2G Mobile Network (PCS, and 2G+)

2G Mobile Network: Second generation mobile network. Refers to the second generation cellular phones that introduced digital technology and carried both voice and data conversations.

Examples of 2G mobile networks:

- CDMA
- TDMA
- GSM (accounts for over 80% of all subscribers around the world!)

2G services are more commonly referred to as Personal Communications Service (PCS in the United States). PCS operates at 1900Mhz. The FCC set aside the frequency between 1850 and 1990 MHz for mobile phone use in 1994 as the original cellular phone band (824 – 894 MHz) was becoming overcrowded. Dual band GSM phones are capable of working in both 850 and 1900 Mhz bands while tri-band or quad-band phones operate in 3 or 4 different frequency bands. While GSM is used throughout the world, CDMA and TDMA are used primarily in the Americas, with PCS being referred to as GSM-1900 outside of the US and in Hong Kong specifically as GSM-1800.

Sources: Newton’s Telecom Dictionary, 22nd edition; 2006 http://en.wikipedia.org/wiki/Personal_Communications_Service


2G+ Mobile Network: Second generation plus mobile network. Refers generically to a category of mobile wireless networks that support higher data rates than 2G mobile networks.

Example of 2G+ mobile network:

- GPRS

1G Mobile Network

First generation mobile network that refers to the initial mobile wireless networks using analog technology only and not carrying data. Advanced Mobile Phone Service (AMPS) is an example of a 1G mobile network standard. Although 1G and 2G both use digital signaling to connect radio towers to the rest of the telephone system, the voice itself is encoded to digital signals in 2G whereas 1G is only modulated to higher frequency (~150 Mhz).

Other 1G standards (other than AMPS) are NMT (Nordic Mobile Telephone) which is used in Nordic countries, Eastern Europe and Russia; TACS (Total Access Communications System) used in the United Kingdom and JTAGS used in Japan. Keep in mind that analog cellular service is quickly being phased out in most places worldwide.

Sources: Newton’s Telecom Dictionary 22nd edition, 2006; http://www.javvin.com/wireless/1G.html

Location Services (re-cap)

Location Services (re-cap):

Over the past few weeks, we have covered many different applications of location services. Since we’ve gone through a wide array of definitions, I thought it would be helpful to summarize what we’ve touched upon over this time. The first group was classified as what we call “network solutions” (think triangulation) while the second group was considered “handset solutions” (think GPS). To summarize these last words of the day, I have divided the terms into the 2 groups to help conceptualize the offerings of location services.

Network Solutions:

- Angle of Arrival (AOA)
- Uplink Time Difference of Arrival (U-TDOA)
- Wireless Location Signatures
- Location Pattern Matching (LPM)
- Multi-path Fingerprinting (MP)
- Enhanced Cell Identity (E-CID)

Handset Solutions:

- GPS
- Assisted GPS (A-GPS)
- Wireless Assisted GPS (WAGPS)
- TV-GPS
- Advanced Forward Link Trilateration (A-FLT)
- Timing Advance/Network Measurement Report (TA/NMR)
- Enhanced Observed Time Difference (E-OTD)

Enhanced Observed Time Difference (E-OTD)

Enhanced Observed Time Difference measures the differences in time that signals from the base stations take to reach both the handset and a fixed point in the network. This information is then sent from the handset and the fixed point to a mobile location center where a latitude and longitude are computed and sent to the PSAP. The main difference between GPS-location services and E-OTD is that GPS uses a constellation of satellites maintained by the U.S. department of defense, whereas E-OTD uses a mathematical algorithm to identify the location of the caller based on the time signal between base stations as well as a triangulation scheme. A major driver for E-OTD implementation in the U.S. was the E-911 mandate made by the FCC.

Image courtesy of Cambridge Position System

Source: http://www.mobileinfo.com/locationbasedservices/E_OTD.htm

Timing Advance/Network Measurement Report (TA/NMR)

This method relies in part on timing advance, which is the maximum amount of time that a TDMA mobile station uses to compensate for propagation delay in order to avoid user time slot overlap when the mobile is far away from the base station. It also employs information from the Network Management Report, which is the measurement done either at the handset or base stations to improve communication flow on the air interface. Various events – such as hand-off, power control, and candidates list – use the Network Management Report. Basically, the “timing advance” value is the length of time a signal from a mobile device takes to reach a particular base station. A TDMA frequency is shared between multiple users (employing TDMA) with each user transmitting periodically for less than 1/8 of the time within one of the eight timeslots. Since the users are various distances from the base station and radio waves travel at a finite speed (speed of light), the precise time at which the phone is allowed to transmit a burst of traffic within a timeslot must be adjust accordingly. Timing Advance (TA) is the variable controlling this adjustment.

