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- 1. Advanced Mobile Phone Service (AI-AMPS)
- 12. AMPS Network Mobile Station Mobile Station 1 2 4 5 7 8 * 0 3 6 9 # Cell Site Cell Site Voice Trunks Data & Control Cell Site Controller Cell Site Controller Radio Radio Radio Radio Radio Radio Mobile Switching Center To Land Line Network Network Databases (HLR, VLR)
- 14. AMPS Network Interfaces PSTN RF Test Equipment 1 2 4 5 7 8 * 0 3 6 9 # 1 2 4 5 7 8 * 0 3 6 9 # 1 2 4 5 7 8 * 0 3 6 9 # CSC CSC CSC MSC MSC MSC Air Interface (U m interface) Not a Standard Interface Base Station (Cell Site) to MSC (A interface) Radio Radio Radio MSC to PSTN (A i interface) MS MS MS Base Station (Cell Site) MSC to HLR (C interface), MSC to VLR (B interface)
- 15. AMPS Air Interface Channels
- 16. AMPS Air Interface Channel Assignment Forward Control Path Reverse Control Path Reverse Voice Path Forward Voice Path Control Channel Voice Channel Mobile Station Cell Site
- 26. Supervisory Audio Tone Signal Call in Progress: Subscriber 1 connected to Base Station 1 on Channel 432, SAT 0 Everything’s OK here! Subscriber 2 initiates call: Base Station 2 responds with channel 432, SAT 1 Normal call setup for Subscriber 2 Co-channel interference: Subscriber 2 receives transmission on channel 432 from Base Station 1, but receives SAT 0 instead of SAT 1 Fade timer starts, after 5 seconds Subscriber 2 call is dropped! Base Station 2 Base Station 1 Subscriber 1 Subscriber 2 3 2 1
- 28. Dual Tone Multi-Frequency (DTMF) Signal Mobile Station Cell Site Mobile Switching Center End-User Device (Answering Machine, etc.) DTMF signaling from mobile station to end-user device PSTN
- 34. AMPS Frequency Reuse F B B A A C D E G
- 35. AMPS Omni Cell Cell Omnidirectional Antenna
- 36. AMPS Sectored Cell Sector Sectored Antenna Cell A1 A2 A3 B1 B2 B3 C1 C3 C2
- 40. Summary
- 41. AMPS Compared to Other Wireless Technologies
Editor's Notes
- © Copyright 2001 Global Wireless Education Consortium All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws. Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners.
- Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039. GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
- After completing this module and all of its activities, you will be able to: Explain how an AMPS network and the AMPS radio interface work. Describe the structure of an AMPS cell and channel. Differentiate AMPS from other standards in the telecommunications industry: N-AMPS, CDMA, TDMA, and GSM. Explain the strengths and weaknesses of an AMPS relative to other wireless technologies.
- The Advanced Mobile Phone Service (AMPS) system was proposed by AT&T in 1958 and technical feasibility was demonstrated in 1971. A network/marketing trial was approved by the Federal Communications Commission (FCC) and conducted between AT&T and Illinois Bell in 1979. The first commercial service was launched in 1983. More recent standards have defined dual-mode air interface protocols that allow interoperability of AMPS with digital wireless technologies (TIA/EIA IS-136 TDMA and TIA/EIA IS-95 CDMA).
- Page Student Notes Global Wireless Education Consortium The diagram above focuses on the 800 MHz spectrum. Because the first cellular phones operated in the analog mode, rules governing frequency allocation were designed around analog. AMPS (Advanced Mobile Phone System) is the set of rules that was developed to cover North American Cellular. ( For additional information, refer to Essentials of Wireless Communications by W.C.Y. Lee, chapter 1, or Wireless Communications by Theodore S. Rappaport, chapter 10.) These AMPS rules stated the following. Each channel must have a pair of frequencies, one for the uplink and one for the downlink. The uplink frequency (also called the reverse channel or reverse frequency ) is the frequency that is transmitted by the mobile and received by the cell. The downlink frequency (also called the forward channel or forward frequency ) is the frequency that is transmitted by the cell and received by the mobile. Using the two frequencies allows true duplex operation, that is, it accurately emulates a landline phone conversation where both parties can talk at the same time. Each channel must have a bandwidth of 30 KHz. The FCC ruled that no one company could have a monopoly (like landline companies) in a given area, so they divided the allocated frequencies into two bands: A Band – Originally reserved for companies that were solely in the cellular business. B Band – Originally reserved for existing landline companies to let them into the cellular business. NOTE: These concepts will be further discussed throughout the module.
