XC002 Intro to Cellular Lesson Notes

XC002_Intro_to_Cellular_Lesson_Notes 26 December 2009

Page 1 of 14 Copyright Carl Chapman



Introduction

Many different technologies and protocols are used around the world to provide cellular coverage.

In this module we will be introducing you to:-

The Base Station

The Cellular System Layout

Cellular Tower Configurations

Cellular Technologies

Cellular System Protocol Allocations, and

An Overview of Cellular Data This module will take approximately 12 minutes to complete.



The Base Station

Cellular systems use equipment called base stations, BTS, or cell sites to provide coverage. They can be identified by the towers that are used.

The cell site is typically high power with 10 to 100 Watts of output power. With the high output power and high towers, the base station can have a range of up to 30 km in rural areas. The range of the base station is designed using the number of subscribers in the area. In urban areas with high subscriber densities, the base station may have a range as small as 500 metres.

The transmit signal from the base station is called the FORWARD or DOWNLINK.

Mobile and portable phones typically have a low output power of 0.5 Watts maximum, and being close to the ground, their range is significantly smaller. The transmit signal from the mobile is called the REVERSE or UPLINK.



Cell System Layout

The cellular system consists of a number of cell sites connected together in a cell like structure, and linked with a computer called a "switch", MSC or MSTO. To provide continuous service, the cell site coverage areas are designed to overlap.

A cellular operator will often have multiple linked switches that control the connected cell sites, log all billing information, and contain current records of all cellular subscribers and calls.

For a cellular to cellular call, the call is originated at the mobile. The signal is sent to the strongest cell site and passed to the switch. The switch then transfers the signal to the cell site with the receiving mobile.

The switch from the cellular company is also connected to the land line telephone company PSTN through a gateway. For a cellular to land line call, the signal is sent to the strongest cell site and passed to the switch. The switch then transfers the signal to the PSTN exchange and onto the land telephone. As the mobile moves from one cell to another, the control is passed to the relevant cell using a process called a "Hand-off".



Cellular Towers

The configuration of the cell site is dependent upon the equipment manufacturer and the design of the operator.

The most common configuration in the world is the 3 sector (or 120 degree) cell site. This can be visually identified by the triangle structure on the top of the tower.

The sectors are typically named in a clockwise manner A, B, C or X, Y, Z or alpha, beta, gamma. The A sector points north as an industry standard.

Each sector uses a different group of channels.

It is common for operators to configure 3 sector cell sites around freeways and main roads as 2 sector sites.

Another configuration is the 6 sector (or 60 degree) cell site. This can be visually identified by the hexagon structure on top of the tower.

The sectors are typically named in a clockwise manner A through to F, or U through to Z. The A sector points north as an industry standard, and each sector has a different group of channels. The third type of cell site configuration is the omni directional site, and is sometimes seen in the rural environment where there are only low subscriber levels. In the omni site, there is only one group of channels available for the whole cell site.



Technologies

There are three core communications technologies used for phones to communicate with the base station over the air. All current cellular protocols are derived from these core technologies.

Analog FM is the original cellular technology. Most analog systems are no longer operating, with their spectrum reused for newer digital systems, but they may still occasionally be found in some third world countries.

The protocols that used analog technology were AMPS or TIA553, NMT, TACS and NAMPS or IS88.

TDMA is a digital technology used in IS54, IS136, GSM, iDEN and DECT systems.

CDMA is a digital technology used in IS95, CDMA 2000, WCDMA or UMTS, and LTE or WiMax systems. The individual protocols derived from a core technology are not necessarily compatible e.g. a GSM phone will not work on an IS54 system.



Analog

All original cellular telephone systems used analog FM transmissions in the 800 to 900 MHz band.

The AMPS system was split to allow for two cellular operators (A and B) and originally only had 666 channels. As it became more popular, this was expanded to 832 channels and consisted of a group of 42 analog control channels (21 for system A, and 21 for system B), and a group of 790 analog voice channels (395 for system A, and 395 for system B). Most modern systems now use up to 1,024 channels.

Control channels are used to control call setup, and communicate with the mobile when it is not in use. The control channel signal transmits continuously so the mobile can always find it.

Voice channels are used to transmit a conversation that is in progress. With a limited number of channels available, the cellular operators use a system called frequency re-use, where the same channel number can be used in multiple base stations.



