4G Drivers and SCENARIOS | WIMAX VS. LTE

The rising mobile subscribers by 2011, estimating over 4 billion, in combination with converged systems and applications are the main contributors of the 4G evolution (GSM world 2009). Several services are expected to drive to the 4G converged ecosystem but the future operators revenues are data services and mainly entertainment services. Three services that exist in today's markets are expected to play a significant role in the future and into a more advanced mode. These are music, mobile games and mobile TV.

The new mobile user's lifestyle is increasing needs capacity, although the ‘walled garden’ might still be a limitation restricting the customer's experience. The users are changed from consumers to producers of content such as photos, videos etc. Several applications will drive the mobile broadband market globally, including:

• Web 2.0,
• Online blogs,
• Mobile music,
• Location Based Services (LBS),
• Multimedia messaging,
• Gambling and
• Mobile TV.

There are a few scenarios discussed including WiBro, which is expected to evolve during 2010 and 2015 and attempting to cover different markets through restructuring and transition into 4G. For the next 5 years Verizon network will evolve into a 28Mbps download speed, leading to an early 4G LTE adoption compared to Vodafone.

These scenarios could be summarized as following:

1. Independent 4G system with one standard, the 3GPP LTE
2. Transition from 3G into 4G with existing (3GPP LTE) or new service providers WiMAX and WiBro
3. Co-existence of different standards
4. Spread of open transmission

To explain the above cases, we claim that history matters and the path dependent concept can really explain the long-term outcome based on initial conditions. The 4G development depends on the initial conditions as shaped from 3G in most of the cases. Based on the ‘Increasing Returns’, and ‘Path Dependency’, where alternatives are possible, and regarding the standards, "the one selected and heavily invested is good enough' or even optimal and remains in use because it becomes established in use". This theory is matching the scenario of different standards coexistence that will interact in the ecosystem and complement each other referring to an advanced LTE or LTE+ and WiMAX that will be established and standardized as 802.16e that offers advanced mobility. This is what usually occurs in technological development scenarios.

WIMAX VS. LTE

The LTE technology that Nokia and the Third Generation Partnership Project (3GPP) are pushing is an upgrade to existing GSM networks. The attraction to this technology had made even the CDMA operator, Verizon Wireless, to join the 3GPP trials. It is also a strategic decision, in order to be compatible with its European, GSM-based parent company, Vodafone. LTE looks like it can heal the GSM/CDMA rift that has divided the industry, as no major carrier has yet signed on with obvious CDMA 4G upgrade technology, Ultramobile Broadband (UMB).

LTE will have the following advantages:

• Fast, with peak data rates of 100 Mbps download and 50 Mbps upload.
• It makes CDMA and GSM debates moot.
• It offers both FDD and TDD duplexing, which means the upload and download speeds don't have to be synchronous, so operators can better optimize their networks to use more upload channels.
• LTE will have lower latency, which makes real-time interaction on high band-width applications using mobiles possible.

3GPP LTE, one of the most advanced mobile communication technologies to date, is currently undergoing 4G technology standardization by the 3GPP This is the most likely technology to become the 4G standard, as many of the world's major operators and telecommunications companies are members of LTE/SAE (Long Term Evolution/System Architecture Evolution) Trial Initiative (LSTI). These companies include operators, such as Vodafone, Orange, T-Mobile, NTT DoCoMo, China Mobile and Telecom Italia and vendors, Ericsson, Nortel, Alcatel-Lucent, Nokia Siemens and LG Electronics. These are also the companies that will be considered to have the advantage in deploying first the 4G services.

WiMAX has certain advantages mainly over the Fiber to the home (FTTH) technology. When bundled with broadband internet access and IPTV, a WiMAX triple play becomes very attractive to residential subscribers. Given the QoS, security and reliability mechanisms built into WiMAX, the users will find WiMAX VoIP as good as or even better than voice services from the telephone company. It also offers a cost effective infrastructure with efficient use of spectrum. Currently, the average cost of WiMAX 802.16-2004 baseband has decreased from $35 to almost $20 today per subscriber.

4G proponents will serve as complements or upgrades to advance the 3G limitation to deliver video/TV and high speed Internet access. For WiMAX, there is a limitation of wireless bandwidth. For use in high density areas, it is possible that the bandwidth may not be sufficient to cater to the needs of a large clientele, driving potentially the costs high. But the main competitor for WiMAX today is the fiber and the wireline network that especially in the US is a real challenge for the residential users as the operators are deploying and growing really fast.

