CSA - Site Survey Method3/Mobility Models

Albert Einstein’s work has influenced and still influences, even today, mobility in cellular networks that we take for granted when applying cell site analysis. Mobile telecommunications is very lucky that it has a fantastic collection of grand luminaries whose influential work from the world of science and mathematics that underpins mobile communications. To understand Einstein’s influence it is useful to firstly understand the mobile radio background which CSA examiners, technicians and student can use to improve the art of performing cell site analysis. It does, though, require keeping a mindset to remain active at all times and that mindset is CSA needs to keep focused on how the mobile network is arranged to react to how the mobile phone is being used whilst moving around. 

     

The goals of mobile network is to keep the mobile phone in touch with the network to maximum the network’s chances of providing services (e.g. revenue generation) to the subscriber customer. The network will do its very best to make coverage available when coping with outages (http://trewmte.blogspot.co.uk/2013/08/operational-audit-check-oac-is-part-of.html)  as it will when radio coverage degrades (http://trewmte.blogspot.co.uk/2008/11/mobile-phones-and-fringe-coverage.html) until it can no longer sustain a revenue service to it (the mobile phone).

Importantly, the goal for available service intensifies the denser the urban area in which the mobile phone is to be found. Having to propagate coverage into a hostile environment requires operators to strive to meet that goal by saturating a particular geographical landscape with more mobile network infrastructure base stations and nodes. 

The monitoring of the switched ON mobile phone that is in use is to ensure a base station/node is available, but that doesn’t just stop there though. Humans move around either as pedestrians on the pavement (sidewalk) or using some form of transportation. Therefore, the movement and the time (called dwell time) that a mobile phone remains in an area will have an influence upon the layer/s of mobile coverage that may be allocated for a particular call. You may remember the image below from a previous cell coverage discussion at my blog: http://trewmte.blogspot.co.uk/2009/08/cell-site-analysis-csa-images-part-2.html.

The above image is a useful guide to CSA examiners, technicians and students as it illustrates how a mobile network operator looks at designing coverage. The guidance it offers is that when conducting CSA merely conducting drive tests during site surveys is an entirely insufficient CSA method as a human walking does not do so at the speed of a human using transport. Whilst walking a person can stop and start dependent upon the intention or circumstances at that time. However, unless the transport is stuck in a traffic jam then we generally comprehend the transport to be moving more quickly/faster whilst on the road than the human on the pavement or crossing the road. 

There have been a huge range of studies that have generated mobility models for incorporation into propagation models. These mobility models are based upon complex study, testing and/or mathematical data endeavouring to predict (mimic) a moving target from which conclusions are generated and later produced for simulation software. The simulation can then be added into programs for designing the coverage (cell planning) for particular geographical locations. 

The image above is by Professor Sami Tabbane from his book detailing Planning Stages Of A Cellular Network: Radio Planning - propagation prediction tools.

Cell planning software tools amalgamate a wide range of models (or iterations of them) that the CSA examiner, technician and student maybe blinded to the compilations/combinations of mobility models that maybe incorporated into program. Identifying mobility models provides such needed knowledge, skill and experience that to ignore them might mean that a CSA examiner, technician and student conclusion/s about results obtained during or following site survey may be flawed. Below is a selection of mobility models that are well known:

- Brownian Mobility Model

- Random Waypoint Mobility Model

- Random Walk Mobility Model (including its many derivatives)

- Random Direction Mobility Model

- Random Gauss-Markov Model

- Gauss-Markov Mobility Model

- Markovian Mobility Model

- Incremental Mobility Model,

- Mobility vector model

- Reference Point Group Model (RPGM)

- Reference Point Group Mobility Model

- Pursue Mobility Model

- Nomadic Community Mobility Model

- Column Mobility Model

- Fluid Flow Model/Morales Mobility Model

- Exponential Correlated Random Model

- Exponential Correlated Random Mobility Model

- Map Based Model

- Manhattan Mobility Model

- Rush Hour (Human) Traffic Model

- Mission Critical Mobility Model

- Obstacle Mobility Model

- Smooth Random Mobility Model

- Post Disaster Mobility Model

- A Probabilistic Version of the Random Walk Mobility Model

- City Section Mobility Model

Albert Einstein

Einstein’s first described mathematically “The Random Walk Mobility Model” in 1926 [ http://users-phys.au.dk/fogedby/statphysII/notes/Einstein%27s-random-walk.pdf ] which later became adopted and used for mobile telecommunications mobility. The Random Walk Mobility Model, its elements are widely used to create simulations, is sometimes also referred to as “Brownian Motion”. 