- AMPS is a fully automated mobile telephone service that uses the 800 MHz to 900 MHz frequency band and has a 30 kHz bandwidth for each channel. It was the first standardized cellular service in the world, and is currently the most widely used standard for cellular communications. It is particularly popular in the United States, South America, China, and Australia. Designed for use in urban areas, AMPS later expanded to rural areas. Early commercial mobile telephone services relied on high-power transmitters and high towers at the cell sites, covering a large area in each cell. With relatively few frequencies available, only a few subscribers in that large area could talk at any one time. AMPS maximizes the cellular concept of frequency reuse by reducing radio power output to allow use of smaller cells to cover a geographic area without interfering with nearby cells using the same frequency. This technique allows more subscribers to be served simultaneously in a given area. As a subscriber moves from cell to cell while a call is in progress, a process called handoff occurs. Handoff allows the call to remain connected even though the frequency used to transmit and receive may change at the cell boundaries. AMPS mobile stations have the familiar telephone-style user interface and are compatible with any AMPS base station. This makes mobility between service providers (i.e., “ roaming ”) easier for subscribers since they can continue to use the same mobile station, even though a service provider’s AMPS base station equipment may be from a different manufacturer than that used by the subscriber’s home network. NOTE: The terms “base station” and “cell site” are used interchangeably in this module to refer to the radio and cell site controller equipment. Occasionally, the term “cell site” may also be used to refer to the particular geographic location of this equipment.
- AMPS uses frequency modulation (FM) to overlay voice and signaling information on a radio frequency carrier wave for transmission. Multiple conversations at each base station are supported by assigning a separate radio frequency to each user. This method is known as frequency division multiple access (FDMA) . In the United States, in order to support full duplex service (that is, allowing both participants in a conversation to speak simultaneously) AMPS transmission from the mobile station to cell sites uses a separate frequency from that used for the transmission from the cell site to the mobile station. Subsequent slides will refer to full duplex AMPS transmission.
- The AMPS network consists of six major components: Mobile station (MS) - Handset or other device used by subscriber to originate and receive calls. Radio-to-radio transmitter/receiver ( transceiver ), including power amplifiers, combiners, etc. Cell site controller (CSC) - Provides channel allocation and local switching so that mobile stations can receive and make calls. Mobile switching center (MSC) - Provides data and control functions, and access to voice trunks by the MS. Network database - For example, home location register (HLR) , visitor location register (VLR). Backhaul to land line network (the public switched telephone network (PSTN) ). The MS communicates over the air interface with the radio transceiver. The CSC provides channel allocation and local switching so that MS can receive and make calls. The radios and the CSC together form the cell site (also referred to as a base station ). Data functions at the MSC allow the MSC to communicate with databases in the network that record a subscriber’s home network, feature information (the HLR), and a subscriber’s current location (the VLR). Control functions include the MSC’s ability to notify a mobile station to change its transmission power level or to tune to a different frequency when a call is handed off from one cell site to another as well as allocation of voice trunks used by a subscriber in originating or receiving a call. All calls going between an AMPS subscriber and a land line network (which account for the greatest percentage of wireless calls) are carried from the MSC to the the PSTN via voice trunks. An AMPS network will always consist of multiple cell sites and will usually have a single, centralized HLR. Depending on the network’s geographic size and the total traffic carried, a single, centralized MSC or multiple MSCs will be required. The VLR function is generally collocated with each MSC.
- Various interfaces connect components of an AMPS network. In North America, the primary body for telecommunications standards is the American National Standards Institute (ANSI) , which accredits other standards bodies such as the Telecommunications Industry Association (TIA) and the Electronic Industries Association (EIA) . The AMPS air interface, which defines the characteristics that make AMPS a distinct radio technology from TDMA or CDMA, is defined by a standard known as ANSI/TIA/EIA 553 . Interactions between MSCs and databases such as HLRs and VLRs, as well as between MSCs and other MSCs, are defined by a standard known as TIA/EIA IS-41. (The current version is the IS-41 revision C). The IS-41 standard is used by TDMA, CDMA, and AMPS networks. This makes it possible for subscribers to roam between AMPS networks and TDMA or CDMA networks, provided that the subscriber’s mobile station supports multiple air interfaces and appropriate agreements have been reached between the network providers. GSM networks, on the other hand, use a signaling protocol known as GSM MAP (instead of IS-41), which is incompatible with roaming to and from an AMPS network.
