OFDM (Orthogonal Frequency Division Multiplex)

Orthogonal Frequency Division Multiplex or OFDM is a modulation format that is finding increasing levels of use in today's radio communications scene. OFDM has been adopted in the Wi-Fi arena where the 802.11a standard uses it to provide data rates up to 54 Mbps in the 5 GHz ISM (Industrial, Scientific and Medical) band. In addition to this the recently ratified 802.11g standard has it in the 2.4 GHz ISM band. In addition to this, it is being used for WiMAX and is also the format of choice for the next generation cellular radio communications systems including 3G LTE and UMB.

If this was not enough it is also being used for digital terrestrial television transmissions as well as DAB digital radio. A new form of broadcasting called Digital Radio Mondiale for the long medium and short wave bands is being launched and this has also adopted COFDM. Then for the future it is being proposed as the modulation technique for fourth generation cell phone systems that are in their early stages of development and OFDM is also being used for many of the proposed mobile phone video systems.

OFDM, orthogonal frequency division multiplex is a rather different format for modulation to that used for more traditional forms of transmission. It utilises many carriers together to provide many advantages over simpler modulation formats.

What is OFDM? - The concept

An OFDM signal consists of a number of closely spaced modulated carriers. When modulation of any form - voice, data, etc. is applied to a carrier, then sidebands spread out either side. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data. As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them. This is not the case with OFDM. Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period.

Traditional view of receiving signals carrying modulation

To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of demodulators, translating each carrier down to DC. The resulting signal is integrated over the symbol period to regenerate the data from that carrier. The same demodulator also demodulates the other carriers. As the carrier spacing equal to the reciprocal of the symbol period means that they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution.

OFDM Spectrum

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission.

In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency. In some systems the peaks are limited. Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.

Data on OFDM

The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload. This reduces the data rate taken by each carrier. The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.

Guard Interval

The distribution of the data across a large number of carriers in the OFDM signal has some further advantages. Nulls caused by multi-path effects or interference on a given frequency only affect a small number of the carriers, the remaining ones being received correctly. By using error-coding techniques, which does mean adding further data to the transmitted signal, it enables many or all of the corrupted data to be reconstructed within the receiver. This can be done because the error correction code is transmitted in a different part of the signal.

OFDM variants

There are several other variants of OFDM for which the initials are seen in the technical literature. These follow the basic format for OFDM, but have additional attributes or variations:

• COFDM:   Coded Orthogonal frequency division multiplex. A form of OFDM where error correction coding is incorporated into the signal.
• Flash OFDM:   This is a variant of OFDM that was developed by Flarion and it is a fast hopped form of OFDM. It uses multiple tones and fast hopping to spread signals over a given spectrum band.
• OFDMA:   Orthogonal frequency division multiple access. A scheme used to provide a multiple access capability for applications such as cellular telecommunications when using OFDM technologies.
• VOFDM:   Vector OFDM. This form of OFDM uses the concept of MIMO technology. It is being developed by CISCO Systems. MIMO stands for Multiple Input Multiple output and it uses multiple antennas to transmit and receive the signals so that multi-path effects can be utilised to enhance the signal reception and improve the transmission speeds that can be supported.
• WOFDM:   Wideband OFDM. The concept of this form of OFDM is that it uses a degree of spacing between the channels that is large enough that any frequency errors between transmitter and receiver do not affect the performance. It is particularly applicable to Wi-Fi systems.

Each of these forms of OFDM utilise the same basic concept of using close spaced orthogonal carriers each carrying low data rate signals. During the demodulation phase the data is then combined to provide the complete signal.

OFDM and COFDM have gained a significant presence in the wireless market place. The combination of high data capacity, high spectral efficiency, and its resilience to interference as a result of multi-path effects means that it is ideal for the high data applications that are becoming a common factor in today's communications scene.

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Technocrats E Services

Jalandhar

GSM Core Network Architecture

GSM network is composed of several functional entities, whose functions and interfaces are specified. GSM core network consists of the several functional modules such as MSC, HLR, VLR, AUC, GMSC etc
These components are used for efficient switching and control of the calls. Previously the core network was supporting the voice calls and slow speed data calls but with the evolution of the technology now core network support high end voice calls and high speed data calls also.

GSM Core Network Components

MSC (Mobile Switching Center)

The central component of the network subsystem in the Mobile Switching Center(MSC). It acts like a normal switching node of the Public Switched Telephone Network (PSTN) or International Switched Data Network(ISDN), and additionally Provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers and call routing to the roaming subscriber. The MSC provides the connection to the fixed network (such as the PSTN or ISDN). Signaling between functional entities in the network subsystem uses Signaling System Number 7(SS7), used for trunk signaling in ISDN and widely used in current public networks.