IPv6 Networks and LTE

In EPC architecture there are two gateways that may be combined into a single network element; the SGW and the PGW (Figure 1). If these two gateways are separate network elements, there are two options for the protocol being used among them: GPRS Tunneling Protocol (GTP) and Proxy Mobile IP v6 (PMIPv6) .

 

Figure 1: All-IP LTE structure

The Release 8 specification identifies EPS bearers similar to the concept of PDP Context, which is defined in 3GPP's Release 7 specifications. An EPS bearer is a logical connection between a UE and a gateway, associated with a specific QoS class. As long as these Service Data Flows (SDFs) belong to the same QoS class, an EPS bearer can carry multiple SDFs.

An EPS bearer can carry both native IPv6 and IPv4 traffic in contrast to the PDP Context defined in 3 GPP Release 7. Therefore IPv4 and IPv6 stacks can be supported by a UE simultaneously as long as it's connected via a single EPS bearer.

Regarding the IPv4 scarce address resources, allocating both IPv6 and IPv4 addresses to a device does not solve the problem of IPv4 exhaustion. A service provider may therefore decide to assign only IPv6 addresses to certain devices, even when the device is able to support IPv4 and IPv6 simultaneously. In that case, NAT-PT or IPv6-toIPv4 http-proxy functionality may be required to connect these IPv6-only devices with legacy IPv4 end-points. Such a decision needs careful consideration and the issues identified in RFC 4966 need to be taken into account.

A User Equipment device (UE) obtains an IP address in one of the following two methods:

1. As part of the attachment procedure
2. Via a separate assignment procedure, such as DHCP or IPv6 Stateless Address Autoconfiguration.

The attachment procedure consists of the following steps:

1. The UE requests an attachment via sending a message to the MME, followed by an authentication procedure involving the HS S. The HSS sends to the MME, subscription data associated with the user as part of this procedure.
2. With a few exceptions, the MME is responsible for selecting the Serving and PDN Gateways that will be used for this UE. It sends a request for the establishment of the default bearer to the SGW, which forwards it to the PGW. This message exchanges results in the establishment of a GTP tunnel or a Mobile IP tunnel segment between SGW and PGW. As long as the user is attached, this segment remains up, even when the UE enters the idle state.
3. The MME orchestrates the establishment of the GTP tunnel segment between SGW and eNB and the (default) radio bearer between eNB and UE. The bearer between SGW and UE is torn down whenever the UE goes to idle state. If the SGW receives IP packets destined for the UE while it is in idle state, the SGW triggers the MME which starts a paging procedure.

When the IP address assignment is part of the attachment procedure, an IP address is allocated by the PGW to the UE according to step 2. This IP address is provided to the UE within the GTP control messages being used for establishing the EPS bearer according to step 3.

When a default bearer is established (disregarding the IP address assignment), DHCP or IPv6 Stateless Address Autoconfiguration (SLAAC) can be performed by the UE for obtaining an IP address. Therefore, a UE could receive an IPv4 address during the attachment procedure and an additional IPv6 address through SLAAC procedure.

PDP Context | LTE AND IPV6

A PDP context is a data protocol structure that is present on both SGSN and GGSN. It contains information regarding the subscriber's session during subscriber's active session. The subscriber-related data within the PDP context information includes: IP address, IMSI information, and Tunnel ID (TEID). Therefore access to the external packet-switching network can be achieved through a PDP context specification. The data associated to the PDP context is consisted of information including the MS PDP address (IP address), the packet-switching network type, the GGSN reference, and the requested QoS. A PDP context is identified and handled by a mobile's PDP address within the following entities: SGSN and GGSN and the MS. Within a given MS several PDP contexts can be activated at the same time.

APDP context is created through a PDP context activation procedure, which may be initiated either by the network or by the MS. Before PDP context negotiation, the MS is always GPRS-attached.

To remove a PDP context a PDP context deactivation procedure is required. This procedure may be initiated either by the network (SGSN or GGSN) or through the MS itself. The PDP context deactivation procedure may be initiated either during the GPRS detach, delete subscriber data procedures, or during an application deactivation.

