Fundamentals Of Cellular Communications Computer Science Essay

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This chapter begins with an introduction to the fundamentals of cellular communications providing the reader with the knowledge of the basic components of a cellular system and the ability to identify and describe digital wireless technologies. We then take a brief look at the history and evolution of mobile communications before initiating an in-depth discussion on LTE.

2.1 Fundamentals of Cellular Communications

In a cellular mobile communications system a large number of low-power wireless transmitters are used to create cells. The geographic service area of a wireless communications system is usually divided into hexagonal shaped cells. The power levels of the transmitter allow cells to be sized according to the subscriber density and demand within a particular region. Each cell is assigned multiple channels (frequencies); frequencies used in one cell can be reused in another cell provided that the same frequencies are not reused in adjacent neighbouring cells.

2.1.1 Network Architecture

Cells (Base Stations)

A cellular network is made up primarily of cells; a cell is the basic geographic unit of a cellular system. Cells are base stations transmitting over small geographic areas. The area of coverage for every cell varies depending on the landscape. Diagrammatically, cells are often displayed as hexagonal shapes that fit together. In reality a cells outer boundary is loosely defined as the signal strength gradually reduces, and towards the boundary (edge) of the cell performance falls.

BS

[hexagonal cell figure]

In a simple cellular system adjacent cells are assigned different operating frequencies. This is because if two adjacent cells use the same frequency interference can and will occur. For this reason cells are usually grouped in "clusters", clusters are usually but not always made up of seven cells and frequency reuse is on a per cluster basis. So channels used in one cluster will not be used within that cluster but can be used in another cluster of cells.

Remember to quote why seven is a reasonable group size - from Cellular Communications Explained: From Basics to 3G.

Seven is a convenient number for a variety of reasons. The first is that it gives sufficient isolation between cells where frequencies are re-used; this is necessary to ensure that the level of signals from nearby cells using the same frequencies is kept to an acceptable level. A further reason is that a group of channels is allocated to e ach cell. As there is a limit on the total number of channels available, it means that a sufficient number of channels can be allocated to each cell [1]

There are different types of cells, each given names to differentiate their purposes. Macrocells are large cells that provide radio coverage served by a high power base station. They cover large and sparsely populated areas, rural areas, highways. Microcells are generally used in a densely populated urban area. Picocells are used for coverage of areas smaller than microcells such as offices, train station or a shopping centre. Femtocells cover an even smaller area in order of meters. A Femtocell allows service providers to service users indoors or possibly at cell edges.

Cell Type

Coverage Area

Macro

35 kilometres

Micro

2 kilometres

Pico

200 metres or less

Femto

In the order of 10 metres

Frequency reuse

The key characteristic of a cellular network is the ability to re-use frequencies to increase both coverage and capacity. The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area that are different from neighbouring cell groups.

Handover

As the subscriber moves from one cell to another cell whilst a call or data transmission is in progress the mobile base station must transfer the call from radio channel to another radio channel. This process of transferring mobile subscribers from one radio channel to another in an adjacent cell is known as a handover. Although the concept is relatively simple to understand it is in reality a challenging process to implement. The system needs to decide when a handover is necessary and to what cell to 'handover' to. All this must be achieved without any noticeable interruption to the call.

2.1.2 Cellular Network Infrastructure

The simple structure of a cellular communications network consists of the following:

The Public Switched Telephone Network (PSTN) - This is the worldwide telephony system, which traditionally uses copper wire for the transmission of analog voice data. The PSTN is a circuit switched network.

Mobile Telephone Switching Office (MTSO) - The MTSO contains the switching equipment (MSC) for routing mobile phone calls. It manages calls, billing information and the mobility of cellular subscribers. The MSC (Mobile Switching Center) is connected to the local telephone exchange; this provides an interface for accessing the PSTN.

Mobile Subscriber Unit (MSU) - The MSU is any equipment that communicates with a cell. These are traditionally mobile phones; though nowadays any device with radio transmission equipment capable of communicating with the cell site is known as a MSU.