The MSC performs the telephony switching functions of the system. It controls the calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channeling and others. The MSC co-ordinates the setting up of the calls to and from the GSM users. It is the telephone Switching office for MS originated or terminated traffic and provides the appropriate bearer services, teleservices and supplementary services.

It controls a number of BSS within a specified geographical coverage area and gives the radio subsystem access to the subscriber and equipment databases. Each MSC manages dozens of cell sites and their base stations. Large systems may have two or more MSCs. The MSC carries out the several different functions depending on its position in the network. When the MSC provides the interface between the PSTN and the BSS in the GSM network it is called the Gateway MSC.

Some important functions carried out by the MSC are Call Processing including control of data/voice call setup, inter BSS & inter MSC handovers, control of mobility management, Operation & maintenance support including databse management, traffic metering and man machine interface & managing the interface between GSM & PSTN network.

This is the main component of the NSS as the BSC co-ordinates with it. This component control the entire network. It interacts with the other databases and PSTN. The MSC set up and release the end to end connections, handles mobility and handover  requirements during the call and takes care of the charging and the real time pre-paid account monitoring. The responsibilities of this entity included checking if a customer has a valid account or not, what profile he is in, what kind of services he had been activated for etc. In short, it provides the needed user registration and authentication information.

HLR (Home Location Register)

The HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM network. The HLRs store details of every SIM card issued by an operator. Each SIM has a unique identity no Called an IMSI which is the most important key or the primary key to each HLR record.

The HLR is a database used for torage and management of subscriptions. The HLR is considered the most important database, asit stores permanent data about subscribers, including a subscriber's service profile, location information and activity status. When an individual buys a subscription from one of the operators, he or she is registered in the HLR of that operator.The HLR contains the master database of the all customers in the PLMN. This data is remotely accessed by the MSCs and VLRs in the network. A PLMN may contains more than one HLR, in which each HLR contains a portion of the total subscriber database. There is only one database record per subscriber. The subscriber data may be accessed by the IMSI or the MSISDN, which is basically nothing but the phone number of the subscriber which he/she uses. The MSISDN is the primary key to the HLR record. The HLR data is stord for as long as a subscriber remains with the operator. Other keys of this database apart from MSISDN and IMSI are: TMSI(temporary IMSI), IMEI( International Mobile Equipment Identity), Name and address of the subscriber, Current service subscription profile, Current location(MSC/VLR address), Authentication and encryption keys, Mobile country code(MCC) and MNC (Mobile Network code).

VLR (Visitor Location Register)

The VLR is a database that contains temporary information about subscriber that is needed by the MSC in order to service visiting subscribers. The data stored in the VLR has either been received from the HLR, or collected from the MS. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.The data includes most of the information stored at the HLR, as well as more precise location and status information. The additional data in VLR are:

Mobile status (busy/free/no answer etc)

Location Area Identity (LAI)

Temporary Mobile Subscription Identity (TMSI)

Mobile Station Roaming Number (MSRN)

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Technocrats E Services

Jalandhar

GSM Core Network Overview

The GSM Core Network consists of several functional nodes for controlling, switching, routing of the audio, video calls, sms and mms services in efficient ways, considering the speed, security, services simultaneously. GSM network broadly divided into three sub parts

BSS: Base Station Subsystem

This system takes care of all the radio related functions between a mobile and the network. It consists of the Base transceiver station(BTS, basically the tower) and a base station controller(BSC).

The BSS is the section of the traditional cellular telephone network which is responsible for handling traffic and signalling between a mobile phone and a Network switching subsystem. The BSS carries out the transcoding of the speech channels, allocation of the radio channels to the mobile phones, paging, quality management of the transmission and reception over the air interface and many other tasks related to the radio network. The BSS is composed of the two parts the BTS and BSC. These communicate across the standardized Abis interface. The Abis interface is generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM subchannels for traffic (TCH), LAPD protocol for the BTS supervicion and telecom signaling and carries synchronization from the BSC to the BTS and MS. The BSC and the Main Switching center communicate across the A interface. It is used for carrying the traffic channels and the BSSAP user part of the SS& stack. Although  there are usually transcoding units between BSC and MSC, the the signaling communication takes place between these two ending points and the transcoder unit does'nt touch the SS& information, only The voice or CS data are trascoded or rate adapted.

