Key elements for structured training

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Abstract

Describe where your training was carried out (company, department, location, etc.) Summarize your training goals, activities, and accomplishments. In addition, describe the key elements you took away from your training. These should be both technical and non-technical (for example, you might describe such things as how your organization was structured, how individuals worked together in your company, how technical employees interacted with customers, what company initiatives you observed, etc.)

INTRODUCTION

The Cyprus Telecommunications Authority (CYTA) is responsible for providing telecommunications to all of Cyprus; this includes providing fixed telephony, internet and mobile communications as well as cable television. These services are provided by separate companies which are all part of CYTA. My internship is based in Cytamobile-Vodafone (the mobile communications provider), specifically in the division of operation and maintenance of GSM and UMTS base stations. CYTA offers both GSM and UMTS services. GSM operates at 2 different frequencies which are 900 MHz and 1800 MHz, while the UMTS operates at about 2100 MHz.

The division of operation and maintenance is divided into 3 separate units in different areas, specifically in Nicosia, Larnaca and Limassol. I was placed with the unit in Limassol which is responsible for half the base stations in Cyprus, which includes almost 400 base stations in the Limassol and Paphos districts and most of the Trodos area. The job of this division is to ensure the operation of the base stations by maintaining and fixing any fault that occurs in any of the base stations.

The base stations are monitored remotely by the base station controllers unit, and every morning a report is made of all faults found in all the base stations, which is handed over to the operation and maintenance unit. The operation and maintenance unit head over to the base stations that have faults in them, taking with them the necessary cards or hardware that might need replacing according to the report made by the base station controllers unit. Then with each faulty part a fault report is written describing the part and the fault and the location. When the number of faulty parts reach a certain threshold they are sent to their main office in Nicosia, where they are shipped back to the company that manufactured them according to the agreement between them, and new parts are brought back replacing the faulty ones.

I was assigned in the operation and maintenance department in the Limassol division, my division had to do with the technical part of the mobile systems in Cyprus. I was also introduced to some other departments during my weeks in the internship.

TRAINING DESCRIPTION

As I was placed in the operation and maintenance department, my first task was to become acquainted with the structure of the GSM and the UMTS networks and the radio site equipment, which is mainly supplied in Cyprus by Ericsson for the GSM system and supplied by Siemens for the UMTS system. Besides the infield visits, where I got some instructions from my colleagues, I got the opportunity to work myself into the topic.

GSM is a cellular radio network, which means that mobile phones connect to it through cells found in their immediate vicinity. The network in Cyprus operates at 900 MHz or 1800 MHz bands. Each broadcasting band is divided into different frequency channels, with a distance between each channel of 200 kHz. Time division multiplexing is applied to create 8 full rate or 16 half rate speech channels per carrier frequency, also called the logical channels. With an 8 channel TDMA frame, a data rate of about 270 Kbit/s is reached. Each cell operates at a different frequency channel than that of its neighboring cells to avoid interference.

Ericsson's GSM network is divided into three major systems: the Switching System (SS), the Base Station System (BSS) and the Operation and Support System (OSS). The main part of the SS is the Mobile Services Switching Center (MSC), which performs the telephone switching functions of the system and controls the calls from its network or other telephone networks. So it is the physical interface between the core and the mobile net and is also responsible for the allocation of the mobile phone numbers. All radio-related functions are performed in the Base Station System. The BSS consists of the Base Station Controller (BSC) and the Radio Base Station (RBS). The BSC is a high capacity switch, which connects the MSC with several RBS's. Different cell configuration data are defined and controlled by the BSC. It handles the connection and the handover of calls to other cells and controls the radio frequency power levels in the different base stations. One BSC is able to handle up to about 85 base stations. The RBS includes all radio and transmission interface equipment which is needed on the radio site. One RBS can serve up to 3 cells. The Operation and Support System has been implemented by Ericsson in order to provide a centralized operation and maintenance for the cellular network. The operator is able to monitor and control the entire hardware as well as several functions of the network. Each technical fault in the network is detected by the OSS, so that the operator can support the operation and maintenance team with a detailed fault description.

Unlike the GSM network, the UMTS network in Cyprus uses a wideband spread-spectrum channel access method called W-CDMA, which operates with 2 carrier frequencies (at 2100 MHz) and supports, due to the asynchronous code division access, more mobile users at the same time. Furthermore it achieves higher data rates than the common GSM network, which reaches up to 21Mbit/s in theory. The data bursts of each mobile station are encoded with a unique spreading code. The information for each mobile station can be filtered out by the correlation of the interfered carrier signal and the right spreading code.

