Generations Of Mobile Cellular Systems

Published Date: 02 Nov 2017

Chapter 2

LITERATURE REVIEW

2.1 Introduction

The era of Fourth Generation (4G) in mobile broadband and cellular networks became a tangible reality in the technology life. Since the baby steps time of mobile networks, the ultimate goal was to fulfill the QoS level to the end-user and provide network of great connection capabilities. However, to some extent, the scope of each mobile generation beside available technologies retard that QoS level of making the network effective enough; but to meet only a current given list of performance requirements on that fixed period of time frame. This in turn; neglected the clue that communication technologies should have a backtracked line or sort of backward compatibility to involve components from different generations in the backhaul system.

As a result, wireless systems concepts were introduced to scale up the end-user QoS performance and add signal mobility for more flexible communication. Wireless networks have been officially emerged in a very early time since 1880s as a postulated wireless propagation. The first wireless system then patented a complete wireless system in 1897 by Marconi. As stated in Wireless, (1995), the British Broadcasting Company (BBC) was among the first to utilize the term wireless, around 1923 manipulated in their program namely Radio Times. After that, the notion of personal communication started to be implemented in practical views. It was implemented thanks to the concurrent advancements in micro-electronic circuits that resulted in rapid mobile and personal communications.

Since then, users could freely move while initiating communication channels with other parties in a wireless mode. In further deployments of the technologies that time, mobile data communication services were introduced in technical systems such as Automatic Vehicle Location, Electronic Mails, and Remote Database Access. The development of mobile systems thereafter, was toward transforming from analog to digital signal communications. The following subsection introduces the generation of mobile and cellular systems.

2.1.1 Generations of Mobile Cellular systems

First generation 1G

The initial mobile 1G system was based on analogue technology (InfoDev, 2012). Nardik Mobile Telephone (NMT), Total Access Communication System (TACS), and Advanced Mobile Phone System (AMPS) in (Washington, 2006); were classified among these generic 1G standards. Some drawbacks were highlighted in 1G system i.e. unencrypted systems suffered from security issues like vulnerability to eavesdropping via a scanner. 1G-based systems were also limited in terms of frequency allocation and spectrum efficiency.

Second Generation 2G

A decade after 1G announcements, 2G mobile systems were introduced in mid of 1990s Wireless, (2012) in light of the frequent mobility issues encountered in 1G. 2G mobile standards were the first transformation from analogue to digital signaling in communication channels. Global System for Mobile Communication (GSM) (Bernhard, 2007), the European candidate standard, and Collision Division Multiple Access (CDMA); the U.S candidate standard were the first 2G cellular digital systems. 2G systems encompassed the concept of phone to network signaling, and lately in 2000, the Short Massaging Service (SMS).

There is a unique stage in 2G technology named as 2.5G mobile. This nested taxonomy was to highlight the radical and rapid improvements of 2G systems. 2.5G mobile introduced more services like General Packet radio Service (GPRS) in GSM (Cai, 1997 and Leppisaari, 2003), and its optimized technique Enhanced Data rate for GSM Evolution (EDGE) to target higher data rates in communication channels.

Third Generation 3G

3G mobile systems were introduced by International Telecommunication Union (ITU) as in (Prasad, 2000 and Markoulidakis, 1997). The specifications of the 3G-based systems are declared in International Mobile Telecommunicatio–2000 (IMT–2000). In 3G-based systems the aim was to support higher data transmission rates and provide enormous cell capacity. In addition to that, 3G promised to process various data such as voice, image, and video by using digital channels. Security and user authentication are also major feature of 3G over the former generations.

WiMAX 802.16 for IEEE candidate and Long Term Evolution (LTE) by the Third Generation Partnership Project (3GPP) are among well-known 3G mobile systems. Data access technologies that operate on WiMAX and LTE are Wideband-CDMA (WCDMA) and CDMA2000 Evolution-Data-only (CDMA2000 EV-DO) used for Internet on Mobile.

Due to the concurrent deployments in mobile systems that time, 3G inherited subsequent stage so-called 3.5G systems. 3.5G were identified by adopting High Speed Downlink Packet Access (HSDPA), and its’ upgraded version High Speed packet Access (HSPA) technologies as stated by (Shah, 2008). These evolutions in turn have scaled up the QoS level by providing higher data rates and faster transferring capabilities that reached up to 14Mbps of peak speed. Currently, the legacy LTE in Release 8 uses HSPA+ to provide peak rate of data up to 300Mbps for the downlink carried on 20MHz channel capacity for the mobile end users. In Figure 1, 3G systems features are highlighted. Adhere, it is noticed that there was a huge evolution in the mobile frameworks that enabled technologies to open more horizon for smarter and more dynamic mobile systems.