NSS: Network Switching Subsystem

This system takes care of the connection, authentication, encryption and supervision of a call. It consists of many elements/entities. the main one being a Mobile Station Controller which handles many BSCs. Other entities being a Home Location Register(HLR), Visitor Location Register(VLR), Authentication center(AUC), Equipment identity register(EIR), Short message services cente(SMSC), Multimedia messaging service center(MMSC) etc.

The first subsytem of the GSM network is the NSS. It carries out switching functions and manages the communications between mobile phones and the Public Switched Telephone Network. it is also responsible for the subscriber data handling, charging and controll of the calls. It is owned and deployed by mobile phone operators and allows mobile phones to communicate with each other and telephones in the wider telecommunication network. The architecture closely resembles the telephone exchange, but there are additional functions which are needed because the phone are not fixed in one location. The NSS also referred to as the GSM core network, usually refers to the circuit switched core network, used for traditional GSM services such as voice calls, SMS and circuit switched data calls. There is also an overlay architecture on the GSM core network to provide packet-switched data services and is known as the GPRS core network. This allows mobile phones to access to services such as WAP, MMS, and Internet access. All mobile phones manufactured today have both circuits and packet based services, so most operators have the GPRS network in addition to the standard GSM core network.

OSS: Operation Support Subsystem

This system responsibility includes operations and management of the whole system.

The Operation and Maintenance Center(OMC) is connected to all the equipment in the switching system and to the BSC. The implementation of the OMC is called the Operation and support system(OSS).

The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of the OSS is to offer the customer cost-effective support for centralized, regional and local operation and maintenance activities that are required for the GSM network. An important function of the OSS is to provide a network overview and support the maintenance activities of the different operation and maintenance organizations. Here are some of the OMC functions:

• Administration and commercial operation(subscription, end terminals, charging and statics).
• Security Management
• Network configuration, Operation and performance management.
• Maintenance tasks.

What is LTE?

LTE: LONG TERM EVOLUTION

Long Term Evolution (LTE) is a radio platform technology that will allow operators to achieve even higher peak throughput than HSPA+ in higher spectrum bandwidth. Work on LTE began at 3GPP in 2004, with an official LTE work item started in 2006 and a completed 3GPP Release 8 specification in March 2009. Initial deployments of LTE began in late 2009.

LTE is part of the GSM evolutionary path for mobile broadband, following EDGE, UMTS, HSPA (HSDPA and HSUPA combined) and HSPA Evolution (HSPA+).  Although HSPA and its evolution are strongly positioned to be the dominant mobile data technology for the next decade, the 3GPP family of standards must evolve toward the future. HSPA+ will provide the stepping-stone to LTE for many operators.

The overall objective for LTE is to provide an extremely high performance radio-access technology that offers full vehicular speed mobility and that can readily coexist with HSPA and earlier networks. Because of scalable bandwidth, operators will be able to easily migrate their networks and users from HSPA to LTE over time.

LTE assumes a full Internet Protocol (IP) network architecture and is designed to support voice in the packet domain. It incorporates top-of-the-line radio techniques to achieve performance levels beyond what will be practical with CDMA approaches, particularly in larger channel bandwidths. However, in the same way that 3G coexists with second generation (2G) systems in integrated networks, LTE systems will coexist with 3G and 2G systems. Multimode devices will function across LTE/3G or even LTE/3G/2G, depending on market circumstances.

Standards development for LTE continued with 3GPP Release 9 (Rel-9), which was functionally frozen in December 2009.  3GPP Rel-9 focuses on enhancements to HSPA+ and LTE while Rel-10 focuses on the next generation of LTE for the International Telecommunication Union’s (ITU) IMT-Advanced requirements and both were developed nearly simultaneously by 3GPP standards working groups. Several milestones have been achieved by vendors in recent years for both Rel-9 and Rel-10. Most significant was the final ratification by the ITU of LTE-Advanced (Rel-10) as IMT-Advanced in November 2010.

The first commercial LTE networks were launched by TeliaSonera in Norway and Sweden in December 2009; as of November 2012, there were 117 commercial LTE networks in various stages of commercial service. Many trials are underway with up to 130 LTE deployments expected in 2012.

LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink, which is well suited to achieve high peak data rates in high spectrum bandwidth. WCDMA radio technology is, essentially, as efficient as Orthogonal Frequency Division Multiplexing (OFDM) for delivering peak data rates of about 10 Mbps in 5 MHz of bandwidth. Achieving peak rates in the 100 Mbps range with wider radio channels, however, would result in highly complex terminals and is not practical with current technology. This is where OFDM provides a practical implementation advantage.

