Network Functionalities | WiMAX Architechture

This section describes the main functions achieved by the WiMAX end-to-end system.

1 Network Discovery and Selection

This function is required for nomadic, portable and mobile WiMAX services where in the same geographical area the MS may have radio coverage access to an ASN managed by a single NAP and shared by several NSPs or coverage access to several ASNs managed by several NAP/NSPs. To perform network discovery and selection, the MS (which has been pre-provisioned with configuration information at subscription) performs a four-step process:

§  NAP discovery. During the scanning of the DCD of the different BSs in the coverage reach from the MS, the MS detects the ‘operator ID’ in the BSID field.

§  NSP discovery. The MS discovers the available NSP through the list NSP ID which is broadcast by the ASN as part of the system information identity message. NSP discovery is also possible via solicited request/response messages.

§  NSP enumeration and selection. Based on the dynamic information obtained in the coverage area and the configuration information from the subscription, the MS selects the appropriate NSP. Manual configuration of the NSP may also be available in the case of a visited NSP.

§  ASN attachment. After selection of the NSP and associated ASN, the MS indicates its NSP selection by sending an NAI (Network Access Identifier) message used by the ASN to determine the next hop AAA where an MS AAA packet should be routed to.

2 IP Addressing

WiMAX networks support IPv4 and IPv6 addressing mechanisms. At the end of the procedure, a PoA (Point of Attachment) IP address is delivered to the MS. The IP allocation address modes depend on the WiMAX access service types.

In the case of IPv4, the dynamic PoA configuration is based on DHCP. The DHCP proxy may reside in the ASN and the DHCP server in the CSN. In the case of IPv6, stateful IP address allocation is based on DHCPv6. The DHCP server resides in the CSN and the DHCP proxy may reside in the ASN. For the stateless CoA (Care of Address), IP address allocations RFC 2462 and RFC 3041  are used.

3 AAA Framework

The AAA framework follows the IETF specifications and includes the following services:

§  authentication: device, user or combined user/device authentication;

§  authorisation: user profile information delivery for sessions, mobility and QoS;

§  accounting: delivery of information for pre-paid/post-paid services.

Authentication and authorisation procedures are based on the EAP (Extensible Authentication Protocol). Between the MS and the ASN (the authenticator function), EAP runs over PKMv2, which enables both user and device authentication. Between the AAA server and the ASN, the EAP runs over RADIUS.

Accounting is also based on RADIUS messages. WiMAX Release 1 offers both offline (post-paid) and online (pre-paid) accounting. In the case of offline accounting, the ASN generates UDRs (Usage Data Records), which are a combination of radio-specific parameters and IP-specific parameters. The serving ASN then sends the UDR to the RADIUS server.

4 Mobility

The mobility procedures are divided into two mobility levels:

§  ASN anchored mobility procedures. This refers to MS mobility where no CoA address update is needed, also known as micromobility.

§  CSN anchored mobility procedures. The macromobility between the ASN and CSN is based on mobile IP protocols running across the R3 interface.

CSN anchored mobility implies that, in the case of IPv4, the MS changes to a new anchor FA (Foreign Agent). WiMAX systems must support at least one of the following mobile IP schemes:

§  Proxy-MIP. In this case, the MS is unaware of CSN mobility management activities and there is no additional signalling/overhead over the air to complete the CSN mobility.

§  Client MIP (CMIPv4). In this case, the MIP client in the MS participates in inter-ASN mobility.

5 End-to-End Quality of Service

The IEEE standard defines the QoS framework for the air interface. The WiMAX architecture specifications extend the QoS framework to the complete network where many alternatives for enforcing the QoS on Layer 2 or Layer 3 may exist.

The end-to-end QoS framework relies on functions implemented in the CSN (PF (Policy Function) and AF (Application Function)) and in the ASN (SFM (Service Flow Management) and SFA (Service Flow Authorisation)). In the CSN, the AF triggers a service flow trigger to the PF based on the information sent by the MS with whom it communicates. The PF then evaluates service requests against a policy database in the NSP. In the ASN, the SFA communicates with the PF and is responsible for evaluating the service request against user QoS profiles. The SFM (located in the BS and responsible for creation, admission, modification and release of 802.16 service flows) mainly consists of an admission control function, which decides, based on available radio resource and other local information, whether a radio link can be created.

Multicast Broadcast Services (MBS) | WiMAX

Multicast Broadcast Services (MBS) may be required when multiple MSs connected to a BS receive the same information or when multiple BSs transmit the same information. Indeed, this allows resource to be saved by allocating a single radio pipe for all users registered to the same service instead of allocating as many pipes as there are users. This is of particular interest for the broadcast TV type of application where, at the same time, several users under the same coverage area are connected to the same service (in this case, a TV channel).

1 Multi-BS Access MBS

The mobile WiMAX system supports MBS as an optional feature for the BS. When the MBS feature is supported, the Multi-BSs Access mode is implemented, as defined in the IEEE 802.16e standard.