Fourth Generation 4G

When it comes to achieve the vision in the Internet of Things (IoT) concept as discussed by (Coetzee, 2011 and Lu, 2010), means that all-IP mobile devices structure should be established. Therefore, 4G have come in place to fulfill this principle and realized real-time data transmission between users. 4G-related system specifications were highlighted by IMT-Advanced in (Loa, 2010). 3GPP organization led the first evolution in 4G by introducing its candidate namely LTE-Advanced (LTE–A) in Release 10. Thereafter, the enhanced WiMAX (WiMAX 2.0) is being under deployment in IEEE standard 802.16m. 4G systems emphasize more in optimizing channel and signal quality metrics such as cell-edge efficiency, cell capacity, and interference index and advanced QoS metrics. Concepts like multi-cell transmissions, heterogeneous network deployments, cell relays, smart antenna techniques and Beamforming are being deployed to further improve 4G systems requirements and the user satisfaction index as well.

2.2 Overview of LTE

Long Term Evolution (LTE) is a Mobile Telecommunication Technology Standard (like WiMAX) target to provide the fastest service delivery to the end users. It is the next generation towards 4G and beyond technologies that support GSM and CDMA cellular carriers (Jimaa, 2011). LTE leverages from System Architecture Evolution (SAE) and Mobility Management Entity (MME), the main components introduced by 3GPP (Sengar, 2011). LTE system mainly operates on Radio Access Network (RAN) to transmit data in high rates benefiting from the radio signal persistence and wide coverage. LTE concepts have been manipulated in most of the 3G mobile devices. The history of LTE evolution toward LTE–A is explained below.

2.2.1 History of LTE/LTE–A

3GPP was initially established in 1998 with a main focus on radio communication, which was motivated by some of the current systems that time such as Universal mobile telecommunication System (UMTS). The target of 3GPP efforts was to meet the specifications of International Mobile Telecommunication-2000 (IMT-2000) project as introduced in (Akyildiz, 2010).

According to Ali-Yahiya, (2011a), 3GPP started its attempts in 2004 to enhance the Universal Terrestrial Radio Access (UTRA) models. By the end of 2008, LTE standard specifications completed and were declared as LTE release 8 (Rel-8). Thanks to the adopted radio access technology, LTE Rel–8 system delivered throughput of 3–4 times equivalent to HSPA as illustrated in Figure 2, hence LTE improvements are tailed by each release.

Simultaneously, a project called system architecture evolution (SAE) was sat off to identify the architecture of (RAN) (Akyildiz, 2010). Then, 3GPP performed minor enhancements such as Dual-layer Beamforming and Femtocells technologies to the current release to come out with release 9 (Rel-9) or what called "Better LTE" targeting to reach 3.9G specifications.

The progressive works on the legacy LTE continued until the first complete and coherent release of 4G and beyond has been declared as Release 10 LTE–Advanced (LTE–A) in (3GPP, 2011).

LTE–A network aims to deliver a system capabilities and QoS level beyond IMT-Advanced. LTE’s Release 10 (Rel-10) was formally submitted to ITU in 2011. Since then; it was approved by ITU as an IMT-Advanced technology standard. In the advanced sub-sections of this chapter, we will see how LTE–A fulfilled IMT–Advanced requirements and even outperformed them in some criteria. Currently, 3GPP initiated the vision to cellular systems beyond 4G and IMT-Advanced on lights of scaling up the features of LTE Rel-10. It is foreseen that in Releases 11 and 12 3GPP working group will realize the big picture of the future Fifth Generation (5G) in a real soon.

2.2.2 LTE System Architecture

Unlike the hierarchical fashion of other traditional mobile systems, the architecture of 3GPP LTE, as proposed in (3GPP, 2008 and 3GPP, 2009) is more to flat distribution in its system components. As described in (Akyildiz, 2010 and Ali-Yahiya, 2011b), LTE architectural model is generally termed as System Architecture Evolution (SAE).

Within SAE Reference model; as depicted in Figure 3, it consists of two main networks: Evolved Packet Core (EPC) network, and Evolved Universal Terrestrial Radio Access Network (E–UTRAN). EPC network usually provides the IP connectivity between User Equipment (UE) and external packet data network utilizing E-UTRAN. While as stated in (Chadchan, 2010), E-UTRAN is characterized by a network of Evolved-NodeBs (eNBs) that supports all services, including real-time multimedia services over shared channels.