Introduction to Evolved Packet Core (EPC) | LTE AND IPV6

Apart of LTE contains EPC, which is a new system design based on the all-IP mobile core network architecture. EPC forms a converged framework based on real-time and non-real-time packet-based services. EPC is specified by 3GPP Release 8 standards.

The EPC is responsible for providing mobile core functionality, which used to be two separate sub-domains in previous mobile generations (2G, 3G). These two sub-domains are: Circuit-Switched (CS) (i.e., supporting voice) and Packet-Switched (PS) (i.e., supporting data). These two distinct sub-domains are used for individual mobile voice and data switching and processing under a unified single IP mobile domain. LTE offers an end-to-end IP-based architecture, which covers from the mobile handsets and other terminal devices offering embedded IP capabilities on top of LTE base stations. The LTE base station is an IP-based device often called Evolved NodeB.

LTE's EPC is an essential functional entity offering end-to-end IP service, as well as allowing the introduction and creation of new business models, including partnerships and revenue sharing with application and third-party content providers. EPC promotes the enablement of new applications and new innovative services.

EPC addresses those fundamental requirements of LTE that deal with media-rich and advanced real-time services offering enhanced Quality of Experience (QoE). Network performance can be improved using EPC. This is done by separating data and control planes by using a flat IP architecture that is able to reduce the hierarchy delay among mobile data elements. For instance, the data path traversing from eNodeB passes only through EPC gateways.

The introduction of the all-IP network architectural EPC in the design of mobile networks has caused various degrees of implications on the following mechanisms:

• All-IP mobile services, such as; IP-based voice, data and video communications.
• New mobile architecture interworking with previous mobile generations (2G/3G)
• Network scalability, which is required by all core elements to address any changes in the bandwidth and the number of user-terminal direct connections.
• Availability and reliability offered by each elements ensuring service continuity.

To address various service and network requirements, the EPC is designed in such a way to change the existing mobile network paradigms.

The following subcomponents are part of the EPC architecture:

• MME (Mobility Management Entity): In LTE, the MME is a key control-node for the access network method. MME is responsible for paging procedure including retransmissions and the UE (User Equipment) tracking idle mode. It is also responsible for match an appropriate SGW for a UE at the time of intra-LTE handover that involves the Core Network (CN) node relocation and at the initial attach time. MME is further involved in the user (by interacting with the HSS) authentication and the bearer activation/deactivation procedures. The MME can terminate the non-Access Stratum (NAS) signaling can generate and allocate temporary identities for UEs. It also checks for the authorization of the UE for camping on the service provider's Public Land Mobile Network (PLMN) and enforcing UE roaming restrictions. The MME handles the security key management and is the termination point in the network for ciphering/integrity protection for NAS signaling. The MME also offers lawful signaling and LTE/2G/3G mobility control plane functions with the S3 interface that terminates at the MME from SGSN. The S6a interface is also terminated by MME towards the home HSS for UEs roaming purposes. The following interfaces were considered for MME:
  • S3: This interface enables bearer and user exchange information for inter 3GPP access network mobility between SGSNs.
  • S6a: This interface enables subscription and authentication data transfer for authenticating and authorizing user access.
• PGW (PDN Gateway): From the UE to the external packet data networks, PDN Gateway provides connectivity by providing the point of entry and exit for the UE's traffic. More than one PGWs may provide simultaneous connectivity for a UE providing multiple PDN access. The PGW performs packet filtering for each user, policy enforcement, lawful Interception, charging support, and packet screening. The PGW may act as the anchor for mobility between non-3GPP and 3GPP technologies, which is nother key role of PGW. These technologies include: 3GPP2 (CDMA IX and EvDO), WiMAX and etc.
• SGW (Serving Gateway): User data packets are routed and forwarded by the SGW. During inter-eNodeB handovers the SGW acts as a mobility anchor for the user plane. It also acts as an anchor for mobility among LTE and other 3GPP technologies, where it can terminate the S4 interface and relay the traffic among PGW and 2G/3G systems. The SGW terminates the DL data path for the idle state of UEs and when DL data arrives for the UE, it triggers paging. The SGW also stores and manages UE contexts, such as parameters of network internal routing information and the IP bearer service. In case of lawful interception, it also performs replication of the user traffic.
• PCRF (Policy and Charging Rules Function): Though PGW, SGW, and MME were introduced in 3GPP Release 8, PCRF was part of 3GPP Release 7 introduction. Architectures using PCRF have not so far been widely adopted by standards, however the interoperability of PCRF's with the EPC gateways and the MME is essential for the operation of the LTE and mandated in Release 8.