Cell site with antenna system - Cell site refers to the actual physical location of the radio equipment used for the transmission of signals. The equipment includes power sources, transmitters and receivers, and antenna systems.

MSO/MSC

PSTN

Mobile Base Station

Mobile Subscriber Unit

2.1.3 Access Schemes

There are various different access schemes used in communications systems, they are known as channel access methods. A channel access method lets several users access the same frequency channel to transmit data and share the available bandwidth. What separates them from each other is the method they use for channel access. Channel access schemes are based on multiplexing methods; a multiplexing method allows several data streams to share the same communication channel by combining the signals. Access schemes commonly take their name from the multiplexing method that they are based on. In this section we'll look at some of the common channel access methods used in communications, TDMA, FDMA and CDMA.

Time Division Multiple Access (TDMA)

The Time Division Multiple Access scheme as you may have already guessed is based on the time division multiplex scheme. TDMA divides the signal into different time slots and assigns the time slots to different users. Each user then transmits, one after the other using its own allocated time slot.

For the time that a user has access to the channel it can use the whole spectrum from transmission. In the above figure there are 4 users and each have been given a transmission time slot, each user then transmits during this time slot.

Frequency Division Multiple Access (FDMA)

Frequency Division Multiple Access is based on frequency division multiplexing. FDM separates the spectrum into non-overlapping sub-bands each of which can carry a separate signal; allowing the transmission of multiple signals at the same time. FDMA assigns users an individual sub-band for communication. Only one user can be assigned to a channel at a time. See illustration for a visual understanding.

As you can see the frequency is divided up and each of the 4 users has been assigned a sub-frequency that they can use to transmit their data.

Code Division Multiple Access (CDMA)

Code Division Multiple Access uses spread-spectrum technology and a coding scheme to allow the spectrum to be used by multiple users. Spread-spectrum technology uses spreading codes to spread the signal over wider bandwidth resulting in a signal that has a much higher data bandwidth than the actual data being sent.

2.1.4 Summary

This section has provided a foundation of cellular communication concepts and technologies. The reader should be armed with the knowledge need to proceed with the rest of this paper. I have discussed the basic components of the simple cellular communication network. Of course modern day cellular networks are more complex but the underlying principles of communications remain the same. I have also presented a discussion on some of the more common and fundamental multiplexing schemes and channel access methods. This paper focuses on LTE with uses OFDMA, a more detailed look at OFDMA will be presented later.

2.2 History and progression of mobile communications

2.2.1 1G Analog cellular networks

The first generation of wireless telephone technology were analog telecommunications standards that were introduced in the 1980s. The focus of first generation wireless networks was voice transmission. Data services were almost non-existent. There were many competing standards for 1G, all incompatible with each other due to the use of different frequencies and signalling. This made international roaming impossible.

NMT (Nordic Mobile Telephone) was the standard used in Nordic countries. AMPS (Advanced Mobile Phone System) was used in North America and Australia. TACS (Total Access Communications System) was used in the United Kingdom. There were many more different standards used for different parts of the world.

1G speeds differed between 28kbit/s and 56kbit/s[reference].

2.2.2 2G

GSM (Global system for Mobile Communications) is a set of standards developed to replace the first generation of analog cellular networks. It originally described a digital circuit switched network for voice telephony and later expanded to support data communications. GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution) was introduced as an add-on to handle data transmissions within 2G networks.

The second generation of wireless communication saw a transition from analog to digital transmission technology, an increase in quality of service and the introduction of wireless data services.

2.5G is the term used to describe 2G-systems that included a packet-switched domain as well as the circuit switched domain. 2.5G saw the introduction of GPRS (General Packet Radio Services) . GPRS provided increased data rates from 56 kbit/s up to 115 kbit/s[reference]. The increased data rates allowed access to new services such as access to WAP (Wireless Application Protocol), MMS (Multimedia Messaging Service) and access to the Internet.

EDGE (Enhanced Data rates for GSM Evolution) introduced 8PSK encoding. This enabled higher bit-rates per radio channel, making EDGE 3 times faster than GPRS. This evolution of GSM is known as 2.75G. EDGE became standardized by the 3GPP as an upgrade of the GSM family.

