3.4 Support for adaptive antenna system
The most important feature of the IEEE 802.16 standard, which principally differ from, say, standards IEEE 802.11 a/b/g, is the presence of built - in support tools for adaptive antenna systems (AAS). Of course, the use of AAS is not a requirement of the standard. AAS is a system with sectored directional antennas (method of forming the antenna patterns in the standard does not specify), i.e. antenna system with multiple antenna elements. The use of AAS significantly increases the potential capacity of the network IEEE 802.16, since different sectors of BS can work in the same channel (frequency and OFDMA). In addition, directional antennas can significantly reduce the total radiated power. The result is reduced and interchannel interference. No less important is the use of multiple antenna systems to improve the passage of signals in fading channels, the so - called methods of space-time coding (explode) STC.
Support for ASS in IEEE 802.16 means a modification of the protocols at the physical and MAC levels, the presence of special control and Supervisory messages to work with adaptive antennas.
Figure 17 - Structure of the frame with the area ААS.
The standard permits within one frame to broadcast both undirected and directed (by AAS) traffic (Fig.6). For the delineation of zones of non-AAS and AAS-traffic uses a special message. The principle use of AAS in modes OFDM and OFDMA (and SCa) are quite similar. More fully described in the standard for the case of OFDMA [3], so focus on it.
The Mechanism Of Diversity-Map Scan. In the OFDMA mode provides two ways of working with AAS - distributed carriers in subchannel (FUSC, PUSC) and consistent with bearing (AMC). Each of the methods at the beginning of the AAS zone provides for the transfer of OFDMA symbol of the preamble AAS zone and header prefix AAS zone. For the transmission of these messages in the AAS zone downward subframe allocated to the special sub-channels (for two senior FUSC/PUSC and the fourth and the last-in AMC subchannels). Messages within these sub-channels may be repeated several times - that if you are not using broadcast broadcast and transmission shifting rays, messages prefixed with reach all the speakers. In the prefix code indicates the beam of the antenna, the type and size of the preamble ASS-zone (uplink and downlink), the area for initial initialization / query strip, and the area in the frame for each AAS connection. The prefix, as in the normal mode, is transmitted through QPSK with rate 1/2 coding and double repeat (within one character). The main purpose of the prefix is to inform the AC about how will be transferred to cards DL/UL channels for the split in the directions of the rays of user groups (obviously, the allocation of channel resources may occur independently in each beam).
For operation in mode AMS-AAS footage can join in supercar lasting not less than 20 ordinary frames. In supercat includes at least one broadcast frame containing the descriptors and maps DL/UL channels. The meaning of this Association is to provide at least a control message for the group shots.
These methods work with AAS use so-called mechanism of Diversity-Map scan (subscriber stations) posted maps of the distribution of channel resources. In the OFDMA mode provides another way of working with AAS - direct method alarm (Direct Signaling Method).
Method Direct Signaling uses a mechanism of sequential distribution of bearing the AMC. It is unique in each frame in AAS zone is allocated from one to four channels of access /allocation of resources (BWAA - bandwidth allocation/access). Each BWAA-channel consists of two sub-channels located in the upper and lower parts of the range symmetrically about the center frequency (if BWAA-channel one, it includes the upper and the lower sub-channels). This channel is used to transmit the prefix downward subframe (for Direct mode Signaling Method), map UL-MAP and DL-MAP for each of the spatially separated speakers or groups of speakers. Due to the precise spatial configuration AAS this method allows a single frame to transmit messages to multiple users.
In the method of direct signaling there are four special coded messages - learning connections back RLT (reverse link training), access to the reverse connection RLA (reverse link access), training FLT direct connection (forward link training) and initiate a direct connection FLI (forward link initiation). The first two messages uses AC, the latter two BS. For initializing or bandwidth request of the MSS sends a message to the RLA in the channel of the BWAA. It precedes the request message strip or initial access of the BS and is used for fine tuning of your antenna system on this speaker. In response, the BS transmits the message FLI is a unique code for each speaker (LH itself may initiate communication by sending FLI). FLI is transmitted in a subchannel allocated to this AC. Each subscriber station scans all the sub-channels and finding the code sequence addressed to her the first initialization message, sends a reply in the same channel in a time interval) sequence RLT designed to fine tune the antenna BS to the MSS in this subchannel. As a result, after completing all of the necessary corrections, the BS and the MSS establish a connection over which data is transferred. Moreover, the data packets are preceded by a training sequence FLT (BS) and RLT (from AC).
4. SERVICES AND ARCHITECTURE Mobile WiMAX NETWORKS
4.1 Services networks Mobile WiMAX technology.
WiMAX networks are designed to provide services to both fixed and mobile users. WiMAX supports the following types of mobility:
1) fixed. In this case, the operator has accepted the user's position in which he receives the service, e.g., concrete honeycomb. Good for that user terminals with a fixed antenna outside the building aimed at the base station.