Sources: Newton’s Telecom Dictionary 22nd edition 2006; http://en.wikipedia.org/wiki/Timing_advance

Advanced Forward Link Trilateration (A-FLT)

A-FLT is a handset-based position location technology (primarily used on CDMA networks) that works by using measurements, taken by the handset, of signals from nearby base stations, and reporting the time/distance readings back to the network, which are then used to triangulate an approximate location of the handset. In general, at least three surrounding base stations are required to obtain an optimal position fix. Unlike GPS (or A-GPS), AFLT does not use GPS satellites to determine location but rather a time difference of arrival (TDOA) technique. TDMA (Time Division Multiple Access) is required to transmit location data and is commonly paired with A-GPS to form a “hybrid solution”; an alternative to purely “handset-based” or “network-based” location technologies. (As AFLT is well-suited for urban environments with close proximity of base stations whereas A-GPS is more advantageous in suburban and rural areas having low base-station densities)

Wireless Assisted GPS (WAG or waGPS)

Generally uses advanced chipsets capable of acquiring very weak GPS signals and integrating the signals very quickly to determine location. WAG distributes GPS functionality between a 1) GPS reference receiver, 2) Location Server, 3) and GPS-enabled client device. The Reference Receiver provides GPS data and precise time to the Location Server. The Location Server computes and provides aiding data to the client, and performs navigation solution upon receipt of GPS measurements from client. The client pre-processes and returns basic GPS measurements along with statistical measures that characterize the signal environment.

Source: http://www.w3.org/

*Bonus WotD: TV-GPS uses synchronization signals from television stations to determine handset location when indoors. Because of the frequency and power of TV signals, they often can be received at indoor locations where GPS signal cannot.

GPS and A-GPS

Now that we’ve seen the network solutions for location services:

- Angle of Arrival (AOA)
- Uplink Time Difference of Arrival (U-TDOA)
- Wireless Location Signatures
- Location Pattern Matching (LPM)
- Multi-path Fingerprinting (MP)
- Enhanced Cell Identity (E-CID)

We can begin to focus on various handset solutions for location services.

GPS: GPS techniques use handsets equipped with GPS receivers. The GPS receiver determines the caller’s latitude and longitude which is sent to the provider’s receivers and relayed to the PSAP (Public Safety Access Point).

Assisted GPS: A-GPS uses techniques and advanced chipsets designed to allow reception of GPS signals indoors. Assisted GPS can be supplemented with an advanced forward link trilateration (A-FLT) system. A-FLT is a network-based location technology that takes measurements of signals from nearby base stations and reports time and distance readings back to the network, which uses them to triangulate an approximate location of the handset.

The benefits of A-GPS over standard GPS include but are not limited to:
· Faster location acquisition
· Less processing power is required by the device
· Saves battery life on your phone
· Location acquisition indoors or in non-optimal environmental settings


Source: http://www.wmexperts.com/articles/gps_vs_agps_a_quick_tutorial.html

*Note: In the picture below, you can see that the AGPS server actually offloads work from the phone/satellites so that most of the CPU and programming required for location identification is performed on the server itself.





Source: http://images.wmexperts.com/articleimages/2008/01/agps.jpg

Enhanced Cell Identity (E-CID)

Enhanced Cell Identity uses a combination of angular information (the cell sector receiving the signal) and timing information to approximate the location of the handset. Similar to Cell Identification, E-CID fixes the location of the user by identifying which cell in a network is carrying the user’s call and translates that information into latitude and longitude. Best used in less dense spaces, E-CID is little more accurate than Cell ID but has the capability of extending to less serviced rural areas.

Multi-path Fingerprinting (MP)

This technology uses features of the physical environment to locate mobile handsets. A wireless signal bounces off a variety of solid objects on the way to its destination (either a base station antenna or a handset), causing what’s called multipath interference. Essentially, the same signal is received multiple times due to the delay caused by bouncing off objects and taking longer paths to the destination. Multipath Fingerprinting takes advantage of this characteristic (which is normally a nuisance) to characterize signals that are received from certain locations. For instance, a base station can record what a handset signal looks like transmitted from a certain intersection of highways. A block away, the multipath signal “fingerprint” will look different, since the location of buildings, trees, and other obstructions has changed. To employ this system, an operator must send test units around to various locations so the base stations can record the fingerprints and create a database for comparison later on. Of course, if new construction occurs in an area the fingerprint will change and must be re-recorded.