2G offered many advantages over 1G. It saw an increase in data rates, utilisation of packet-switching technology, which naturally lead to the introduction of data services such as SMS, WAP, MMS, and internet browsing.

2.2.3 3G (Third Generation)

As the use of 2G phones became more widespread and as it became part of users daily lives, the demand for data services increased. The technological differentiator between 3G and 2G was the use of packet switching rather than circuit switching for data transmission. Voice data was still carried via circuit-switching.

2.2.4 4G

2.2.5 Summary

2.5G: Adding Packet Services: GPRS, EDGE

_ 3G: Adding 3G Air Interface: UMTS

_ 3G Architecture:

_ Support of 2G/2.5G and 3G Access

_ Handover between GSM and UMTS technologies

_ 3G Extensions:

_ HSDPA/HSUPA

_ IP Multi Media Subsystem (IMS)

_ Inter-working with WLAN (I-WLAN)

_ Beyond 3G:

_ Long Term Evolution (LTE)

_ System Architecture Evolution (SAE)

_ Adding Mobility towards I-WLAN and non-3GPP air interfaces

2.3 Long Term Evolution (LTE)

2.3.1 Background

There has been rapid increase in the use of the internet to provide a plethora of services since the 1990s started. Access to the internet has become more and more widespread and popular over the last couple decades and so has the use of 2G and 3G phones. It naturally occurred that the next step would be to make those internet services available on mobile devices; this is known today as mobile broadband. The ability to support the same IP based services that users use at home through their fixed-line broadband connection is a major challenge and a fundamental driver for the evolution of LTE.

The first data services over GSM were circuit switched with packet-based GPRS being added on later. This was the same as 3G which was also based on circuit-switching and had packet-switched services as an add-on. The evolution of 3G into HSPA and now LTE is the first instance where packet-switching services and IP are primary.

[Talk about system architecture evolution(SAE) (Evolved Packet Core) - ]

The tasks of specifying the LTE system that meets the design targets falls on 3GPP. 3GPP is a partnership project formed by the standards bodies ETSI, ARIB, TTC, TTA, CCSA, and ATIS. 3GPP writes specifications for 2G, 3G, and 4G. In 2004 a workshop was held to organize and initiate work on 3GPP Long Term Evolution radio interface. A study item was created work had begun on defining the requirements for LTE. This was documented and released in a technical report TR 25.913 [R* = "Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)" (Technical Report TR 25.913)"] and approved in June 2005.

The justification of the study item was, that with enhancements such as HSDPA and Enhanced Uplink, the 3GPP radio-access technology will be highly competitive for several years. However, to ensure competitiveness in an even longer time frame, i.e. for the next 10 years and beyond, a long-term evolution of the 3GPP radio-access technology needs to be considered.

Important parts of such a long-term evolution include reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. In order to achieve this, an evolution of the radio interface as well as the radio network architecture should be considered.

Considering a desire for even higher data rates and also taking into account future additional 3G spectrum allocations the long-term 3GPP evolution should include an evolution towards support for wider transmission bandwidth than 5 MHz. At the same time, support for transmission bandwidths of 5MHz and less than 5MHz should be investigated in order to allow for more flexibility in whichever frequency bands the system may be deployed. [http://www.3gpp.org/ftp/Specs/html-info/25913.htm]

The motivation for LTE can be summarised:

The need for a packet switched system

The need for higher data rates

High quality of services

Cheaper infrastructure (Reduce costs for operators)

Capacity, Improving spectral efficiency

(figure x)

2.3.1 LTE Requirements and Key Features (Enabling Technologies)

The technical requirements defined in TR 25.913 are as follows:

Peak Data Rate - E-UTRA should support significantly increased instantaneous peak data rates.