The IPv6 Transition Mechanisms | Long Term Evolution (LTE)

IPv6 was designed with a long transition period in mind. Therefore there is a myriad of IPv4 to IPv6 transition mechanisms have been defined in the various RFCs. The transition mechanisms can be grouped into a few categories (3G Americas, 2008):

1. Dual Stack
2. IPv6 tunneling over IPv4
3. IPv4 - IPv6 translation
   
   

1 Dual Stack

In a dual stack mechanism, the device supports both IPv6 and IPv4, which means that the device is able to obtain both IPv6 and IPv4 addresses from the network and is able to choose either IP version to use to communicate depending on the IP version and the peer supports. The IP version and the peer supports can be discovered, for example, using a DNS service. 

2 IPv6 Tunneling over IPv4

When two IPv6 domains are not directly connected over IPv6 but instead are connected through an IPv4 network, which is often the case during the initial transition of the Internet to IPv6, IPv6 traffic will be tunneled over IPv4. The following tunneling techniques that may be relevant to IPv6 migration are:

• Configured tunnels: This is used for connecting two IPv6 domains that have native IPv6 connectivity. The tunnel (typically from router to router) is configured via administrative means. IPv6 packets are encapsulated in IPv4 packets.
  
  
• Tunnel broker: Tunnel broker is a technique that uses an IPv6 domain (a network or an individual host), which establishes an IPv6 connectivity using such a tunnel broker, serving as a virtual IPv6 ISP.
  
  
• 6to4: This is used for deploying IPv6 in a network without waiting for the administrator of the network to provide native IPv6 connectivity. A 6to4 router advertises a global unicast IPv6 prefix to the network, constructed from a public IPv4 address assigned to the network. The 6to4 router also acts as a tunnel endpoint for the network. IPv6 packets are encapsulated in IPv4 packets. The IPv6-in-IPv4 tunnels between 6to4 sites are established automatically. The 6to4 relay routers allow 6to4 sites to communicate with native IPv6 sites.
  
  
• Teredo: Teredo is used for deploying IPv6 in a network without waiting for the administrator of the network to provide native IPv6 connectivity (similar to 6to4). This is particularly important when the network is behind a NAT and the NAT system is not upgradeable to provide 6to4 functionality, since the 6to4 router cannot sit behind an IPv4 NAT. In Teredo, a Teredo server deployed in front of the NAT, serves as the IPv6 router for the network, transmitting IPv6 router advertisements with a global unicast IPv6 prefix, constructed from a public IPv4 address assigned to the network. This helps the hosts behind the NAT (the Teredo clients) learn about their assigned public IP address and UDP port and enable hole punching through the NAT for Teredo clients between sites to establish direct connectivity. Teredo relays allow Teredo sites to communicate with native IPv6 sites.
  
  
• ISATAP: The ISATAP is an IPv6-in-IPv4 tunneling technique that allows dual-stack hosts and routers within a network segment to communicate over IPv6 when the IP infrastructure within the network segment does not support native IPv6. One example scenario is to allow a host to access the Internet via IPv6 when a site has native IPv6 access to the Internet, however the host happens to be within a network segment that has not been upgraded to IPv6.
  
  

3 IPv4 - IPv6 Translation

When an IPv6-only host needs to communicate with an IPv4-only, some form of protocol translation is needed. The following translation techniques have been developed:

• NAT-PT (Network Address Translation — Protocol Translation) and SIIT (Stateless IP/ICMP Translation Algorithm).
  
  
• IPv6-to-IPv4 Transport Relay Translator: A TRT is a TCP or UDP relay which additionally performs IPv4 — IPv6 translation.

INTRODUCTION TO IPV6 | Long Term Evolution (LTE)

IPv6 is, by far, one of the most important and significant technology and network upgrades in the communication history. This upgrade is growing slowly and will eventually terminate IPv4's network deployment at the end of the transition phase. IPv6 is designed to work with high speed network and low bandwidth networks, particularly suitable for wireless networks. The IPv6 design accommodates new technology requirements, such as QoS, security, and of course extended addressing.