Evolved Packet Core Network

In this part of LTE SAE system, the packet communication with the Internet is managed and controlled (Pelcat, 2013). EPC is designed for supporting much higher data rates, significant low system latency, and optimizing packet flows through bandwidth rotation and charging schemes. Moreover, EPC explicitly supports multiple radio access technologies in seamless interest that in turn provide a feature of system backward compatibility. Table 1 explains the functional features of each component in the network. Where by the components are interacted with each other to provide a management orientation systems.

Evolved Universal Terrestrial Radio Access Network (E-UTRAN)

E–UTRAN is the air interface of LTE system that connects UEs to network services. It manages radio resources and guarantees the accessibility to user data (Pelcat, 2013). Basically, E-UTRAN is a distribution of several eNBs, in which each eNB manages several cells attached to it. Multiple eNBs are also connected to each other via X2 interface as shown in Figure 4. This linkage is important for handover processes, and also for advanced resources management approaches like multi-cell packet scheduling as we will see in later sections.

As shown in Figure 3, a closer picture of E-UTRAN network is illustrated by identifying the interactive interfaces between components. The core part in E-UTRAN is the eNB. It is the air interface to the user and control plane protocols as described in (Larmo, 2009). Several eNBs are connected together through X2 Interface. There are two special types of eNB: Home eNB (HeNB), is also called Femtocell, with low cost and power capability that provides indoor coverage. The second is the Relay eNB (ReNB) that was deployed for purposes of increasing signal coverage and network performance enhancement. UEs can either directly connect to the main eNB or to one of its variants low cost ones for having more traffic relaxing.

According to (Alcatel, 2009), E-UTRAN is responsible for many functions like: radio resource management, IP header compression and data encryption techniques, resources scheduling and allocation in both uplink and downlinks, and coordination and handover with other adjacent eNBs.

2.2.3 LTE/LTE–A System Features

Since the announcement of LTE Rel 8, 3GPP targets to upgrade the features of the legacy LTE to meet the 4G requirements stated by ITU under the term IMT-Advanced. LTE-A was approved by ITU in 2011 as the 3GPP’s first 4G mobile system that fulfilled ITU needs. As shown in Table 2, the major specification items which are defined by IMT-Advanced compared with LTE-A system features that outperformed them.

From Table 2, it is obvious that LTE-A provide a high system capabilities by supporting wide range of bandwidth to carry and process multiple UEs. Cell efficiency as a core criteria in assessing cellular systems has been given optimized to the double in LTE-A. This in turn enabled LTE-A to accommodate more traffic without any congestions of compromising to the relative QoS metrics. Moreover, system latency was also reduced to the double-lower than IMT-Advanced. These capabilities were achieved thanks to the deployed technologies and concepts in LTE system as will be explained below.

In table 3, an illustration of the general QoS performance metrics in LTE-A with respect to the legacy LTE Rel-8 is performed. LTE-A shares the same based concepts as in LTE Rel-8; but with more scaled up features like mobility, data rate, and other QoS-related metrics (Wannstrom, 2012). Nevertheless, it is quite reasonable for LTE Rel-8 to reach up to that level of performance shown in the table, as it is anyhow identified as 3G-3.5G mobile system unlike the 4G LTE-A.

2.2.4 Enabled Features and Technologies by LTE/LTE-A

LTE and LTE-A system were designed with some distinct features and concepts that make them of a different architecture; so far; from the other cellular systems like WiMAX. In the following, some of these enabling technologies are discussed.

Carrier Aggregation (CA)

As described in (LIN, 2012 and Yao-Liang, 2011), CA was initially proposed by 3GPP in LTE Rel-8. The purpose of CA is to support more bandwidth in order to increase the system throughput and achieve higher peak data rates and coverage. This is done through aggregating multiple existing three types LTE Component Carriers (CCs) in the eNB as in (Akyildiz, 2010). The efficiency of CC depends on channel condition, and UEs QoS. Many studies in (LIN, 2012; Yao-Liang, 2011; Dongwoon, 2012) discussed and analyzed the improvement in CC approach at LTE-A systems.

Deployed Relay eNBs (ReNBs)

The motivation behind deploying this special type eNBs, is to improve the cellular system coverage, exactly in the remote regions (Akyildiz, 2010; Agilent, 2011).

Relaying concept reduces the cost in network design as there is no wired backhaul assumed. In addition to that, the cell power is wisely controlling especially when many remote UEs are connecting to the network. Figure 4 shows a simple relaying scenario planned in LTE-Advanced. "Uu" is the interface to connect UE to Relay eNodeB (RN), whereas RN is connected to the donor cell eNB throughout "Un" interface.

