Forensic Radio Survey Techniques for Cell Site Analysis

Joseph Hoy

• Publisher: Wiley
• Year: 2014
• Print ISBN:
  9781118925737
• Edition: 1
• eBook ISBN:
  9781118925751
• Topic: Technology & Engineering
• Subtopic: Mobile & Wireless Communications
• Index: Technology & Engineering

This book is intended to be used as both a text book and as an aide memoire handbook by forensic radio survey engineers, particularly those working for official police agencies. The book provides a simple but detailed overview of the operation of cellular networks (GSM, UMTS and LTE, US CDMAOne/CDMA2000, amongst others). In addition, the author also provides an overview of the technical theories that underpin cellular radio systems – basic radio theory and a simple explanation of the mathematical concepts that underlie measurements scales such as dB and dBm. The main part of the book, however, focuses on radio surveys, the various types of survey, the techniques employed for each survey and the considerations and potential problems that can be encountered when surveying different types of network. The final section deals with processing and interpreting the results of radio surveys and examines the information that can be gained from them.

1
Forensic Radio Surveys for Cell Site Analysis

1.1 Cell Site Analysis

Cell site analysis attempts to provide evidence of where a mobile phone may have been located when certain significant calls were made.

Mobile phone networks consist of a large number of radio ‘cells’, each of which covers a limited geographical area. Each cell is assigned a unique ‘Cell ID’, which is captured in the billing record (CDR or Call Detail Record) when calls are made.

Network operators are able, under tight regulatory guidelines, to provide details of the calls made by ‘target’ phones and can also provide details of the locations of the cells used by those phones.

Cell site analysis is designed to enable an investigator to determine whether calls made at or around the time of an incident or offence used cells that are located near the location of that offence.

1.2 Forensic Radio Surveying

Forensic radio surveys are designed to provide solid evidence to back up the assumptions made by investigators and cell site analysts.

Forensic radio survey equipment captures details of the cells that can be detected at a location and can indicate which cells would be selected for use by a phone being used at those locations.

Forensic radio survey results can be used to prove that particular cells provide coverage at significant locations and can therefore indicate whether it is possible for a phone using those cells to have been at or near those locations when particular calls were made.

The only totally definite conclusion that can be drawn from cell site analysis is that the use of a particular cell by a target phone means that the phone must have been within the serving coverage area of that cell at the time.

Forensic radio surveys can set approximate limits to the area within which the target phone must have been located. This type of evidence can be very useful when attempting to prove or disprove an alibi or other statement.

Overall, forensic radio surveys add empirical rigour to an area of investigation that would otherwise fall prey to assumptions and wishful thinking.

Cell site analysis, based on a combination of CDR, cell location details and forensic radio survey results, can provide compelling evidence to support the allegations made by investigators.

2
Radio Theory

Cellular networks use communications methods based on basic RF (Radio Frequency) transmission principles.

2.1 RF Propagation

2.1.1 Radio Theory

Radio signals are created when an alternating electrical current is applied to an antenna.

Any electrical current applied to a conductor generates a magnetic field around the conductor. This field extends for only a short distance.

As shown in Figure 2.1, if the electrical current through an antenna is made to alternate – that is, to change its direction of flow from forwards to backwards, which causes the electrical current to move through a cycle of positive and then negative values – the entangled electrical and magnetic (or ‘electromagnetic’) field generated around the antenna begins to extend far beyond the antenna and turns into a radio signal.

Figure 2.1 Alternating current

As the current travelling through the conductor alternates, the electromagnetic field generated around the antenna expands to match each peak positive value and then collapses back towards the antenna, it then expands again to match the peak negative value and then collapses, and so on for each cycle of alternations.

If the rate of alternation (i.e. the number of cycles of changing positive to negative values per second) is sufficiently fast, each instance of the electromagnetic field that is generated does not have time to fully collapse before the instance generated by the next cycle of alternation begins to expand.

A conceptual way of imagining the effect of these alternating cycles could be as follows: A change in the electric current sets up a disturbance in the magnetic field close to the conductor. In turn, that disturbance causes the electric and magnetic fields further out from the conductor to change. Continuation of this process leads to a ripple of electric and magnetic fields travelling away from the conductor, which take the form of an electromagnetic wave. The whole process is very like the formation of a water wave when a stone is dropped into a pond.

As the source signal continues to cycle, wave after wave of electromagnetic fields are pushed out from the antenna as a phenomenon that we term ‘radio waves’. This is demonstrated in Figure 2.2.

Figure 2.2 Generating a radio wave

Each alternation of the source electrical signal is termed a ‘cycle’ and the ‘frequency’ of a signal is calculated by counting the number of ‘cycles per second’.

One cycle per second is known as 1 hertz (after Heinrich Hertz, the scientist who first demonstrated the existence of electromagnetic waves in the late nineteenth century) and is abbreviated as 1 Hz.

1000...

Table of contents

• Cover
• Title page
• Copyright page
• Dedication page
• About the Author
• Preface
• Acknowledgements
• Glossary
• 1 Forensic Radio Surveys for Cell Site Analysis
• 2 Radio Theory
• 3 Wireless Technologies and Deployments
• 4 Cellular Theory
• 5 3GPP Network Types
• 6 Other Cellular Network Types
• 7 Forensic Radio Surveys
• 8 Cell Site Analysis
• 9 Summary and Practical Activities
• Index
• End User License Agreement