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The good architecture specifications provide a frame work which is backward compatible in which future service additions and expansion of system capabilities without loss of backward compatibility and also supports legacy technology. Based on this IEEE 802.20's (Mobile Broadband Wireless Access) frame work is done. It is consistent with operator requirement and equipment. It provides a framework for rapid development of cost effective, multi-vendor, interoperable, broadband, mobile, wireless access. This compatibility standard is used in wide variety of frequency bands.
OFDMA(Orthogonal frequency divison multiple access) technique is used in wideband mode and this is designed to operate time divison duplex (TDD) and freuency division duplex(FDD) bandwidths from 5 MHz to 20 MHz. For other system having more than 20MHz other multicarrier that can take use of larger bandwidth are defined on wideband mode.
AIR INTERFACE INTRODUCTION:
The air interface was designed in layers and each layer was defined with interfaces, as well as with each protocols. One or more protocols can also be specified for layers for accomplishing their central function. The layers of IEEE 802.20 are grouped into PHYSICAL and MAC Layers. This form well defined and enables compatibility with wide range of upper layer architecture and application.
AIR INTERFACE INTRODUCTION:
Multi-route standards are defined by the MBWA Wideband mode in which each route has a unique path to network with an independently operating but identical protocol stack to it. For forward link (FL) and reverse link (RL) at a given time only one route can be used. To enable rapid and efficient transfer of control and data between access networks, systems deploying wideband mode supports a network based route-tunnelling which has capability of maintaining and creating virtual connections between two access networks. Inter Route Tunnelling Protocol (IRTP) supports air interface of such network architecture.
The below figure depicts the layering architecture for each route of air interface in wideband mode. Each sublayer consists of different protocols that perform function of sublayer.
IEEE 802.20 Physical Layer: In Wideband Mode.
Generally IEEE 802.20's PHY layer provides the following for forward and reverse link. They are:
The physical (PHY) layer of 802.20 comprises of
Two duplex modes:
Time Division Duplex (TDD)
Frequency Division Duplex (FDD)
Two forward link hopping modes:
Symbol Rate Hopping
Two synchronization modes:
Two multi-carrier modes:
Modulation uses OFDM with QPSK, 8PSK, 16QAM, and 64QAM modulation formats.
In wideband mode PHY layer supports both FDD and TDD deployments thereby utilizing similar baseband waveform for both.
FDD mode is provided for enhancement of backward compatibility on paired spectrum.
TDD mode supports emerging unpaired spectrum.
For both forward and reverse link to improve throughput it's specification provides modulation signal up to 64 QAM with Synchronous Hybrid Automatic Repeat Request (HARQ). It supports different coding schemes like convolution codes, turbo codes and a low-density parity check (LDPC) in different environments.
The PHY Layer in reverse link has a special and unique feature that is a portion of signalling from Access Terminal (AT) to Access Network (AT) occur over a CDMA (code division multiple access) controls a segment embedded in subscriber of OFDMA although it is based on Orthogonal frequency division multiplexing. Therefore techniques developed for CDMA and soft handoff techniques can be made used by continuous signalling from AT to AN. The Access Network (AN) makes easily broadband measurements required for improved interfaces because CDMA segment is hopped over entire broadband channel.
IEEE 802.20 MAC LAYER: In wideband mode
The MAC layer of 802.20 consists session, convergence, security, and lower MAC functions. The lower MAC sub layer controls operations of data channels: Forward Traffic Channel and Reverse Traffic Channel. Radio link sublayer which has protocols provides many services as follows:
Reliable and insequence delivery of application sublayer packets.
Quality of service (QoS)
Multiplexing of application sublayer packets.
The air interface support MAC protocols On, Hold and Sleep states with fast transitions among them.
In the On state, client uses the system resources actively to transmit and receive data.
In order to get higher system efficiency, the Hold state should be initiated whenever the client doesn't use the system.
The sleep state is initiated when the user is completely inactive.
To bring the user from sleep state to ON state Paging mechanism is used to wake up. This mechanism allows a mobile station to conserve energy with the help of Sleep state and still allows for the mobile station to receive incoming packets.
MBWA air interface should send paging signals once every 100ms to reduce delay associated with waking a user up.
Transport of required data from Access Network (AN) and the Access Terminal (AN) are supported by multiple application protocol provided by the application sublayer. For transportation of packets to or from other configured routes over the in use route is done by IRTP which is specified by application layer. Extensible Authentication Protocol (EAP) which is support protocol is also included in application sublayer for security functions such as for privacy, authentication and authorization for services subscribed. Interfaces to IP (IPv4 or IPv6), to protocols for transporting packets from other air interfaces or networks and to Robust Header Compression (RoHC) support protocol are all also supported by application layer to support user data transmission.
Air link connection establishment and maintenance service are provided by The Connection Control Plane.
