fair sharing of resources to desired system performance such as high throughput and short delay.

Reduce Scanning Time and Optimization of Ranging Process in WiMAX Network Entry

Reduce the channel scan approaches during the scanning time are able to achieve a fast scanning process but this approaches may not guarantee Subscriber Station (SS) to acquire a precise information for the purpose of association with Base Station (BS).

While optimization of ranging process may improve the connection but it will sometimes disregard the performance of WiMAX network traffic flow.

WiMAX

What are we know about WiMAX?

1. WiMAX bandwidth must be shared among multiple users and affect the speed in practice.
2. WiMAX is similar to WiFi but in can enable usage at much greater distances.
3. The Asia Pacific region has surpassed the North American region in terms of 4G broadband wireless subscribers.
4. WiMAX is the most energy-efficient pre-4G technique among LTE and HSPA+.
5. Throughput between clients and base station determined largely by distance.

Long Channel Scan and Collision in Ranging Process in WiMAX Network Entry

How the effect of long channel scan and collision in ranging process during scanning to ranging steps in network entry is managed? One method is by 'reduce scanning time' and another one is 'optimization of ranging process'.

Reducing scanning time approaches have had success in reduce a number of channels to scan so that fast scanning is achieved while optimization of ranging process approaches was appealing in term of optimizing the connection probability and average connection delay.

Other approaches also exist such as to upgrade the IEEE 802.16 protocol performance regarding the delay during subscriber network entry process, analyzing the collision probability and analytical modeling of network entry process.

Most of the approaches are primarily focused on modification of scanning and ranging procedure. All of these approaches correspond to the IEEE 802.16 standard but most of them are not easily implemented and ignored the ability of WiMAX system parameter setting. Most of them also not considered to implicate with better communication system's QoS for WiMAX network.

The overview of these approaches has shown that while they may be relevant for the associated environments, they do not provide solutions that could help in both process of scanning and ranging in WiMAX network entry. The question is, what is the appropriate technique that can be used to relate the specified process of scanning to ranging in network entry that can deal with the problem of long channel scanning and collision in ranging process?

Scanning to Ranging Problem during WiMAX Network Entry

The problem during scanning to ranging steps in network entry will degrade the access time for initialization and recovery of service in WiMAX network. For example, a lengthy channel scanning will causes a service disruption between the SS and BS. The collision in ranging will lead to disconnection between SS and BS.

As part of functionalities mechanism for network entry in MAC CPS, this could result unexpected delay and under utilization of WiMAX link.

One way to manage the delay during scanning to ranging steps is by evaluating the WiMAX network entry system parameter setting.

Most of the current approaches are too focused on scanning and ranging process and has ignored system parameters setting that can influence scanning to ranging steps delay in network entry.

Network Entry Process of WiMAX MAC CPS

Network entry process is part of MAC CPS sub layer. It is the first step for Subsciber Station (SS) joining the WiMAX network. It referred to the early procedure SS perform to register themselves with the Base Station (BS) that controls the 802.16 network.

There are several steps associated with network entry as following:

• Scanning to ranging.
• Intersection of SS's and BS's capabilities.
• Authentication and authorization.
• SS registration and connection establishment.

WiMAX MAC CPS Functionalities

The main functionalities of WiMAX MAC CPS are:

1. Network entry.
2. Connection management.
3. Quality of Service (QoS) control.
4. Air-link control
5. Protocol Data Unit (PDU) operation.
6. Mobility and power management.
7. Multicast and broadcast service.

The MAC CPS provides the fundamental MAC funtionality of system access, bandwidth allocation, connection establishment, and connection maintenance.

The MAC management messages are part of MAC CPS component. The IEEE 802.16 standard defines three types of connection for each messages those are broadcast connection messages, basic management connection messages, and primary management connection messages.

WiMAX IEEE 802.16 Standard

The worldwide Inter-operability for Microwave Access (WiMAX) is a telecommunication  technology based on IEEE 802.16 standard. WiMAX supports two types of network topologies which is Point-to-Multipoint (PMP) and Mesh. In the PMP, the link connection is only between Base Station (BS) and Subscriber Station (SS).

The standard was designed to evolve with media access control (MAC) layer consists of three sub layers. The three sub layers are:

1. Service specific convergence sub layer (CS)
2. MAC common part sub layer (CPS)
3. Security sub layer