100Mb/s within a 20 MHz downlink spectrum allocation

50Mb/s within a 20 MHz uplink spectrum allocation

Significantly reduced latency

The system should be able to support a large number of users per cell

Spectrum efficiency

Target for spectrum efficiency (bits/sec/Hz/site), 3 to 4 times Release 6 HSDPA in the Downlink

Target for spectrum efficiency (bits/sec/Hz/site), 2 to 3 times Release 6 Enhanced Uplink

Mobility

E-UTRAN to support mobility across the cellular network optimized for low mobile speed from 0 to 15 km/h

Higher mobile speed between 15 and 120 km/h should be supported with high performance

Mobility across the cellular network shall be maintained at speeds from 120 km/h to 350 km/h (or even up to 500 km/h depending on the frequency band)

Coverage

E-UTRA should be sufficiently flexible to support a variety of coverage scenarios whilst meeting performance targets

Radio Resource Management

Enhanced support for end to end QoS (Quality of Service)

Efficient support for transmission of higher layers

Support of load sharing and policy management across different Radio Access Technologies

Cost-related

Backhaul communication protocols should be optimized.

The E-UTRAN architecture should reduce and balance the cost of future network deployment by maximizing the usage of existing site locations [, interfaces, and protocols].

UE complexity and power consumption shall be minimized/optimized. Complicated UTRAN architecture and unnecessary interfaces should be avoided.

LTE aims to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio access technology.

There are some key new technologies introduced by LTE. They enable LTE to operate more efficiently with respect to the use of the spectrum. These key technologies allow LTE to meet its requirements especially the much higher data rates that are being requested. A much detailed presentation of these is to be discussed later.

OFDM (Orthogonal Frequency Division Multiplex)

Multiple Access Schemes (OFDMA, SC-FDMA)

MIMO (Multiple Input Multiple Output)

SAE (System Architecture Evolution)

2.3.2 LTE Radio Access Network Architecture

The EPS (Evolved Packet Switched System) provides IP connectivity between a UE and an external packet data network using E-UTRAN (Evolved Universal Terrestrial Radio Access Network). The EPS consists of an EPC (Evolved Packet Core) and Evolved UTRAN (E-UTRAN). This section focuses on the Radio Access Network (E-UTRAN) in LTE. There is a functional split between the E-UTRAN and EPC as illustrated in figure [below]

*A bearer is an IP packet flow with a defined QoS between the gateway and the User Terminal*

Figure 0: Functional Split between E-UTRAN and EPC

The E-UTRAN architecture is illustrated in Figure 4 below.

Figure 0: Overall Architecture [2]

The E-UTRAN consists of base stations called eNBs (evolved Node B) interconnected with each other via the X2 interface. The S1 interface connects the eNBs to the Evolved Packet Core (EPC).

eNodeB (Evolved NodeB)

eNBs perform the following functions defined [2]:

Radio Resource management: Dynamic allocation of resources to al UEs in both downlink and uplink, Radio Bearer control, Radio Admission Control, Connection Mobility control.

IP header compression and encryption of user data stream

Selection of an MME at UE attachment when no routing to an MME can be determined from the information provided by the UE;

Routing of User Plane data towards Serving Gateway;

Scheduling and transmission of paging messages (originated from the MME);

Scheduling and transmission of broadcast information (originated from the MME or O&M);

Measurement and measurement reporting configuration for mobility and scheduling

X2 Interface

The X2 interface connects eNodeBs together. The main reason for this interface is to reduce the packet loss due to user mobility. As the user moves through the network, unsent packets stored in the old eNB can be forwarded to the new eNB via the X2 interface. Handovers are also performed via the X2 interface.

2.3.3 LTE System Architecture Evolution (SAE)

The system architecture Evolution is the evolution of the core network. The new architecture has been developed to support E-UTRAN by providing a much higher level of performance to meet LTE requirements. The main component of the SAE is the EPC (Evolved Packet Core).

Features

The key features and advantages of the System Architecture Evolution are:

Improved data capacity: This is to meet the much higher levels of data that will be flowing through an LTE system. It is imperative that the system architecture is able to handle the increased data.