The mathematical proposition states since many entities in nature move in extremely unpredictable ways, the Random Walk Mobility Model was developed to mimic this erratic movement. For this mobility model mobile node (MN), a common expression used for human (target) movement, moves from its current location to a new location by randomly choosing a direction and speed in which to travel. The new speed and direction are both chosen from pre-defined ranges, [speedmin; speedmax] and [0;2p] respectively. Each movement in the Random Walk Mobility Model occurs in either a constant time interval t or a constant distance travelled d, at the end of which a new direction and speed are calculated. If an MN which moves according to this model reaches a simulation boundary, it “bounces” off the simulation border with an angle determined by the incoming direction. The MN then continues along this new path.

In A Survey of Mobility Models for Ad Hoc Network Research the authors Camp, Boleng and Davies comment that many derivatives of the Random Walk Mobility Model have been developed including the 1-D, 2-D, 3-D, and d-D walks. In 1921, Polya proved that a random walk on a one or two-dimensional surface returns to the origin with complete certainty, i.e., a probability of 1.0. This characteristic ensures that the random walk represents a mobility model that tests the movements of entities around their starting points, without worry of the entities wandering away never to return. The 2-D Random Walk Mobility Model is of special interest, since the Earth’s surface is modelled using a 2-D representation.

So how might a CSA examiner, technician or student apply Einstein’s Random Walk Mobility Model? One key element to remember that this model is a memory-less mobility pattern because it retains no knowledge concerning its past locations and speed values. The current speed and direction of an MN is independent of its past speed and direction. This characteristic can generate unrealistic movements such as sudden stops and sharp turns which if undesired for simulation purposes can be addressed using e.g. Gauss-Markov Mobility Model. Because there is a memory-less occurrence of random movements it is the investigation into the mobile phone usage that requires analysis.  

Points to consider for that analysis can be those that I raised to the 2005 consultation by the Legal Services Commission regarding the Use of Experts in Public Funded Cases: 

In any assessment of the evidence the Defence expert at first instance seeks to correlate all the evidence to identify consistency or discrepancy regarding the data from the devices compared to the data obtained from mobile network operators and/or third parties. To assess that against the opinion of the Prosecution expert and/or the findings of the examiner at first instance. To then check the information against the Defence case. Eliminate the points agreed and to deal with those aspects concerning usage and the services obtained against the radio network in the geographical locations where the mobile telephone is alleged to have been. For instance, information not seen or ignored could result in an inaccurate opinion. By way of illustration, a Defendant is alleged to have been at a certain location where a murder occurred and cell site identity of the Mast used to make/receive mobile calls is presented as a justification for the Defendant being at the location.  Experience teaches one not to accept that as absolute, but to consider the radio coverage and how the Defendant might use there mobile phone is daily life. The key is 'daily life', referring to regular or irregular movements of the human being in a locale and the purposes of him/her being there. An experienced Defence expert should be looking for evidence that may assist where the Defendant may have been so as to assess that evidence against the allegation. That can require knowing for instance whether the Defendant visited a burger bar outlet or café. Knowing whether the defendant used a cash machine, purchased petrol or used their Nectar card and so on. Knowing the aforementioned information it is then required to conduct a site visit and conduct radio test measurements at those locations identified by the Prosecution and Defence, where these types of events took place. A Prosecution expert is unlikely to know aspects of the Defence case as the Defendant's Proof of Evidence comes after the case material has been served and the Defence's consideration of it. Proof of Evidence is never served to the Prosecution expert or to the examiner for that matter.

Another key element to be reminded for the site survey is if the specified time (or specified distance) an MN moves in the Random Walk Mobility Model is short, then the movement pattern is a random roaming pattern restricted to a small portion of the simulation area. Some simulation studies using this mobility model set the specified time to one clock tick or the specified distance to one step. This can require taking account of e.g.  Location; Time of Day/Night; Call duration [sCall;eCall]; Base stations/nodes used and so on.
 

Using the cell coverage layers image at the beginning of the discussion we can use a description of a dense urban area (e.g. a high street) and the target (mobile phone user) is a pedestrian on the pavement/sidewalk. In the vicinity, at below roof level, are three micrcocells (uBTS) providing lower layer coverage.

All things being equal the MS moving in a uniform linear direction may pass (handover) a call from BTS1 to BTS2. Bearing in mind we are discussing Random Walk Mobility Model the target whilst walking changes direction (turns left) which brings the MS into an area covered by BTS3. It maybe along the road covered by BTS3 there is a shop window the target stops to look in whilst chatting on the phone or decides to use a ATM cash machine to withdraw cash which is next to the shop. The call records show BTS1 and BTS 2 are used for the shop window browsing but BTS1 and BTS 3 have been used for the ATM cash machine usage. Why might this occur?  