The W-CDMA Radio Access Network from Siemens consists of the Radio Network Controllers (RNC), the Radio Base Stations (RBS) and an Operation and Maintenance System, similar to that in the GSM network. The OSS is applied for handling operation and maintenance tasks in the W-CDMA network. The main accumulation of the RNC is to manage the user data transport and to optimize the radio network resources by controlling a number of RBS's. It is also the physical link to the core network and is connected to the RNC via an interface. The RBS includes all equipment for maintaining the radio link and connects the mobile station, for example a mobile phone or a laptop with the network.

As I got well acquainted to both radio systems used in Cyprus, I was taken with my colleagues on-site to different base stations that had some specific fault. Each day the department of operation and maintenance receives a report from the base station controllers specifying faults detected in different base stations. As the fault report was checked by the department, the critical faults were chosen first and split up between the different members in the department. According to the location of the faults some were given more than one base station. I was given the chance to go on-site with all the members throughout my training period and visited most of the base stations that the department was responsible for.

When the source of a fault was know, it included just a simple replacement of the faulty hardware card installed in the base station, which ranged from Transmission Receiver Unit (TRU) cards up to channel cards and the Combining and Distribution Unit (CDU) cards. Sometimes the source of the fault is not known, that is when an important tool called the Operation and Maintenance Terminal (OMT) is used.

The OMT is a software tool for installation, testing, site acceptance and diagnostics of the base station. A computer, connected to the Radio Base Station, enables the field engineer to read out the Installation Data Base file (IDB). The IDB includes all information about the applied hardware in the RBS. Furthermore all settings and adjustable parameters are saved and controlled by the IDB, and can be monitored by the OMT. Different kinds of views can be displayed by the software. The Cabinet View shows the hardware items in the RBS. The MO View shows the application objects and the service objects. These objects are also in the BSC's model of the RBS. The Hardware View shows the RBS's schematic structure. The hardware view is the most useful to detect a fault or a failure in the hardware of the whole RBS.

Some base stations we visited had unknown faults and thus we experienced the use of the OMT software, monitoring the values that passed through the different parts of the system. After debugging the system using this software, the fault is easily detectable, and the process returns to the simple replacement of the faulty card.

Sometimes the fault involves the cable and antenna system. The cable and antenna system plays an important role in the overall performance of the cell site. Small changes of the antenna system can affect the signal coverage area and eventually cause dropped calls. Furthermore, replacing a whole feeder cable and the antenna system is not as easy as replacing faulty cards of the RBS. The Wiltron is a portable frequency analyzer to measure and characterize the cable and antenna system. Several different measurement methods are used. The return loss and VSWR measurements are key measurements used in making cable and antenna measurements on the field. These measurements show the user the match of the system and if it conforms to system engineering specifications. The difference is that the VSWR displays the match of the system linearly and the return loss is specified in dB, with a maximum value of 60 dB as a perfect match. If the antenna reflects too much power the quality of the receiving signal decreases and the transceiver power amplifier can be destroyed. A return loss of a maximum of 15dB is a common limit for a cable and antenna system. Furthermore the Wiltron measurement system is equipped with a cable loss mode that displays the average cable loss of the swept frequency range. By placing a short on the end of the cable the measurement signal is reflected and the average loss can be computed. Return loss or VSWR measurements characterize the performance of the overall system. If one of them fails, the distance-to-failure (DTF) measurement can be used to check the system and to indicate the exact area of the fault. The DTF measurement sweeps in the frequency domain and using the Inverse Fast Fourier Transform (IFFT), the data can be converted from the frequency domain to the time domain. Together with the constant phase velocity, the distance to fault is calculated.

Another task done by the department of operation and maintenance is monitoring signal strengths at different locations. The company usually gets a phone call by some customer complaining about call drops or signal weakness in their area. The complaint is forwarded to the department of operation and maintenance, where then a team of usually two people go out to the specified area using a GPS system along with a mobile phone connected to a laptop. A software is used which monitors the signal through the cell phone, by attempting to make a call and checking the strength of the signal as well as monitoring the handovers occurring during a call, and when a call drop occurs. A report is then written regarding the problem at that area and is forwarded to the department of cell planning, where then they figure out a solution by either increasing the signal at a certain base station or even sometimes building a new base station.

If a cell phone is switched on but not in operation, it measures all radio frequencies of the GSM network and records the different signal strengths. Then the cell phone tunes the strongest signal, monitors the neighboring cells and transmits the collected information to the BSC, this is called the ideal mode. During a conversation, the mobile phone and the RBS perform signal strength measurements and send them also to the BSC. If a better signal is tunable, the BSC will accomplish a handover onto another cell, here the cell phone is in the active mode. To collect these data, the engineer needs a mobile measurement system. The system used is the TEMS equipment, consisting of a cell phone connected to a laptop and a special software tool. With the TEMS equipment the engineer is able to evaluate the signal strength of each neighboring base station. The software displays all information about the tuned and neighboring cells, namely the signal strength, the base station identification code, the used synchronization and the traffic channels. The software allows also the monitoring and control of handovers to other cells. For test runs to record the signal strength in an area, the TEMS equipment is combined with a GPS receiver, that way the field engineer is able to allocate the quality of the signal to the area on the map.

