Add a note hereAt this moment it is very difficult to predict the exact architecture of the 4G mobile communication system. Looking at the present scenario of the 3G and the likes of WiMAX etc. we can only predict the probable architecture of the fourth generation architecture. However in labs and on an experimental basis there are already some of the architectures available for the 4G. Of course, with the advances of the technology in both the UMTS and the CDMA 2000 and their evolved versions the architectures will be updated. Here some of the experimental architectures and some of the predicted models of the 4G architecture have been presented.

Add a note here1. The OSI Model for 4G
Add a note hereThe best way to represent any communication system architecture is the OSI model, and here the probable OSI model of 4G model has been presented (Figure 1) with the understanding that may be some differences in specific future 4G systems. The OSI model of 4G can properly explain the different operations of and the underlying technologies. It is similar to the various layers found in the OSI model of internet, but as a result of basic differences they are arranged in a different fashion and some of the layers are absent. Here the physical layer and the MAC (medium access control sublayer) are quite important.

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Figure 1: The OSI model of4G
Add a note hereThe OSI model of 4G depicts the different layers and their functions in a proper sequence. The physical layer, or bottom one, deals with the signals in the OFDM format. Above it lies the transmission convergence sublayer (CS), which is between the physical layer and MAC layer. On top of that layer is the MAC layer, which has three sublayers. The uppermost layer of MAC, the convergence sublayer, supports both ATM services as well as IP based services. In 4G the MAC layer at the base station (BS) is responsible for the allocation of bandwidths to different users both in the uplink and downlink. MS only occasionally takes the control of bandwidth allocations when it has multiple sessions or it has connections with the BS. This is quite different from other services and ensures better quality of service. Most of the services of 4G would be IP based; as a result the optimization and QoS related improvements are done as per the IPv6 configuration and structure. ATM service facilities are also provided for the compatibility with other existing networks.
Add a note hereWhen we look at the first release of WiMAX standard in 2001, the IEEE 802.16 standard proposed applications for a fixed wireless scenario in licensed frequency bands in the range between 10 and 66 GHz, where the use of directional antennas were mandatory to obtain satisfactory performance figures. But difficulties were encountered in metropolitan sub-areas where line-of-sight operations cannot be ensured due to the presence of obstacles, buildings, towers etc. Thus, subsequent amendments to the standard (IEEE 802.16a and IEEE 802.16-2004) have extended the 802.16 air interface to non-line-of-sight applications in licensed and unlicensed bands in the 2-11 GHz frequency range. Additionally, after the revision of IEEE standard document 802.16e, some necessary mobility support will be provided. Revision of IEEE 802.16f is intended to improve multi-hop functionality, and 802.16g is supposed to deal with efficient handover and improved QoS. This revision also increased the range of WiMAX technology; according the WiMAX forum, it can reach up to a theoretical 50 Km coverage radius and achieve data rates up to 75 Mb/s. Of course, actual IEEE 802.16 equipment is still far from these performance figures, but it has been proved that with the use of MIMO antennas and OFDM based technologies the data rates can be made really high. For example, with 5 MHz bandwidth, a data rate of 18 MBPS is possible using this advanced MIMO technique. After looking at the success of these technologies in WiMAX, the 4G development research groups are ready to follow the same path. Most of the settings of 4G would be according to the IEEE 802.16m standards, and the new MAC layer bridging is waiting for some amendments of IEEE 802.16k.
Add a note hereDuplexing or bidirectional data transmission is provided by means of either Time Division Duplexing (TDD) or Frequency Division Duplexing (FDD). In TDD, the frame is divided into two sub-frames, one devoted to downlink and the other to the uplink. A Time-Division Multiple Access (TDMA) technique is used in the uplink subframe. The BS is in charge of assigning bandwidth to the SSs, while a Time Division Multiplexing (TDM) mechanism is employed in the downlink sub-frame. In case of FDD, the uplink and downlink sub-frames are concurrent in time, but are transmitted on separate carrier frequencies. There are supports for half-duplex FDD SSs, at the expense of some additional complexity. Each subframe is divided into physical slots. Each TDM/TDMA burst carries MAC Protocol Data Units (PDUs) containing data towards SSs or BS, respectively. The transmission convergence sublayer operates on top of the physical layer and provides the necessary interface with the MAC. This layer is specifically responsible for the transformation of variable length MAC PDUs into fixed length physical blocks. Here in 4G, the RR and the MM layers are different from the GSM RR and MM layers. Here the use of MIMO enabled antennas can manage the resources quite efficiently.
Add a note hereThere is a big demand for secure data transmissions, which has led to the native inclusion of a privacy sub-layer in 4G (which is very similar to the WiMAX), at the MAC level. There are some well-organized protocols to take care of the security related processes. Those protocols are responsible for encryption/decryption of the packet payload, according to the rules defined in the standard. IEEE 802.16 uses a wireless medium for communications, and one of its main targets of the MAC layer is to manage the resources of the radio interface in an efficient way, while ensuring that the QoS levels negotiated in the connection setup phase are fulfilled. Of course the IEEE 802.16 MAC protocol is connection-oriented and is based on a centralized architecture. There is a need for segmentation and resemblance of frames for proper security monitoring of all the packets. The common part sublayer is responsible for this segmentation and the resemblance of MAC service data units (SDUs), the scheduling and the retransmission of MAC PDUs. The common part sublayer also provides the basic MAC rules and signaling mechanisms for different system access, bandwidth allocation and connection maintenance. The core function of the protocol is bandwidth requests/grants management. A SS may request more bandwidth, by means of a MAC message, to indicate to the BS that it needs (additional) up-stream bandwidth. The request of bandwidth is processed on a per-connection basis to allow the BS uplink scheduling algorithm, to consider QoS-related issues in the bandwidth assignment process. The bandwidth granting methods as per the original 2001 standard encompassed two operational modes: Grant per Connection (GPC) and Grant per Subscriber Station (GPSS). Later in the 2004 release, the term "grant" refers only to the GPSS mode. Whereas, in the GPC mode, the BS allocates different scalable bandwidths to individual destinations. With this revision, BS got the control of all the centralized mechanisms, with all the intelligence placed in the BS, while the SSs act as merely passive stations. On the other hand the bandwidth, in the GPSS mode, is granted to each individual SS, which is then in charge of allocating the available resources to the currently active flows.
Add a note hereConvergence sublayer (CS) is the uppermost sublayer of the MAC layer. The CS associates the traffic coming from the upper layer with an appropriate Service Flow (SF) and Connection Identifier (CID) which gives the idea about its destination. The CS also provides payload header suppression when some entity is sent and reconstruction at the receiving entity. CS delivers the resulting CS PDU to the MAC Common Part Sublayer to confirm the negotiated QoS levels.
Add a note hereThe 4G standard defines two different Convergence Sublayers for mapping services to and from IEEE 802.16 MAC protocol like the WiMAX. The ATM convergence sublayer is there solely for ATM traffic, while the packet convergence sublayer is specific for mapping packet-oriented protocol suites, such as IPv4, IPv6, Ethernet and Virtual LAN etc. The IP Sublayer is as the name suggests is there to provide all IP enabled services. The classification of IP traffic and ATM traffic is done in the CS sublayer. However the system's IP architecture will be based completely on IPv6.

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