- Interfaces shown in the diagram above are identified by standard letter designations used in IS-41 and in related standards documents (e. g., the C interface refers to the interface between the MSC and the HLR). The air interface (designated in the IS-41 reference model as the U m interface ) is the interface between mobile stations and the base station radios. It is based on a standard, allows all mobile stations to communicate with systems developed by different manufacturers, and is specified for AMPS by the TIA/EIA IS-553 standard. The interface between the radio and CSC is not standardized. Its implementation is proprietary and specified by individual manufacturers. The standard protocols for the interface between the BS and the MSC (the A interface ) for IS-41 compatible radio technologies have been defined for an SS7 or frame relay interface in TIA/EIA IS-634 and for an ISDN interface in TIA/EIA IS-653. Standardization of this interface allows a network provider to purchase radio equipment and the MSC from different manufacturers, at least in theory. (Existing AMPS networks may contain equipment developed prior to the standardization of the A interface.) The interface between an MSC and the PSTN ( A i ) in a network using IS-41 protocols has been defined in TIA/EIA IS-93.
- To support full-duplex transmission, AMPS assigns all channels (also known as channel sets ) between the mobile station and cell site as pairs of frequencies (one to transmit, one to receive), each 30 KHz in bandwidth, for a total of 60 KHz for each mobile station actively involved in a call. In an AMPS network, channels are numbered according to their order in a particular cell and their overall order in the frequency plan of a service provider. The total number of channels available to a service provider depends on the amount of spectrum for which that service provider is licensed. In the United States, cellular frequencies (the frequencies in which AMPS is deployed) cover a total of approximately 50 MHz, from about 824 MHz to 849 MHz and from about 869 MHz to 894 MHz. This results in a total of 832 full-duplex AMPS channels (1,664 frequencies using 30 kHz of bandwidth each). Two service providers are licensed in each area, giving each approximately 25 MHz of spectrum, or 416 AMPS channels. In AMPS, 21 channels per service provider are typically reserved for control channels . The remaining channels are referred to as voice channels and are used to support voice or data transmission and the associated signaling. For both control channels and voice channels, the transmission path from the cell site to the mobile station is called the forward path (or forward channel , which is used as a verbal shorthand for “forward control channel”, instead of “forward path on the control channel”) or downlink . The transmission path from the mobile station to the cell site is called the reverse path (or reverse channel ) or uplink . Cell sites transmit in the frequency range from 869 MHz to 894 MHz and mobile stations transmit in the frequency range from 824 MHz to 849 MHz, so that separation of frequencies on the downlink and uplink in a given frequency pair is always 45 MHz. An AMPS mobile station can transmit and receive only on one pair of frequencies at a time. The control channel performs call setup and remains active until a voice channel is assigned. After the mobile station is assigned a voice channel, all subsequent messages, tones and conversation are transmitted on the voice channel. The following slides describe these functions in more detail.
- There is only one AMPS control channel allocated per cell, but that control channel supports a number of functions. The main functions of the AMPS control channel include the following: Registration - The set of messages that allows a mobile station to notify the AMPS network of its current location. Paging - The process by which a particular subscriber’s mobile station is notified of an incoming call. Call setup - The process by which a call originating or terminating at a subscriber’s mobile station is connected to the AMPS network and the PSTN. While the term channel refers to a pair of frequencies used to transmit and receive signals in AMPS, specific terms used in discussing control channel functions also incorporate the word “channel” to refer to a continuous stream of data being transmitted in each direction on the AMPS control channel. By populating the various fields in this data stream, the base station and mobile station can transmit all of the information required for registration, paging, and call setup. The two data streams involved are referred to as: Forward control channel (FOCC) - Data stream from the base station to the mobile station. Reverse control channel (RECC) - Data stream from the mobile station to the base station. The FOCC and the RECC are discussed in detail in the following slides. All transmission in AMPS (on control channels and voice channels) is analog. However, signaling messages sent on the control channel (and later on the voice channel) contain information that must be expressed in binary format, that is, as a series of 1’s and 0’s. The method used to translate these 1’s and 0’s into a format that can be transmitted on the analog channels is a form of frequency modulation (FM) known as frequency shift keying (FSK) .
- When mobile stations are not involved in an actual conversation, they monitor the FOCC data stream being transmitted by the base station. All information transmitted from the base station to the mobile station necessary for registration, paging, and call setup is sent by setting the various parameters in the FOCC messages. The data stream consists of continuous repetition of the FOCC messages, which include: Mobile station control message - May contain information indicating an incoming call for a particular mobile station or a voice channel assignment for call setup. Overhead message - Provides identifying information concerning the base station and can indicate whether the base station has digital capability. Control filler message - Used to fill unequal message blocks (for example, if stream A and stream B are of different lengths). The FOCC data stream consists of three discrete information streams, time-multiplexed together to form a message word. Messages to mobile stations with the least significant bit of their mobile identification number equal to “0” are sent on stream A (containing the mobile station control message). Messages to mobile stations with least-significant bit of their mobile identification number equal to “1” are sent on stream B (containing the mobile station control message). The busy-idle stream contains busy-idle bits interspersed with other messages, which are used to indicate whether or not the reverse control channel is busy receiving information from a mobile station. The bit must indicate the idle state (= 1) before a mobile station will try to transmit a message to the cell site over the reverse control channel.