TDMA

TDMA or Time Division Multiple Access is a form of digital wireless technology, which expands the capacity of the available radio channels by dividing the older analog channel into separate time-based divisions or time slots.

Currently each channel supports between three and eight time-slots, and can therefore transmit up to eight conversations simultaneously.

The easiest way to understand TDMA is to think of a train.

At Station 1 there are three people with messages to be transported to three other people at Station 2.

The people at Station 1 place a message into each carriage. The train then goes to station 2, and the people there retrieve their message from the carriages. The train then returns to Station 1, and this process continues until the whole message has been transmitted. In technical terms each carriage represents a time slot.

The TDMA IS136 systems (also called DAMPS) are still found in some areas in North and South America, although many operators have changed to GSM. GSM is the most popular system globally, and has all digital channels using TDMA technology.



CDMA

CDMA or Code Division Multiple Access was originally designed for military spread spectrum communications.

The best way to understand CDMA is to think of a room full of people where many conversations are taking place at one time, but each conversation is in a different language. A listener who can understand the language is able to follow a particular conversation, but to someone who cannot understand the language it is just noise.

With CDMA, individual conversations are digitised and then encoded. Each telephone or data call is assigned a unique code, and as long as the receiving device has the correct code, it can pick its conversation out from the crowd.

Many CDMA systems have evolved from the original IS95 to CDMA 2000.

New technologies such as WCDMA or UMTS use a CDMA air interface, but are actually regarded as GSM systems, using GSM protocols to communicate with the network switch.

LTE or WiMAX also have a CDMA air interface, but utilize Internet Protocols for network communication rather than a radio channels based system. CDMA protocols are also used for some satellite systems such as GlobalStar, Sky Terra and TerreStar.



iDEN

iDEN or Integrated Dispatch Enhanced Networks are based on TDMA technology.

The proprietary iDEN system was developed by Nextel and Motorola.

It enables users to take full advantage of the capabilities of two-way dispatch trunk radio, commonly called push to talk, and full-duplex telephone calls in one phone.

This has been very popular with subscribers, as they can use the dispatch radio function to provide free phone to phone service.

The operation band is typically around 800 MHz, just below the cellular band, however there is a 1,500 MHz system in Japan. Dual mode iDEN phones have the capability to switch to CDMA for telephone calls, but stay in iDEN mode for push to talk calls.



Cellular Systems

Cellular systems around the world use a variety of technologies, protocols and frequencies.

Many of the technologies we have discussed can be used in multiple frequency bands. e.g. GSM can be used at 850, 900, 1800 and 1900 MHz. When talking about a particular cellular operators system, it is important to specify both the protocol and frequency band to avoid confusion, e.g. GSM 900, or TDMA1900.



Cellular Data

Cellular systems can also be used for data communication.

SMS or short message service is used by all cellular protocols, and consists of a text message of up to 160 characters.

The data protocol for analog systems was CDPD or Cellular Digital Packet Data.

GSM initially used General Packet Service GPRS, which was eventually upgraded to Enhanced Data rates for GSM Evolution (or EDGE).

CDMA has evolved with EVDO and 1xRTT updates for improved data performance. With new 3G and 4G technologies it is now possible to get high data rates giving broadband performance.



3G and 4G

Thanks to marketing spin from the various cellular operators, there is often a lot of confusion over 3G and 4G technologies.

Earlier operators used the term 3G to describe systems using CDMA2000 with EVDO data.

3G is also commonly used to refer to WCDMA systems using HSDPA (High Speed Downlink Packet Access) data.

Even though they are both called 3G, there is a significant difference in speed and capabilities of the two 3G technologies.

The current talk of the town is 4G. This will be the first cellular all digital system using Internet Protocol for addressing.

The two 4G systems are LTE (Long Term Evolution), and WiMax.

Depending on system implementation, both can provide similar performance.

Many operators still using CDMA2000 will upgrade to 4G quickly so they do not lose market share to the much faster WCDMA. Operators currently using HSDPA are expected to upgrade to 4G sometime in the next 5 years.



Summary

Congratulations on completing this module.

Many different technologies and protocols are used around the world to provide cellular coverage.

In this module we have discussed:-

The Base Station

The Cellular System Layout

Cellular Tower Configurations

Cellular Technologies

Cellular System Protocol Allocations, and An Overview of Cellular Data

Documents

XC002 Intro to Cellular Lesson Notes