LTE Network Architecture

The high-level network architecture of LTE is comprised of following three main components:

·         The User Equipment (UE).

·         The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).

·         The Evolved Packet Core (EPC).

The evolved packet core communicates with packet data networks in the outside world such as the internet, private corporate networks or the IP multimedia subsystem. The interfaces between the different parts of the system are denoted Uu, S1 and SGi as shown below:

Facts about LTE

• LTE is the successor technology not only of UMTS but also of CDMA 2000.
• LTE is important because it will bring up to 50 times performance improvement and much better spectral efficiency to cellular networks.
• LTE introduced to get higher data rates, 300Mbps peak downlink and 75 Mbps peak uplink. In a 20MHz carrier, data rates beyond 300Mbps can be achieved under very good signal conditions.
•  LTE is an ideal technology to support high date rates for the services such as voice over IP (VOIP), streaming multimedia, videoconferencing or even a high-speed cellular modem.
•  LTE uses both Time Division Duplex (TDD) and Frequency Division Duplex (FDD) mode. In FDD uplink and downlink transmission used different frequency, while in TDD both uplink and downlink use the same carrier and are separated in Time.
• LTE supports flexible carrier bandwidths, from 1.4 MHz up to 20 MHz as well as both FDD and TDD. LTE designed with a scalable carrier bandwidth from 1.4 MHz up to 20 MHz which bandwidth is used depends on the frequency band and the amount of spectrum available with a network operator.
•  All LTE devices have to support (MIMO) Multiple Input Multiple Output transmissions, which allow the base station to transmit several data streams over the same carrier simultaneously.
•  All interfaces between network nodes in LTE are now IP based, including the backhaul connection to the radio base stations. This is great simplification compared to earlier technologies that were initially based on E1/T1, ATM and frame relay links, with most of them being narrowband and expensive.
•  Quality of Service (QoS) mechanism have been standardized on all interfaces to ensure that the requirement of voice calls for a constant delay and bandwidth, can still be met when capacity limits are reached.
•   Works with GSM/EDGE/UMTS systems utilizing existing 2G and 3G spectrum and new spectrum. Supports hand-over and roaming to existing mobile networks.

Advantages of LTE

• High throughput: High data rates can be achieved in both downlink as well as uplink. This causes high throughput.
•  Low latency: Time required to connect to the network is in range of a few hundred milliseconds and power saving states can now be entered and exited very quickly.
•  FDD and TDD in the same platform: Frequency Division Duplex (FDD) and Time Division Duplex (FDD), both schemes can be used on same platform.
• Superior end-user experience: Optimized signaling for connection establishment and other air interface and mobility management procedures have further improved the user experience. Reduced latency (to 10 ms) for better user experience.
• Seamless Connection: LTE will also support seamless connection to existing networks such as GSM, CDMA and WCDMA.
•  Plug and play: The user does not have to manually install drivers for the device. Instead system automatically recognizes the device, loads new drivers for the hardware if needed, and begins to work with the newly connected device.
• Simple architecture: Because of Simple architecture low operating expenditure (OPEX).

Bibliography

HCIG Reference Guide

4GAmericas

Ericsson LTE 

With Regards
Technocrats E Services
Jalandhar

Technocrats E Services, Jalandhar has organized a Seminar on HCIG course i.e. Huawei Certified ICT Graduate(powered by indovision) among various technical institutions in and around Jalandhar in the month of September 2013. The theme of this seminar was awareness to technical / engineering graduates about the above said  industrial course benefit. HCIG* consist of modules that benefits both Computer science and Electronics students in the same way. It's an open access to job market without categorizing any specificity of the particular skill.

The delegates representing Huawei conducted this seminar with the help of Technocrats E Services to shift focus from the mass students selecting the industrial courses either without proper consultation or without any knowledge of job market scenario. The main points of discussion were

• Benefits of HCIG 
• Cloud computing and futuristic scope
• Telecom industry working ethics
• Job / Market scenario  Vs available skillset

Students at various college, attending the seminar were engaged in discussion and the overall response was very positive. The fresh graduates seemed to be ready to make a change but the only thing missing was a complete course set that makes an individual a master. In this view they all appreciated Technocrats E Services for introducing them to HCIG course that completes every checklist of being Job Ready!!

We have conducted the seminar in CT Institute of Technoclogy, Jalandhar ; DAVIET, Jalandhar ; Shiv Shankar Institute of Technology, Amritsar;

We are thankful to the college management for allowing us for the seminar and to consult students about the skill set required to attain a better job in the market.

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Technocrats E Services

Jalandhar