After spending more than half my training period in the operation and maintenance department, I began to visit the other departments in the company. I was given lectures by the different departments as they explained to me the main concept of their work and the basics of how it all works from their part of the company. One of the departments I visited was that responsible for the GPRS system.

GPRS is an applied update of the GSM network, to increase the data rate between the mobile station and the radio base station. The Base Station Subsystem requires a software upgrade of the transceiver unit as well as some other units. In addition, a new hardware unit called a Packet Control Unit (PCU) is added to the BSS to manage the packet data transfer between user devices and the GPRS core network. Furthermore, GPRS adds two new components to the network: the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). These nodes interact with the network subsystem and the internet. Also a special channel is reserved in each cell for the control data of the GPRS applications. This package data channel (PDCH) is mapped onto a single time slot and is utilized by the same physical channel structure as the other circuit-switched GSM channels.

The GSM radio technology is based on 200 kHz wide radio channels with a data throughput rate of 9.6 or 14.4 Kbps, depending on the coding scheme supported by the network and terminal device. To increase the data rate, GPRS ties the information into data packets and makes use of unused TDMA timeslots. In theory, up to 8 timeslots can be allocated to one data channel during packet transfer. The maximum theoretical data rate under perfect radio conditions is 171 Kbps. In practical application, the number of allocated channels is reduced by the capability of the mobile station and the number of users in the network. At the moment, mobile stations can use up to 4 channels for uplink and downlink, but a maximum of 5 timeslots at the same time. Depending on the quality of the radio transmission the applied channel coding sequence reduces the data traffic speed again. In this case, one time slot is statically assigned to GPRS. The additional timeslots are dynamically assigned by the BSC, depending on the circuit-switched traffic, which has a priority because it is more time critical than the data traffic of GPRS. Under good radio conditions, this yields speeds of approximately 50 Kbps downstream and 20 Kbps upstream. This is more than three times faster than current 14.4 Kbps GSM networks and roughly equivalent to a good landline analogue modem connection. With its faster data transfer rates, GPRS enables higher bandwidth applications like Multi Message Service (MMS), Push To Talk (PTT), and a faster Wireless Application Protocol (WAP). Moreover, the transfer of Short Message System (SMS) via GPRS is much faster.

The Mobile Stations are connected to the RBS by a virtual “always on” connection, which eliminates the lengthy delays required to reconnect to the network to send and receive data. These means the radio channels are used on demand, so that information can also be pushed to the end user in real time. GPRS allows providers to bill by the packet, rather than by the minute, thus enabling cost-effective “always on” subscriber services. In both cases, for GSM and GPRS, Gaussian Minimum Shift Keying Modulation (GMSK) is used. It is a continuous-phase frequency-shift keying modulation scheme. It is encoded with bits alternating between quaternary components, with the Q component delayed by half the symbol period, while each bit is modulated as a half sine wave. Depending on the previous symbol and the transmitting symbol, the signal is modulated continually which reduces problems caused by non-linear distortion. Furthermore the digital data stream is first shaped with a Gaussian filter before being applied to a frequency modulator. This reduces the bandwidth of the signal to decrease the sideband power and prevent out-of-band interference. GMSK has a high spectral efficiency, but it needs a higher power level than other common modulation schemes.

I then visited the department responsible for Mobile Switching Center (MSC), where I was given a lecture about the function of the MSC. The MSC is the main delivery node for GSM and is responsible for voice calls as well as other mobile services. The MSC is connected to many base station subsystems as well to other MSC's to handle handovers as mobile stations move from one MSC to another area connected through another MSC. The MSC also has a record through all the base stations it is connected to of what mobile stations are connected to it, through its Home Location Register (HLR). The HLR has information about all the SIM and mobile services ISDN number connected through its base stations. The MSC is also connected to the Visitor Location Register (VLR), which keeps data about the location of the other mobile stations.

ANALYSIS

Throughout the weeks I spent in my training at CYTA, I learned a lot in

Describe the usefulness of your training work in advancing your understanding of the engineering profession. What did you learn from this experience? Include both the technical and non-technical elements (for example, you might describe such things as how your organization was structured, how individuals worked together in your company, how technical employees interacted with customers, what company initiatives you observed, etc.)

Analyze your performance during the training. What are your primary strengths? Give examples of what you did well. What are the areas that you would like to improve? Give examples of what you would do differently. Ask your supervisor for input on this aspect of the report.

CONCLUSION

Summarize the usefulness of your summer training in advancing your understanding of the engineering profession, both technically and organizationally. Summarize what you learned from the experience. Discuss the benefits acquired from the training experience and the ways in which it enriched your knowledge. Any deficiencies in your education and suggestions for improvements of the training program should also be mentioned.

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