Deployments on Self-Organizing Networks (SON)

SON is a new trend introduced by 3GPP to simplify the network management and save operational expenditure (Feng, 2008). As discussed by (Marwangi, 2011), SON concepts are implemented through three functions Self-Optimization, Self-configuration, and Self-healing. In LTE-A, SON has been taken advantage of in the design of femto-cells and HetNets in such a way to manage and reduce interference and reduce energy consumption.

Hybrid Automatic Repeat Request (HARQ)

HARQ technique is meant for buffer management in LTE-based systems (Berggren, 2012; Liu, 2009). HARQ aims at improving the packet transmission by reusing the data from previous transmissions rather than discarding them. In (Gao, 2012), it is stated that LTE-A is deployed with HARQ buffer management to maintain higher throughput and lower latency in multi-transmission scenarios like MIMO, Coordinated Multipoint Transmission and Reception (CoMP), and enhanced Inter-Cell Interference Coordination (eICIC). However, some challenges in developing HARQ still efficiently unaddressed in terms of storage requirements described in (Dongwoon, 2012); due to the huge amount of bits stored.

2.3 Resources Scheduling in LTE/LTE-A

Scheduling in LTE-related networks is referred to the procedures of allocating radio resources and managing them in such a way that multiple users are simultaneously served. We should keep in mind that, the scheduling is performed with aim of maintaining stable data rate, QoS requirements and the mitigation of eNBs signal interference as much as possible on the other hand. Like the other modern cellular networks, in LTE scheduling processes take place in the base station (eNB).

As investigated by Kwan, (2010), the major difference between packet scheduling in LTE and other earlier radio systems like HSDPA is that LTE- schedule resources for users in both Time Domain (TD) and Frequency Domain (FD), whereas HSDPA involves only in TD. There are usually two types of links to be scheduled in LTE network: (1) links between eNBs and the UEs. (2) Links between multiple eNBs connected to each other. The latter is an obvious type in which advanced scheduling technologies like ICIC and CoMP are invoked under a concept of "cooperative scheduling" to manage multiple UEs as it will be discussed later.

2.3.1 Downlink (DL) Scheduling

As discussed in (Gavrilovska, 2011), the design of DL scheduling is a hard task. Hence, some challenges should be considered like maximization of system capacity, boosting spectral efficiency, and ensuring calibrated fairness. In LTE/LTE-A, OFDMA is used as radio transmission module in downlink. This allows several UEs to share OFDM sub-carriers in parallel way. As investigated by (Yao-Liang, 2011), OFDMA is an efficient variant of OFDM to be applied in LTE downlink because of the high spectral efficiency that can be achieved, bandwidth scalability, and also robustness against multi-path fading.

The process of DL scheduling model for RBs is clearly illustrated in Figure 5. In DL channel, MAC scheduler uses Physical Downlink Control Channel (PDCCH), and Physical Shared CCH (PSCCH) functions for scheduling radio resources. The Scheduler receives the CQI (number between worst 0 to best 15) reported by UE to accordingly indicate the most efficient MCS to be used.

2.3.2 Uplink (UL) Scheduling

Unlike DL scheduling, work flow in UL is differs to some extend from the DL explained above. Whereby, a simple procedures are involved since eNB does nort require any additional information of the UL channel quality. Authors in (Delgado, 2010) claimed that the OFDMA variant suffers from a major issue in spite of its famous features it added to the system capacity. The issue here is about the instantaneous transmitted RF power that may vary significantly within a single OFDM symbol. This entails provide a large Peak to Average Power Ratio (PAPR) to end up with more power consumption. Therefore, SC-FDMA was introduced as an access modular for the uplink LTE channel.

Although on other essence, (Capozzi, 2012) stated that SC-FDMA limits the scheduler freedom to transmit RBs only in a single carrier mode. This is translated to less channel capacity and data rates. Obviously, all the introduced UL schedulers are divided into two scheduling types: Time Domain Packet Scheduler (TDPS) and Frequency Domain Packet Scheduler (FDPS) as described in (Salah, 2011).

2.4 Scheduling Algorithms in LTE/LTE-A

LTE mobile network in Release 8 and before, are classified as 3G systems. This means that the network specifications can cope; to some extent; with the other 3G systems such as WiMAX. Scheduling algorithms and concepts can therefore be inherited to LTE for resource management and interference mitigation.

However, such legacy approaches could not be exactly adapted when the latest releases of LTE-A were announced as a 4G technology. Whereby, channel capacity is scaled up beyond the algorithms measurements. Therefore, scheduling techniques should be deployed to suit the current system requirements and keep the QoS in the satisfaction level with optimal trade-off between utilization and fairness as discussed in (Ali-Yahiya, 2011e). In the following subsection we provide a brief discussion on opportunistic scheduling as the traditional packet transmission techniques in LTE systems.