   

Microcell coverage usage has parameters included to detect the speed of the MS/dwell time of the MS in an area. The key is slow MS movement in an area is handled differently by the network than a fast MS movement. In this regard the network may apply techniques for homogeneity of speed discrimination in lower layer and upper layer cells. The network does this because the MS speed is detected based upon the signals received by the network. Microcell coverage has problems with street corners and can create fast ping-pong effect in the network to handover between Microcells; hence why I have often stated previously to this discussion that Microcells don’t go around corners. 

A coping mechanism in the network is to use emergency handover (HO) to an upper layer of coverage that is more suited to handle traffic signalling for a period of time to decide the best handover candidate for the rest of an MS call.

The use case of BTS1 and BTS2 for the shop window scenario could be as a consequence that the shop was on the corner of the road and the period of target static time was short due to one to three steps (clock ticks) where as the BTS1 to BTS3 dwell time for the ATM cash machine scenario was much longer and requires handover via an upper layer cell down to BTS 3 to remove/reduce chances of fast ping pong on street corners and subsequent call drop.

Whilst the above discussion used Einstein’s Random Walk Mobility Model there are other Mobility Models that have been highlighted and each can be used to good effect to broaden CSA examiners, technician and student knowledge skills and experience.

CSA - Site Survey Method 2/ITU

The International Telecommunications Union (ITU) combines standards making capability and also has regulatory functions specific to mobile telecommunications. Therefore the ITU goes beyond standards making that may not create obligations in contrast with issuing regulatory measures that clearly do create regulation for it members who are signatories to the Convention.

 

 

Three important functions of the ITU are:

1) Regulation and Recommendations

Where there is an international aspect involved the ITU has the responsibility to manage the radio-frequency spectrum. The ITU allocates frequency bands to certain applications that would make use of the RF bands (see list below) e.g. Radio/Television Broadcast; Microwave Links; Radio-Astronomy; Mobile Telephony. The technical means and the physical nature of the frequency bands form the basis of the allocations. That is to say where a frequency band can be used and doesn't interfere with prescribed wide-ranging criteria; and the technical means exists or can be developed that enables the physical radio medium to be manipulated for use. Member states are bound to this allocation prepared by the ITU but assigning the frequencies to users is within the power and autonomy for each member state. ITU decisions are, in principle, binding to its members. The relevance behind that statement is that the ITU origins began to facilitate and enable subsequent amendments of the agreements to be agreed upon made at the Interntaional Telegrahy Convention of 1865. The principle of being bound only comes into effect when member states ( http://www.itu.int/online/mm/scripts/mm.list?_search=ITUstates&_languageid=1 ) ratify the text of an evolving Convention. Changes to any text in the Convention thereafter also need to be ratified. A member state ( http://www.itu.int/en/membership/Documents/signatories-pp10.pdf compare with http://www.itu.int/en/membership/Documents/signatories-pp06.pdf ) failing to ratify new text is not bound by it thus watering down the effects of any binding powers over national sovereignty.

Essentially, whilst understanding ITU's can make decisions when it comes to Band allocations we know that ITU does not hold regulatory functions when it comes to standards. When dealing with interntional bodies like ITU the term standard, as we commonly understand it, is more profound at the ITU's level. This is because international technical issues are being addressed. Were the standards to be made binding that could have political implications / ramifications regarding national sovereignty etc. However, a standard adopted by the ITU are called "recommendations". The recommendations carry a voluntary adoption by members states. The recommendations can though become directly or indirectly binding if it is incorporated into member states legislation where the legislation refers to a particular ITU recommendation. That would have a direct binding agreement. An indirect binding agreement could be where European legislation does not mention ITU recommendation per se but refers to CEPT or ETSI standards that become recorded that are in-turn derived from ITU recommendations. Were there to be an inextricable link requiring identical wording for CEPT/ESTI standard/ITU recommendation then that may amount to an indirect binding agreement with or to the ITU recommendation. It may be accepted that *CEPT/ETSI might be in the driving seat but isn't this nothing more than that old adage 'What is in a name? That which we call a rose by any other name would smell as sweet...(Romeo and Juliet)?'

* At an appropriate juncture in another discussion CEPT/ETSI will also be discussed.