2) vagus (nomadic), i.e. with a variable location. The user has the ability to connect to the operator's network from any location where the operator provides coverage. During one session, the user must be stationary.
3) mobile (portable). The user has the ability to move at speeds up to 5 km/h without losing the established session, including optional network) to move from one cell to another (handover). During the handover permitted breaks in data transmission (up to the value for maximum service current TCP/IP session), up to 2 s. Allowed data loss during handover, the contracted quality of service, QoS, is restored only after the completion of handover.
4) limited mobility (simple mobility). The user can move, including the move from honeycomb to honeycomb, with speeds up to 60 km/h without compromising the quality of service, and up to 120 km/h are permissible with the gradual deterioration of service quality. For applications unreal (non-real time) time (work with e-mail, the web, watching videos buffered data, transferring files using FTP, IPsec/VPN) quality of service is guaranteed. The handover time should not exceed 1 when switching between IP subnets and 150 MS in the same subnet, the interruption time of data transmission does not exceed 150 MS. Mandatory support standby (idle), sleeping (sleeping) modes of operation of user terminals, and user call (paging), see the relevant sections.
5) full mobility full mobility). The user can move, including the move from honeycomb to honeycomb, with speed up to 120 km/h without compromising quality of service. Guaranteed quality of service for real-time applications (VoIP, video telephony, video without buffering) and unreal (see the limited mobility). The handover time is not greater than 50 MS, the interruption time of data transmission does not exceed 5 MS (or no more than the duration of one frame).
4.2 Principles of WiMAX networks
The following are the principles of WiMAX (Release 1 version 4):
1) WiMAX is based on 802.16 standard
2) WIMAX network architecture includes 2 global parts: ASN - subnet access, CSN - subnet that provides connectivity to IP networks, see below. Support 802.16 standard is fully implemented in ASN. Usually subnet CSN is owned by the ISP, IP services, NSP - Network Server Provider, subnet ASN - radio access provider, NAP - Network Access Provider. NSP and NAP can be a provider.
3) one subnet access, ASN, can be used multiple service providers (NSP), i.e. to a single ASN can be connected to multiple CSN, and one provider of IP services, the NSP may use several different subnets access, i.e. to one CSN can be connected several ASN.
4) the architecture provides standard interfaces (reference points, Reference Points), see below, in particular between an MS, ASN, CSN to ensure efficiency in the use of equipment from different manufacturers.
5) the network needs to support mobile telephony based on VoIP, multimedia services, as well as the mandatory features defined by the regulator, such as emergency, legal, listening, etc.
6) the network must provide the services in accordance with the agreed service level agreement (SLA) to support, if required, multiple voice sessions for a single user, simultaneous voice and data, prioritization of emergency calls.
7) the network must be able to interact with the gateways, providing the existing services based on IP: SMS over IP, MMS, WAP, and others;
8) the system must support broadcast and multi-user (multicast) services;
9) the system must support mutual authentication of MS and the network, as defined in the 802.16 standard;
10) the system must support user authentication using a login/password, SIM, USIM, RUIM;
11) the system must support confidentiality (confidentiality) and integrity of transmitted data by using the functions implemented in the ASN;
12) the system must support the establishment/removal of a VPN (Virtual Private Network), MS initiated
13) the network should not prevent you from switching (handover) multi-standard MS network other technology - Wi-Fi, 3GPP, 3GPP2, DSL;
14) the system must support IPv4 mobility or IPv6;
(15) the network should not prevent roaming between service providers (NSP). The network must allow the access provider (NAP) to serve the MS, using different domain names (served by different service providers) ;
16) the system must support seamless handover at speeds of traffic;
17) the system must support different levels of QoS;
18) the system must support interoperability with other wireless (3GPP, 3GPP2) or wired (DSL) networks. The interface used for such interaction should be based on the protocols of the IETF and IEEE;
19) the system must support roaming with other operators WiMAX
20) the system must support change settings and update subscriber devices over the air Over-the-Air (OTA) ;
21), the network architecture must ensure interoperability of devices from different manufacturers;
within ASN (BS and transport network), between different ASN, as well as different elements of the ASN and CSN;
22), the network architecture must support the following types of CS (from the list of CS defined in 802.16).
Enlargement WiMAX network consists of the following logical objects:
1) SS (Subscriber Station) ;
2) ASN (Access Service Network) ;
3) CSN (Connectivity Service Network) .
Each object can be implemented in a single physical module (e.g., SS) or multiple (ASN, CSN).
Several CSN can be connected to the same ASN, and Vice versa; several ASN can be connected to a single CSN. ASN and CSN can belong to the same operator or different:
A network architecture according to WiMAX, shown in figure 17:
Figure 17 - Architecture the WiMAX network.
Figure 18 - Components of the WiMAX network architecture.
The operator may own WiMAX network fully or partially. The operator providing radio access is called NAP - Network Access Provider. He may belong to one or more ASN. The operator providing network services (Internet, voice, access to certain content) is referred to as NSP - Network Service Provider, he may belong to one or more CSN.