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.