2) Recommendations as Standards

ITU draws up standards (recommendations) and provides them to the telecommunications community that are relevant for telecommunications between countries.  There are numerous diverse tasks requiring standards under the umbrella and responsibility of the ITU that are prepared and developed by numerous advisory groups that are split into divisions.

As this discussion relates to CSA and identified recommendations listed here ( http://cellsiteanalysis.blogspot.co.uk/2014/07/csa-survey-method-2.html ) the detail below highlights the radio subject matter from the division ITU-R.

Radio:
http://www.itu.int/en/ITU-R/Pages/default.aspx
-- http://www.itu.int/pub/R-REG
--- http://www.itu.int/en/membership/Documents/WRC-12.pdf
--- http://www.itu.int/en/membership/Documents/signatories-wrc07.pdf
---- http://www.itu.int/pub/R-REC

Individual Recommendations for allocated bands
BO - Satellite delivery
BR - Recording for production, archival and play-out; film for television
BS - Broadcasting service (sound)
BT - Broadcasting service (television)
F - Fixed service
M - Mobile, radiodetermination, amateur and related satellite services
http://www.itu.int/rec/R-REC-M/e
P - Radiowave propagation
http://www.itu.int/rec/R-REC-P/e
RA - Radio astronomy
RS - Remote sensing systems
S - Fixed-satellite service
SA - Space applications and meteorology
SF - Frequency sharing and coordination between fixed-satellite and fixed service systems
SM - Spectrum management
SNG - Satellite news gathering
TF - Time signals and frequency standards emissions
V - Vocabulary and related subjects

Telecommunications
http://www.itu.int/en/ITU-T/Pages/default.aspx

Development
http://www.itu.int/en/ITU-D/Pages/default.aspx

Whether a member state has signed up to using the standards (recommendations) or not predominantly it is inescapable the technical information in the standards provides useful advice to countries and industry concerning: interconnection, access, terminal device standards, reference standards etc. Certainly in the areas of GSM, TDMA, CDMA, WCDMA/UMTS-UTRA etc many of those standards specific to these technologies constantly refer to ITU recommendations, thus further underpinning how useful ITU recmmendations are to use as references and guidance for cell site analysis.

3) Forums and Facilitors

Agreements between members require a commonality in understanding as to the reliability of international services available; thus technical services and commercial agreements need to be acceptable to both parties. ITU offers forums that help facilitate international agreements.


Final thoughts

The intention of this mini-overview about ITU recommendations was to demonstrate the value they offer given that they have weight due to the requirement of reliability to assist commonality and provide useful guidance for member states for that purpose. When dealing with CSA we are usually not involved at the member state level but at the operational performance of radio communications and services at the local level of which the ITU recommendations can and do provide useful reference material for reports and useful knowledge, skill and experience when conducting in the field surveys.

CSA - Site Survey Method 2

The purpose of these CSA - Site Survey Method discussions invites examiners, technicians and students to consider the wider area analysis involved in cell site analysis (CSA) beyond simply conducting radio test measurements at site and producing test results from particular Masts.

The wider area analysis enables examiners, technicians and students to suggest why coverage is being detected at particular locations. The coverage detected may not be LOS (line of sight) but due to NLOS (non line of sight). With these two radio scenarios there are a wide range of propagation models/components that are commonly referred to and used for mobile (cellular) communications.

Uniformity for CSA surveys is not impossible and has been established by industry and mobile radio network operators and radio architects/designers. It is simply due to lack of consensus in the forensic community that has largely stopped consensus. The latter state has arisen because of the way investigations have been subjected to intervening factors/limitation imposed, caused by constraints: financial constraints; knowledge, skill and experience constraints; timing constraints; combination of constraints. Can you imagine a DNA or Blood specialist giving evidence in court and stating the results I obtained are these but I have no clue as to why those results would be obtained at a particular juncture/point in the examination / survey and there is no consensus in industry as to what I should refer to as a criteria or norm.

CSA can be defined by five practical forensic headings:

1) Call-Billing Records/Cell Details/Operational Records/Network Records;
2) Radio Coverage and Mast-Tower BTS;
3) Radio Coverage and Mobile Station-Smartphone;
4) Radio Coverage and Geo-Clutter;
5) Radio Coverage and Scene of Crime.