2.4.1 Opportunistic Scheduling

The study of opportunistic scheduling in LTE was started in an early time of 3GPP evolutions. Wherein, in LTE network schemes deployed focus more on certain QoS metrics like throughput, delay, and fairness to be fulfilled. As stated in (Al-Rawi, 2008); authors defined the opportunistic scheduling as a specific behavior of the channel toward controlling transmitted data in such a way overuse the entire available bandwidth to allocate resources with a high throughput.

From the literature, (Ali-Yahiya, 2011e) conducted evaluation studies on the main three well-known opportunistic schemes: Proportional Fair (PF), Maximum–Largest weighted Delay First (M-LWDF), and Exponential PF (EXP-PF) to indicate the efficiency level in each one over the other. These schemes attempt to make the best trade-off as possible between several performance metrics like cell throughput, fairness, and delay. Next up, we briefly explain on the main concept of each scheme.

Proportional Fair (PF)

The concept of PF was proposed by Kelly in (Kelly, 1997), and firstly introduced for single-cell cellular network. PF scheduling is suitable for managing non-real time traffic. Therein, experience channel quality and past user throughput are two factors considered in radio resource assignment. PF aims at maximizing the total network throughput and guarantee fairness among various flows.

Modified Largest Weighted Delay First (M-LWDF)

In spite of the fairness level that PF manage to maintain, the algorithm was limited for non-real time traffic only. Hereby, the motivation of M-LWDF was formulated. M-LWDF was initially introduced in (Andrews, 2000) to treat delay sensitive traffic (real-time traffic) as stated by (Monghal, 2009). It also supports multiple data users with various QoS requirements in TDPS. In some other contributions stated that there are the two reasons behind preferring M-LWDF for delay-aware scheme: (1) the optimal gained throughput. (2) The use of time stamping makes it easy to be implemented. The main idea in this approach is to induce the scheduler to balance the weighted delays of packets and make an efficient use of the channel state information.

Exponential Proportional Fairness (EXP/PF)

This algorithm was proposed by (Jong-Hun, 2003) based on the traditional PF in (Kelly, 1997). Unlike PF and M-LWDF, the algorithm schedules both real and non-real time traffic with more focus in prioritizing delay sensitive traffic. The main idea is to combine the benefits of the exponential rule in (Shakkottai, 2000) with the PF explained above with the aim of balancing between guaranteeing high system throughput and maintaining a level of fairness.

2.4.2 ICIC Scheduling Techniques

ICIC was introduced as a provisioning technology in LTE/ LTE advanced resource management. It was highlighted in LTE Rel-8, and thereafter officially implemented in LTE Rel-10 (LTE-A). ICIC was deployed to mitigate the issue of cell-edge inter-cell interference. Therein, as shown in Figure 6, coordination mechanisms are defined among the neighboring cells to allocate orthogonal resources to the overlapping highly interfered area.

Unlike the previous resource scheduling approaches, ICIC consider both of frequency and time domain packet scheduling. That means more cell efficiency and bandwidth utilization is provided to the coordinated services. Thanks to principle of multi-carrier resource management in ICIC, eNB is able to simultaneously transmit data UE in several sub-carriers; using some enabling LTE technologies like MIMO, Femto-cells, and relays.

In light of the inter-cell interference phenomenon that was described above in Figure 6, (Calabrese, 2011) discussed that, the rule of ICIC is to control the link power allocation in frequency domain with respect to the time domain scheduling in order to provide the UEs with low-interference resources. On the other hand, ICIC is considered as a promising technology for mitigating the degradation in cell-edge bit rate that results from interfering adjacent eNBs and improving the cell-edge throughput.

Summary

The era of 4G wireless has already realized by the technology communities. Whereby, features of data integration and seamless services with more mobility support are now achieved features. Therefore, it has been obvious for LTE to fulfill a satisfied set of performance requirements to provide a high level of QoS for the end users. Scheduling as a main concept in mobile system is supposed to be given a due care to guarantee data delivery and network efficiency.

In LTE network the nature of deploying heterogeneous environment makes it a bit complicated to fit on the traditional scheduling concepts from similar cellular technologies like WiMAX and WiFi. Therefore more over the network architecture and the protocol stack in LTE was uniquely constructed to support different access modules for both downlink and uplink.

Nevertheless, it still applicable to leverage from the legacy scheduling concepts and adapt them in LTE channels considering system architecture that may support the logical policies in channel. Therefore we will be able to realize the issues of resources allocations, with fair share level in multi-use mobile scenarios.