Protocol negotiation and protocol configuration services are provided by Session Control Plane. Also configurable internal parameter required by AT and AN are established by this session control plane.
An interface to enable OAM (operation, administration and maintenance) systems is provided by MAC and PHY management information base (MIB) for unicast transport. This system gathers statistics from all protocols and management planes like bytes successfully transferred, access attempts, dropped connections, and so on. Such information makes required functions in large and complex network to be performed by network management system.
625k-Multicarrier Mode: (625k-MC):
This mode was developed to get maximum gain from adaptive multiple antenna signal processing. This is Time Division Duplex (TDD) air interface. Multiple TDD RF carriers are supported by this mode to increase peak data rates available on a per user basis. It is enhancement to iBurst system as given by High Capacity Spatial Division Multiple Access (HC-SDMA). Based on HC-SDMA specifications on commercially deployed system it is fully backward compatible.
The transfer of IP traffic, including broadband IP data is enabled by 625-MC mode which is designed around multiple antennas with spatial processing and SDMA. Spatial training and correlated uplink and downlink interference are provided by physical aspect of protocol. Fast make before brake, low overhead and inter cell handover all are also supported.
Baseline Features of HC-SDMA:
The Layer 1 (L1) of HC-SDMA which is corresponded by physical layer of 625k-MC mode is characterised by TDD or TDMA structure with 5ms frames. These each frame contains 3 uplinks and three downlink bursts. Many aspects of L1 are designed to support the effective use of adaptive antenna arrays to provide high spectral efficiency. Spatial channel is a basic physical resource in the system. This spatial channel consists of a carrier, uplink and downlink time slot pair and a spatial channel index.
The L2 and L3 functional specifications of the HC-SDMA system defines features MAC and PHY which are encompassed in data link layer of 625k-MC mode. Logical sessions for the efficient transport of IP packets are created by L3 of HC-SDMA. Specifications for reliable transmission including MAC, RLC and logical channel structure are defined by L2 of HC-SDMA. Control functions and dynamic access management to map and transport logical channels onto PHY layer bursts are provided by the L2 MAC sublayer. To coordinate the power control and link adaptation required to maintain an RF link the logical connection and registration management functions are defined by L3 protocol of HC-SDMA. With authentication and air interface confidentiality the L3 protocol also ensures a robust security infrastructure. According to the ISO/International Electrotechnical Commission (IEC) 9796 standard authentication for both BS and UT is based on using digital certificates signed using RSA algorithm (Revist Shamir Adelman) as signature primitive.
PHY LAYER ENHANCEMNET OF 625-MC:
The 625-MC mode defines extra modulation techniques like 32QAM and 64QAM, error control coding for higher throughput deliverance with peak uplink data rates of 571.2kb/s and a peak downlink data rates of 1.493 Mb/s. To support additional modulation classes additional RF specifications are provided by 625-MC mode.
Data Link layer (MAC and LLC) Enhancements:
For efficient voice over IP (VoIP) packet transmissions and to support the enhanced cell coverage there are additional MAC and RLC specifications.
To deliver short message broadcast services and multicast streaming services additional protocols are defined by 625-MC mode.
Three classes of QoS corresponding to assure forwarding (AF), expedited forwarding (EF), and best effort (BE) are defined by 625-MC mode air interface. To control the stream shutdown timing each sessions QoS is exchanged by BS and UT.
AES advanced encryption standard is used for security purpose in this system.
Power conservation for the UT in sleep or idle mode is enabled by Sleep mode control which is offered by five paging activities defined by 625k-MC mode.
MIB is defined by 625k-MC mode to provide Radio network quality monitoring and control functionality. It is comprised of attributes, actions , managed objectsand notifications required for managing a BS.
625k-MC Support for Adaptive Antenna Arrays:
For each downlink transmission, current spatial training data is provided by uolink slots preceding the downlink in a frame as shown in above figure. Downlink slot is paired with each uplink slot. This pairing ensures that interference environments will be highly correlated. To prevent channel condition from degrading the duration is small between uplink and downlink. The performance of the adaptive antenna array is degraded by this. Low complexity multi antenna signal processing algorithms are enabled by carrier bandwidth which is relatively narrow about 625 KHz.
Unique training sequence is assigned to each user on a given carrier time slot pair. Of the propagating channel for appropriately accurate estimation is designed by training sequence. A unique training sequence is used by coventional channel via SDMA which shares each UT from a selected set with good correlation properties. Between the training sequences the coss correlation property is very low. For the non zero lags of the training sequence the autocorrelation property is very low.
625k-MC Mode Air Interface Handover:
UT directs the make before break handover scheme of the air interface. Based on signal power and many other factors the broadcast channels from surrounding BSs and rank candidates is monitored by each UT. While exchanging traffic channel data with its current serving BS a UT performs this type of measurements. It also registers with new candidate serving BS. Make before brake is hand over for the user data. In this after successful registration the traffic channel redirects to new serving BS.