Since the mid 90's many developers and researchers have been working on IPv6 and many RFCs are directly or indirectly discussing this technology. RFC 1883, released in 1995, is the first RFC in regards to IPv6. These efforts have been initiated and monitored by the Internet Engineering Task Force (IETF).

In the past several years IPv6 has deeply penetrated into the architecture of the Internet. Figure 1 (adapted from Gallaher & Rowe, 2005) presents a penetration estimates of IPv6 in the United States, which shows that by year 2010, almost 30% of ISPs and 18% of the users will be switching to IPv6.

Figure 1: IPv6 penetration in vendors, ISPs, and users

 

The main issue with IPv6 integration is its interoperability with IPv4. IPv4 is going to be around for at least a decade before all the networks are purely running over IPv6. Therefore it is essential to ensure IPv6 traffic flows are not going to have any negative impacts on IPv4 and vice versa. Dual-stack systems are considered a solution for the IPv6/IPv4 interoperability issue, which are going to be utilized in the design and implementation of systems for sustaining both IPv4 and IPv6 parallel processing to guarantee interoperability amongst the two protocol suites.

Mobility is another major features that is going to be impacted by this transition. 3G networks have been growing immensely in the past few years and IP connectivity has become an inseparable part of the 3G networks. The involvement of IP is going to increase more as 4G networks are becoming realizable. These requirements are being considered in the IPv6 applications.

This chapter will explore IPv6 features, in particular, in conjunction with LTE architecture. At first, IPv6 is going to be discussed and compared to IPv4. Following this introduction, IPv4-to-IPv6 transition, IPv6 security, Mobile-IPv6, and QoS-IPv6 are covered.

ARCHITECTURE OF 4G

At 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.

1. The OSI Model for 4G

The 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.

 

Figure 1: The OSI model of4G

The 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.

When 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.

Duplexing 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.

There 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.

Convergence 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.

The 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.

FEATURES OF 4G WIRELESS SYSTEMS

There are numerous features of 3G which needed to be modified for the future applications of the mobile communication networks and their service sectors. These collections of advanced versions, along with some others new advanced features, have been proposed for the forth coming 4G system. Of course, the complete picture is not clear yet, though it is believed that by the end of 2010 a better view of the complete 4G features will be known. Here we have captured the main features of the existing experimental 4G system and some of the essential future versions.

As per the announcements of the 4G working groups, the infrastructure and the transreceiver terminals of 4G system will have almost similar structures like that of 2G and 3G except for some advanced features. The previous legacy systems will be in place to keep the existing users. The major change in the infrastructure for 4G will be "all packet-based system" and the technology on which it will be based is the IPv6. There are some other proposals for an open platform in which the new innovations and evolutions of the future can fit. One of the first technology really fulfilling the 4G requirements as set by the ITU-R will be LTE Advanced as currently standardized by 3GPP. LTE Advanced will be an evolution of the 3GPP Long Term Evolution. The higher data rates needed are for instance, achieved by the aggregation of multiple LTE carriers that are currently limited to 20MHz bandwidth and there are many such changes have been recommended. In the following sections we have listed some of the important features of the 4G systems.

1 OFDM Based Physical Layer

The main aim of 4G technology is to Provide high speed wireless broadband services. Airport lounges, cafés, railway stations, conference arenas, and other such locations are required to have high speed internet services; in those places, 4G can play an important role. 4G is equipped with the proper arrangements at the physical layer to meet all the demands of those various scenarios. There are many difficulties, however, in providing high speed wireless internet services in these environments, such as multipath fading and the inter-symbol interferences generated by the system itself. As a result, OFDM technology is used to handle this problem.