An all IP architecture: In 3G networks voice data is carried as circuit-switched data. The move from circuit-switching to packet-switching has necessitated the adoption of IP network configurations for the system architecture.

Reduced Latency: To meet requirements of LTE, the SAE has been initiated to ensure reduced latency and responsiveness from 3G LTE applications.

Reduced operational and capital expenditure: The flat architecture of the SAE allows for easier set-up and commissioning time. This allows operators to benefit by reducing the cost associated with running an LTE system.

Figure 2 : SAE (System Architecture Evolution)

The main components of the EPC are:

The Serving Gateway

The PDN Gateway

The MME (Mobility Management Entity)

The HSS (Home Subscriber Server)

The PCRF (Policy and Charging Rules Function) Server

Serving Gateway (SGW)

The SGW primary function is to route and forward user data packets as well as managing user-plane mobility. It also acts as the boundary point of separation between the Radio Access Networks and core networks (EPC). The SGW is the mobility anchor point for inter-eNodeB handovers and for mobility between LTE and other 3GPP technologies such as 2G and 3G.

Packet Data Network Gateway (P-GW)

The Packet Data Network Gateway is the demarcation point of the packet data interface towards the Packet Data Networks. The PDN-GW supports policy enforcement features, per-user based packet filtering, lawful interception, UE IP address allocation, transport level packet marking in the uplink and the downlink, UL and DL service level charging.

Mobility Management Entity

The MME is main signalling node within the LTE EPC. The LTE MME is responsible for initiating paging and authentication of the UE (User Equipment). It is also responsible for the tracking of each user and the selection of the right gateway. MME is connected to the eNB via S1 interface and to the S-GW via S11 interface and also the HSS via the S6 interface. A more detailed functionality listing of the MME can be found in the 3GPP technical specification [reference - TS36.300]

Home Subscriber Server (HSS)

The HSS is the main IMS database and also act as a database within the EPC. It contains user related and subscription related information. The HSS is based on HLR (Home Location Register) and AuC (Authentication Center).

PCRF (Policy Control and Charging Rules)

The PCRF is responsible for policy control decision-making. It is also responsible for controlling the flow-based charging functionalities in the Policy Control Enforcement Function (PCEF). [LTE_NETWORK_ARCHITECTURE_ WHITE_PAPER]

2.3.4 OFDM

Orthogonal frequency Division Multiplex is the modulation format of choice for LTE. Its use is not limited to LTE and can be found in other communications system such as WiMAX, WLAN, DVB and DAB. OFDM has been chosen as the signal bearer format for LTE as it is very resilient to interference. It is also suitable for the transmission of high data rates which is one of the main requirements of LTE.

The basic idea behind OFDM is to divide the available spectrum into narrow-band parallel channels (sub-carriers) and use it to transmit information at a low signalling rate. The sub-carrier frequencies are chosen so that they are overlapping and orthogonal to each other. This mitigates inter-symbol interference (ISI) and makes the inter-carrier guard bands unnecessary.

http://www.blinqnetworks.com/media/uploads/Figure%201%20-%20OFDM%20Sub-Carriers.png

Figure (x) OFDM signal narrow sub-carriers spacing

To eliminate the remaining impact of ISI caused by multipath propagation; a guard period must be inserted at the beginning of each OFDM symbol by adding a Cyclic Prefix (CP). A CP makes sure that the delayed symbols don't overlap with the following symbol.

http://www.blinqnetworks.com/media/uploads/Figure%202%20-%20OFDM%20Symbol%20with%20Cyclic%20Prefix(1).png

The modulation of data symbols is the equivalent to an Inverse Fast Fourier Transform operation (IFFT). At the receiver end the reverse operation is applied to retrieve the data stream. This is the equivalent to a Fast Fourier Transform (FFT).

Image of frequency-time representation of an ofdm signal

What is orthogonality? - answer this show some equations

Advantages of OFDM

Flexible utilisation of the frequency spectrum

High spectrum efficiency as a result of orthogonality between sub-carriers

Easily scaled up to wide channels that are more resistant to fading

Channel equalizers are much simpler and easier to implement as an OFDM signal is represented in the frequency domain rather than the time domain.

