Sunday, November 06, 2011

Signal Strength and Distance

Signal Strength and Distance

Reading an article on Susan Brenner's blog lay-testimony-on-cell-phone-radio-waves  about the case U.S. v. Kale, 2011 WL 4361531 (U.S. Court of Appeals for the 3d Circuit 2011) I read with interest the comments of the network operator's lay witness, in the article, recorded as having stated:

" Jeff Strohm, a custodian of records at Sprint Nextel Communications (Sprint), testified that Kale's cell phone used signals from a cell tower located in Pennsauken, New Jersey, and that `the biggest indicator’ of which tower has the strongest signal is `distance.’ "

" Jeff Strohm explained that a “cell phone is constantly searching for the strongest signal” and that the strongest signal is usually determined by `how far away you are from the cell phone tower.’ "

If one is speaking very, very generally to school children or novices etc where those people learning the information are hardly going to make a decision upon learning that info, may well be sufficient, but as influencing testimony for a criminal trial? Is that really enough to provide low-level of information that may be used to decide someone's innocence or guilt?  It is important to enter the caveat that other things may have been said by this witness or other material looked at, but this is the only info included in the article and, thus, by omission of any other info the discussion is based upon what is stated in the article.

Technically, when we deal with received signal strength (MS-BTS/BTS-MS) radio engineers - as opposed to back office call record staff - generally consider it to be a composite of three discrete effects:

- path loss
- slow fading
- fast fading

Those discrete effects take no account of and omit other fundamentally important data a mobile phone requires to have received, decoded and understood in order to 'camp on a cell', that is even before the mobile phone is receiving a communication or instructed to communicate.  

'Signal strength' and 'distance' are most certainly not sufficient for either GSM or spread-spectrum transmission technologies such as CDMA or W-CDMA to suggest are the main key factors to determine which Mast will handle a particular mobile phone's communications.

Within the realms of the GSM transmission technology it is made fundamentally clear as part of the mandatory requirements that a mobile phone shall detect 'signal strength', but what use is ranking signal strength in order of merit without understanding issues, such as:

- cell selection algorithm C1?
- cell reselection algorithm C2?
- if the mobile phone fails to decode parameters in 'control channels' e.g. RACH control parameters in the SYS_INFOS BCCH_INFOS 1–4. Is the Cell Barr Access bit = 1 (trace cf: 9D 00 00 & 9F 00 00) or the Access Control Class not equal 0?
- etc?
- And what about BTS capacity and directed retry?

When a mobile phone is engaged in a mobile call then the newtork instructs a mobile phone which Mast to use via control info through eg SDCCH.

Turning to issues associated with distance. If a mobile phone is located in-building, which can have affect on radio signals and latching to a particular Mast; where the landscape surrounding the mobile phone affects radio signals; or the clutter on the landscape affects the radio signals or a combination of any of the aforementioned, distance between MS and BTS gets blown out of the window as a reasonable suggestion in opposition to other matters that ought to be mentioned to a court of law.

As Sprint operates spectrum-spread transmission technology CDMA mobile services and iDEN transmission technology TDMA mobile services, I need to take some care here not to portray, too much, the wrong image of identical scenarios with GSM.

Wiki records Sprint's activities (Sprint_Nextel)
Sprint Nextel Corporation (NYSE: S) is an American telecommunications company based in Overland Park, Kansas. The company owns and operates Sprint, the third largest wireless telecommunications network in the United States, with 52 million customers, behind Verizon Wireless and AT&T Mobility. Sprint Nextel also owns a separate wireless division, Sprint Prepaid Group which offers prepay wireless services as Boost Mobile and Virgin Mobile USA.

Sprint is a global Internet carrier and makes up a portion of the Internet backbone. In the United States, the company is the third largest long distance provider and also owns a majority of Clearwire, which operates the largest wireless broadband network.

The company was renamed in 2005 with the purchase of Nextel Communications by Sprint Corporation. The company continues to operate using two separate wireless network technologies, CDMA and iDEN (for Nextel and some Boost Mobile subscribers). In 2006, the company spun off its local landline telephone business, naming it Embarq (which was subsequently acquired by CenturyTel). In 2009, Sprint reached an agreement to outsource management of its wireless networks to Ericsson.

Sprint Nextel launched its first WiMAX wireless card on December 21, 2008 (the Franklin Wireless u300 broadband card), and the first WiMAX phone available in the United States (the HTC Evo 4G) on June 4, 2010, utilizing its WiMAX technology from Clearwire Corp. A recent Consumer Reports survey tied Sprint with perennial front-runner Verizon Wireless in terms of customer satisfaction, a big improvement over previous years.

Thus, Sprint Nextel is a company with not only good quality indicators, but has outsourced management of its wireless networks to Ericsson, according to Wiki. The latter company (Ericsson) is known for its high technical competence, quality and originality in telecommunications and mobile communications, and adds further gravitas to my discussion that using 'signal strength' and 'distance' are insufficient to suggest that those elements should be used in isolation to other factors how a mobile phone may use a particular Mast. Ericsson, one could say, as an owner of patents, a developer of mobile networks/devices and transmission technology are the expert's expert. I am convinced, therefore, that companies like Sprint Nextel and Ericsson would hardly endorse in isolation to anything else the simplicity of using 'signal strength' and 'distance' as the indicators to inform a court of law about why a mobile phone would use a particular Mast.
In the next discussion I shall go further to open up and explore CDMA/TDMA parameters and protocols required for a mobile phone and Mast to connect for communications purposes.

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