2 Inter-Symbol Interference Due to Time Delay

In a multipath environment, the signals and their delayed versions arrive at different times. When the time delay between the different delayed signals is a large enough fraction of the transmitted signal's symbol period (actual time allotted for one symbol transmission), a transmitted symbol may arrive at the receiver during the next symbol period. This is well known as inter-symbol interference (or ISI). At higher data rates, the symbol period or duration is shorter; hence, it takes only a small time delay to introduce ISI. In case of broadband wireless, ISI is a big problem and reduces the quality of service significantly. In conventional situations, statistical equalization is the method for dealing with ISI, but at high data rates it is quite complex and requires considerable amount of processing power. OFDM appears as a better solution for controlling ISI in broadband systems like 4G

OFDM deals with this problem in a very intelligent way by introducing a guard interval before each OFDM symbol. This guard interval is the duration in which no information is transmitted. Digitally, it is nothing but a certain number of zeros transmitted between each couple of symbols. Whatever signal comes during that interval is discarded by the receiver, but when the guard interval is properly chosen then the OFDM signal can be kept undistorted.

3 Effective Use of Bandwidth through OFDM

OFDM has the ability to optimize the consumption of resources. Extraneous bandwidths in the form of guard bands can, with proper implementation, be reduced to zero. Due to the orthogonal nature of the carriers used for different channels, it is possible to overlap the bands on each other and still recover them in the receiver without losing any quality. Because of this, OFDM is very effective in saving bandwidth. In low bandwidth systems where the demand for spectrum is very high, OFDM comes naturally as the first choice. The bandwidth saving has been shown in Figure 1

Figure 1: OFDM and bandwidth use

Figure 1: OFDM and bandwidth use 

Besides the above advantages, OFDM based systems provide other facilities for digitalization and protocol supports. Processes like error correction and interleaving are easily supported by OFDM.

4 Software Defined Radio for the 4G System

Software defined radio (SDR) is an emerging radio technology that can be used in various digital networks and can be controlled and programmed through software.

According to the SDR Forum, SDR technology is "radio that provides software control of a variety of modulation techniques, wide-band or narrow-band operation, communication security functions (such as hopping), and waveform requirements of current and evolving standards over a broad frequency range." SDR has the ability to support wireless applications in various networks like Bluetooth, WLAN, GPS, radar, WCDMA and GPRS.

Currently, all of the major operations such as modulation, demodulation, coding, decoding, interference management, channel allocation and capacity management are done through the control software. One of the biggest advantages of the SDR is that it can ensure a secure communication network through implementation of encryption systems like the AES (Advanced Encryption Standard). This means that SDR is very reliable and useful for military and other high-level, secret communications. Due to these features, SRD is the most suitable method of data handling at the higher levels in 4G. With the link protocol standards now moving into 3G and 4G, networks differ dramatically in many ways. This is a big problem for both consumers and service vendors; while it can be handled through upgrading the handset, upgrading is usually not a good choice due to the high cost. Additionally, the wireless network operators face many interfacing problems during the migration of a network from one generation to another. Finally, the use of incompatible systems in different countries can hinder global communication. Through the use of SDR, all these scenarios can be handled smoothly.

The SDR system uses a generic hardware platform which has its own programmable units, microprocessors, digital signal processors, field programmable gate array and analog RF modules. The software modules of the SDR that implement link layer protocols and modulation/demodulation operations are called radio applications, and these applications provide link layer services to higher layer communication protocols such as WAP and TCP/IP. SDR has the ability to significantly reduce the life-cycle costs and can also support advanced capabilities in different portable networks. The SDR technology is also reconfigurable; it allows several software modules to co-exist, and also permits dynamic configuration on the handset as well as in the back-end equipment. As a result of this flexibility, the problem of discrepancies due to legacy handsets is solved, and the extra cost for a new handset is not required. SDR can also handle the implementation of multi-mode, multi-band and multi-standard terminals. All of these demonstrate that SDR is clearly the most desirable technology for 4G.

5 MIMO Antenna Systems for 4G

4G like its predecessor 3G would use the advanced versions of the MIMO Antennas. The antennas used for the 3G system were smart enough to take care of many advanced operations at the signal level. This system must continue for 4G as well, and may even be made more sophisticated for 4G, as the number of signal-level decisions would be far greater in the case of 4G compared to 3G

6 IPv6 Based Packet Transmission

The all-packet infrastructure is quite popular in the wireless communication, and now it is also true for the 4G systems as well. The biggest difference between 3G and 4G is the all-IP network (AIPN) structure of 4G, which means that all communication will be controlled by TCP/IP protocols. As a result, the whole communication will be packet switched and the circuit switching part will be taken out of this advanced version. Not only can this make the system compatible with all digital devices, but internet access will be quite flexible and high data rates can be achieved. According to the 3GPP LTE team, this target will be achieved by the end of 2008. Similarly, the 3GPP2 LTE teams are also busy trying to keep pace with their competitors.