In a Multi-BS MBS system, several BSs in the same geographical area transmit the same broadcast/multicast messages at the same time on the same frequency channel. These BSs actually belong to MBS_ZONE. An MBS_ZONE is a unique identifier, which is transmitted from each BS of the set on the DCD message. An example of an MBS_ZONE allocation is provided in Figure1. It has to be noted that a BS may belong to different MBS_ZONEs.

Figure: Example of MBS zone deployment and MBS_ZONE allocations. All the MS registered to an MBS_ZONE can receive MBS signals from any BS of the MBS_ZONE

A Multi-BS MBS operation requires from the BSs belonging to the same MBS_ZONE:

§  time synchronisation (frame number and symbol level);

§  use of the same CID for broadcast/multicast messages;

§  use of the same Security Association (SA) for the encryption of the broadcast/multicast messages.

Using a Multi-BS MBS solution provides two major advantages. First, the MSs that successfully registered to a MBS service can receive MBS information from any BS of the MBS_ZONE without needing to register to a specific BS of that zone (even MS previously in the Idle mode). Secondly, the MS receives MBS signals from multiple BSs simultaneously. This provides a macrodiversity gain and performance enhancement for the MBS signals. This is actually similar to the Single Frequency Network (SFN) concept that can be found in DVB-H systems.

2 MBS Frame

When the MBS is activated on a BS, it can use a dedicated frequency or it can use only a dedicated zone in the frame, as illustrated in Figure 2. The descriptions of existing MBS zones in a frame are indicated using the DL-MAP in the extended DIUC 2 field (DUIC = 14) by MBS_MAP_IE, which includes (among others) the MBS_ZONE identifier, the symbol offset where the MBS zone starts, the permutation to be used (PUSC or other) and the size of the MBS zone (in symbols and subchannels).

Figure 2: Example of the 802.16 frame with the MBS service zone. The presence of the MBS zone is indicated in the DL-MAP message (in a MBS_MAP_IE field). The exact details of the MBS zone are then described in the MBS_MAP message at the beginning of the MBS zone

Each MBS zone starts with MBS_MAP, which describes the MBS connections available in the MBS zone. In particular, it contains, among others, the CIDs used by the multicast/broadcast connections, the channel configurations (modulation, coding and eventually repetition coding) and the logical channel ID. Logical channel IDs are used to distinguish the different MBS connections inside a MBS zone that belong to the same MBS CID (for instance, this may be used to differentiate between different TV channels with different subscription rights).

The MBS PDUs in the MBS region follow the order of the combination of multicast CID/logical channel CID, as described by the service association between the MS and the BS. Of course, from the time when the MS goes into the Idle mode or when the MS moves across different BS areas of the same MBS_ZONE there is no need for the MS to be registered to the BS to receive the MBS content. The mapping between the multicast CID and the logical channel CID must be the same in the cells belonging to the same MBS_ZONE. The procedure to provide consistency of the identities used by the MBS channels is beyond the scope of the IEEE 802.16 standard.

MIMO (Multiple-Input Multiple-Output) Solution

MIMO Basics

MIMO systems use multiple input and multiple output antennas operating on a single channel (frequency). At the transmitter side, the signal is space–time encoded and transmitted from N_T antennas. At the receiving side, the signals are received from N_R antennas (see Figure 1). The space–time decoder combines the signal received by the N_R antennas and transmitted from the N_T antennas after having estimated the channel matrix (N_T×N_R).

Figure 1: Generic MIMO block diagram for the downlink

The objective of the MIMO solution is to exploit the space and time diversity of the channels on the different radio paths between each combination of transmit/receive antennas to improve the reception sensitivity and/or to improve the channel capacity. There are several families of MIMO solutions. The two extreme ones are the spatial diversity MIMO schemes and the spatial multiplexing MIMO schemes.

In addition, several MIMO schemes exist that are a mix between SM and spatial diversity schemes. The diversity order and capacity increase depends on the space–time code and number of antennas.

More recently, MIMO schemes using pre-coding have been defined. In these cases, the space–time code depends on a feedback from the receiver on the channel states. Indeed, this solution requires a closed-loop operation and additional signalling between the receiver and the transmitter.

Finally, MIMO can also be generated from signals transmitted from different BSs (virtual MIMO). This requires time synchronisation of the BS but also a synchronisation of the scheduler of the BS involved in the transmission. 

System Design Aspects of BS and MS

As indicated in Figure 1, MIMO operation has a significant impact on the design of the BS and the MS. Indeed, in addition to the MIMO codes, the algorithms for encoding and decoding MIMO signals, there is the requirement to implement several transmitter and several receivers both at the BS and MS sides. This may be critical for the MS side. Indeed, implementing several receiver chains implies several antennas on a small device and additional hardware and processing power. Moreover, several transmitters at the MS side also mean significant additional power consumption. Efficient implementation of MIMO at the MS side has then some technological challenges to be solved.

The preferred antenna configuration for MIMO is when the multiple antennas are perfectly uncorrelated. In that case, the performances of MIMO are optimal. Good correlation is obtained if at the BS side the antennas are separated by 10 to 20 λ and at the MS side by at least λ. The latter requirement makes solutions with more than two antennas at the MS impractical.

In some cases, the additional effort needed for the space–time decoder is significant. This also imposes some limits on the number of transmit antennas at the BS side.