It is agreed that each heading will have its own subset headings but for each of the main five headings it is possible to produce primers for each. It is also accepted that another reason for naming convention consensus yet to be achieved in the forensic community is due to the disparate range of definitions by industry. Mobile Forensics must develop a generic title and statement. This would not only assist the prosecution and defence to have firm ground upon which to question/cross examine a witness but aid the court to understand submissions. And this is only right and proper. With medical principals and practices it roughly takes 10-20 years to accept/understand each medical terms, yet we find after 30 years of cellular radio technology the courts and legal profession still struggle to get to grips with mobile cellular terminology and techniques. Thus using headings such as those above with appropriate descriptions attached to them should reduce or remove that problem. The following discussion illustrates established norms that are available that can be used consensually between expert, forensic and legal parties. 

The discussion in CSA - Site Survey Methods form subsets of 2), 3), 4) and 5). By way of illustration in the discussion here [ http://cellsiteanalysis.blogspot.co.uk/2014/06/csa-site-survey-method-1.html ] it refers to affects to radio coverage. The examiner, technician and student having obtained all the information at 1) above may wish to then consider 2) and 4) above.

                                      

The above photo of a Mast-BTS/NodeB/eNodeB shows an example of identified radio transmssion technology that the examiner, technician and student should identify at first instance to comprehend (a) communications transmissions available and (b) the services to be obtained from this type of multi-basestation.

The examiner, technician and student are ultimately considering scenarios into which radio-coverage will be propagated. These will not just simply be the standard radio coverage frequencies allocated to GSM/W-CDMA/CDMA/LTE etc but also microwave links for backhaul of the sites traffic where no landline backhaul is possible.

It follows therefore background knowledge about propagation, models and survey profiles etc can assist how an examiner, technican and student may plan on-site surveys and radio test measurements or those components that might be involved in the results detected. Some examples are:

Walfish-Ikegami
Basic algorithm: COST 231 Model (ETR 364, COST 231 Final Report)
Type: Point-to-area (multipoint)
Frequency: about 800 MHz - 2 GHz
Distance: up to 5 km
  
SUI
Basic algorithm: IEEE 802.16
Type: Point-to-area (multipoint)
Frequency: about 2 GHz - 5 GHz
Distance: up to 70 km

Hata
Type: Point-to-point
Frequency: 150MHz - 1500 MHz
Distance: 1Km to 20 MHz
Allows for correction factor where mobile is different from baseline height 1.5m

Okumura-Hata
Type: Point-to-multipoint
Frequency: ~ 150 MHz - 2 GHz
Distance: up to 100 km

Line of Sight
Basic algorithm: ITU-R P.452-14
Type: Point-to-point and Point-to-multipoint
Frequency: about 700 MHz - 40 GHz
Distance: up to 100 - 150 km

Additional:
ITU-R P.1411 provides guidance on outdoor propagation for systems that operate under distances 1 km, and over the frequency range 300 MHz to 100 GHz

ITU-R P.1546 provides guidance on outdoor propagation for systems that operate over distances of 1 km and greater, and over the frequency range 30 MHz to 3 GHz

Effective Antenna Height:
Absolute
Profile
Average
Relative
Slope

Analysing point to point/area components:

Examiners, technicians and students should note I have used in these examples the ITU (international telecommunications union) recommendations adopted around the world by its members. This means whether the CSA is located in Europe, Middle East, Asia, Africa, North America etc these are useful recommendations to consider and refer to in final reports. Put another way there should be nothing in a final report that should have be made-up (false statement) by the report's author.

Free space path loss: ITU-R P.525-2
Fresnel zone ellipsoids: ITU-R P.526-11
Path clearance: ITU-R P.530-13

Reflection, Diffraction, Scattering and Attenuation:
Specific attenuation: ITU-R P.676-8
Preciptation attenuation: ITU-R P.837-5
Specific Rain attentuation: ITU-R P.838-3
Rain Height Model ITU-R P.839-3
Hydrometeors attenuation: ITU-R P.530-13
Fog attenuation: ITU-R P.840-5
Single knife-edge: ITU-R P.526-11
Deygout: ITU-R P.526-11
Average: ITU-R P.530-15
Spherical Earth: ITU-R P.526-11
Reflection: ITU-R-REC-P.527-3
Multipath: ITU-R-REC-P.1407-5
Scattering due to RET: ITU-R P.833-5
Vegetation: ITU-R-REC-P.833-8
Polar and Desert Dry Temperatures: ITU-R-REC-P.841-4
Buildings: ITU-R-REC-P.1812-3
Building Materials and Structures: ITU-R-REC-P.2040-0 

There are of course other regional specific cellular transmission technology standards for North American, Europe etc e.g. ETR 364: Digital cellular telecommunications system; Radio network planning aspects. These shall be referred to in another discussion.

PLEASE NOTE: The page will be updated with other ITU recommendations from time to time.