High spectrum efficiency

Disadvantages of OFDM

Sensitive to frequency errors, phase noise and Doppler shift. This is because the subcarriers are closely spaced.

High peak-to-average signals, for this reason a modification to the technology is used in the uplink.

Summary:

OFDM is a widely used and mature technology

It utilises CP in order to avoid ISI.

It uses orthogonal sub-carriers to avoid spectrum *wastage* there by resulting in efficient spectrum usage.

Easily extended to a multiple-access scheme.

2.3.5 OFDMA

Orthogonal Frequency Division Multiple Access is a multi-user version of the OFDM modulation scheme described above. OFDMA distributes sub-carriers to different users at the same time; this allows multiple users to receive data simultaneously.

C:\Users\RnD\Google Drive\ofdma.gif

[reference] http://mobilesociety.typepad.com/mobile_life/2007/05/an_introduction.html

To create the transmitted signal, each sub-carrier (a-e) in (figure above) is passed to the IFFT (Inverse Fast Fourier Transform) which outputs a summation of all sub-carrier time-domain signal ready for transmission. The signal is then modulated then amplified from then transmission begins. On the receiver side the signal is de-modulated and amplified. The Fast Fourier Transform is performed on the signal to convert the signal from the time-domain to the frequency domain. A detector function is then applied to separate the sub-carriers and obtain the data-stream for each sub-carrier.

OFDMA FDD Frame Structure

OFDMA Resource Block Structure

OFDMA time-freq multiplexing

2.3.5 SC-FDMA (Single-Carrier Frequency Division Multiple Access)

Single-Carrier Frequency Division Multiple Access is the channel access method chosen for LTE uplink transmission. Signals in OFDMA are susceptible to high peak-to-average power ratio, these signals require highly linear power amplifiers to avoid excessive inter-modulation distortion [reference: Single Carrier FDMA for Uplink Wireless Transmission]

SC-FDMA is an extension of OFDM and can be viewed as DFT-coded OFDM where time domain data symbols are transformed to frequency-domain by a discrete Fourier transform before going through the standard OFDM modulation[reference: Single Carrier FDMA in LTE - ixia.com whitepapers]. This means that SC-FDMA inherits all the benefits of OFDM whilst also benefitting from low peak-to-average power ratio of single-carrier systems.

C:\Users\RnD\Google Drive\scfdma.gif

[figure: processing step] [reference: http://mobilesociety.typepad.com/mobile_life/2007/05/an_introduction.html]

SC-FDMA spreads the data signal over all the sub-carriers. This is the function of the additional processing block. The data stream is fed into a Fast Fourier Transformation function; the output of this function is then fed into the IFFT and the process from then is the same as OFDMA. On the receiver side the signal is handled the same as in OFDMA but the resulting signal after processing by the FFT block has to be processed again by IFFT to retrieve the original bits. The illustration below shows a comparision between OFDMA and SC-FDMA

[modulation diagram]

2.3.6 MIMO

MIMO in wireless communication is the use of multiple antennas for transmitters and receiver to increase communication performance. The basic idea behind MIMO is to use multiple signal paths for the data being sent in-order to increase performance metrics such as error-rate and data rate. By using multiple signal paths it is possible to provide the receiver with multiple versions of the same signal, thereby greatly reducing the probability of a corrupt signal (due to interference) and even improving the signal to noise ratio.

2.3.7 Protocol Stack

layers

IP packets are passed through multiple protocol entities:

PDCP

RLC

MAC

PHY

Communication channels

Radio Link Control (RLC) Layer

Medium Access Control (MAC) Layer

Physical (PHY) Layer

- OFDMA with cyclic prefix in the downlink and SC-FDMA with a cyclic prefix in the uplink

Duplex modes: full duplex FDD, half duplex FDD and TDD

A radio frame has length of 10ms and contains 20 slots (slot duration is 0.5ms)

Modulation schemes : QPSK, 16QAM, 64QAM

2.3.6 Radio Resource Management

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