7 Presence of TDD and FDD

TDD (Time Division Duplex) and FDD (Frequency Division Duplex) are different modes of CDMA. In FDD transmission mode, both the transmitter and the receiver transmit simultaneously. This simultaneous transmission is possible because they are both on different frequencies. In TDD mode of operation either transmitter or receiver can transmit at one time. This is because they use the same frequency for the transmission.

At present all the major 3G Networks are using FDD mode of operation, but in the 4G system both the FDD and TDD will co-exist.

In the FDD mode of operation, the uplink and downlink use separate frequency bands. These carriers have a bandwidth of 5 MHz and are divided into 10-ms radio frames; each frame further id divided into 15 time slots. The frequency allocation consists of one frequency band at 1920-1980 MHz and one at 2110-2170 MHz. These frequency bands are used in FDD mode both by the UE (user equipment) and the Network. The lower frequency band is used for the Uplink (UL) transmission and the upper frequency band is used for the Downlink (DL) transmission. The frequency separation is specified with 190 MHz for the fixed frequency duplex mode and with 134.8MHz to 245.8MHz for the variable frequency duplex mode.

The TDD mode differs from the FDD mode in that both the uplink and the downlink use the same frequency carrier. There are 15 time slots in a radio frame that can be dynamically allocated between uplink and downlink directions. Thus the channel capacity of these links can be different which is very advantageous especially when people are downloading stuff on their mobiles. The chip rate of the normal TDD mode is also 3.84 Mbps, but there exists also a "narrowband" version of TDD known as TD-SCDMA. The carrier bandwidth of TD-SCDMA is 1.6 MHz and the chip rate 1.28 Mbps. TD-SCDMA has been proposed by China and potentially has a large market-share in China if implemented.

8 Self-Organizing Characteristics of 4G

The resource and duty management operations of 4G would be quite different from the present scenario. Extensive automation in the system and self-organizing characteristics can create an intelligent management. This is a quite strange feature unique to 4G that could lead the system to a complete new level.

9 Two-Tier Coverage

In case of 4G, the geographical coverage would consist of at least two tiers. The normal coverage would be through normal macro cells, but in order to handle the traffic and resources properly during the peak-hours, microcells would be kept in place. Depending on the traffic distribution, the transmission and control duties are switched to the appropriate cells. In some hot spots the coverage layering would be composed of multiple layers to improve the quality of service and resource management.

10 4G Uplink and down Link Frequencies (Proposed)

Though the spectrum of 4G is still under planning, we have a rough idea about the uplink and downlink frequencies from the early developers. OFDM is used to divide the whole spectrum or bandwidth into thousands of small narrow bands. each having different frequencies. By doing this, the system becomes resistant to multipath fading and thus capable of providing better quality of service.

The 4G system also uses OFDMA for the downlink and single carrier FDMA (or SC-FDMA) for the uplink. It optimizes the data rate by using four MIMO antennas per station, which we have seen can provide tremendously high data rates. The channel coding schemes are chosen to be suitable for the OFDM signals. Turbo codes are preferred in this application.

10.1 Downlink

The OFDM system for the downlink uses maximum 2048 subcarriers. The subcarrier spacing in OFDM downlink is 15 kHz. The mobile device must have the ability to receive all the 2048 subcarriers but the base station needs only 72 subcarriers for transmission. The transmission is divided into sub frames of 1.0 ms duration and each time slot is of 0.5 ms duration. The net length of a radio frame is 10ms. For downlink the popular modulation formats are QPSK, 16 QAM, 64 QAM and 256 QAM. The spectrum for the downlink has not been finalized; but it is expected to be wider than the mobile WiMAX and in the similar range of WiMAX.

10.2 Uplink

For uplink, the proposed multiplexing method is SC-FDMA, and proposed modulation methods are QPSK, 16 QAM and sometimes 64 QAM. SC-FDMA is used to suppress the high PAPR, as in the case of OFDMA. For high data rates the constellation size may go up to 256 QAM. Of course it is still under review and the current road map is considering 64 QAM as the proper choice. Uplink spectrum of 4G would be in the same range as the WiMAX but it would have more bandwidth for faster data rate.