- In each message, the 28 left-most bits of the 40-bit field are the content bits . The first two content bits identify the particular type of message being sent. Each repeat of the FOCC messages starts with a 10-digit sequence (known as a dotting sequence ) with alternating “1”s and “0”s (1010101010), followed by an 11-bit word synchronization (synch) sequence . This is followed by five repeats each of the two data streams. Mobile stations use the dotting sequence and the word synch sequence to get synchronized with the overall data stream. The mobile station is then able to decode the information contained in the data streams.
- The reverse control channel ( RECC) messag e consists of up to five different words, each repeated five times. The RECC contains fields for identifying information for the mobile station (used for registration and other purposes), called number information (for call origination), and other fields. Messages sent on the RECC are coordinated among the competing mobile stations by using the busy-idle bits from the FOCC data stream to indicate when the reverse control channel is idle. A 48-bit word contains 36 content bits in the left-most field. All messages begin with a 30-bit dotting sequence, an 11-bit sync sequence, and the coded digital color code (DCC) . The DCC is an identification tag on control channels. It is transmitted to the mobile station by the base station as one of the parameters in the FOCC data stream, and is duplicated in messages sent by the mobile station back to the base station in the RECC data stream. Using DCC ensures that the control transmission path is reliable and that the mobile station is communicating with the correct base station. The base station will recognize the correct mobile station if the returned DCC is identical to the transmitted value. All information transmitted from the mobile station to the base station necessary for registration, paging, and call setup is sent by setting the various parameters in the RECC message. The following slides discuss applications of the FOCC and RECC data streams.
- The AMPS mobile station automatically begins the registration process when it is powered on. Rather than having to scan all of the frequencies used for base station transmissions in a particular AMPS network, the mobile station can identify the 21 frequency pairs used for control channels in its own network and then it needs to scan only those 21 downlink frequencies. The mobile station selects the FOCC with the strongest signal (usually the FOCC from the nearest base station). Using the identifying base station information in the FOCC data stream, the mobile station responds to that base station by sending the mobile station identifying information in the RECC data stream. The MSC authenticates the mobile station’s information using the AMPS databases, and then registers the mobile station’s current location. After registration takes place, the subscriber may originate a call from the mobile station, or may simply leave the mobile station powered on in anticipation of incoming calls. Moving out of range of one base station and into range of another causes a mobile station to re-register. Similarly, after a pre-set period of time, a mobile station with the power turned on will re-register. While the mobile station is powered on and registered, it continuously scans the FOCC data stream on the FOCC. The preceding slides discussed the format of the FOCC data stream (data transmitted continuously from the base station to the mobile station) and the RECC data stream (a continuous data stream shared by all mobiles monitoring the FOCC data stream of a particular base station). The following slides discuss in more detail, specific applications of messages sent from the base station to the mobile station using the FOCC data stream and messages sent from the mobile station to the base station in the RECC data stream.
- Information sent from the base station to mobile stations using the FOCC data stream consists of two types: Messages intended for all mobile stations Information sent to mobile stations includes information about the system and how the mobile stations should access the system ( overhead information ). An example is the system identification (ID) , which is used by the mobile station to turn on or off its “Roam” indicator light. The inability of the mobile station to use the dotting sequence and word sync sequence to synchronize with the message stream is used by the mobile station to turn on its “No Service” indicator light. Messages intended for specific individual mobile stations If a mobile station is in the process of originating a call, the forward setup channel is used to notify the mobile station of a specified voice channel ( frequency pair ) that it should tune to in order to complete the call. If a mobile station is idle and a call comes in for it, the mobile station is paged using the messages in the FOCC data stream. (While a paging message is intended for a specific mobile station, indicated in the FOCC data stream by its mobile identification number (MIN) , the FOCC data stream is being scanned by all mobiles that are powered on and within range of the transmitting base station. Each individual mobile station must determine whether the MIN contained in the paging message corresponds to the MIN stored at the mobile station. When the correct mobile station has identified itself to the cell site (via the RECC, as discussed in the next slide), the cell site transmits additional information to the mobile station via the FOCC data stream, indicating the voice channel to be used and other information necessary to complete the call setup process.
- Because the RECC data stream must be shared among all mobile stations registered at a particular cell site, mobile stations transmit data in bursts to the cell site to allow other mobile stations to share the same RECC. The two main message types transmitted by mobile stations using the RECC data stream are origination messages and page response messages. Origination messages are sent when the user dials a directory number and presses the “send” button. The origination message contains the directory number of the “called telephone” and other information about the originating mobile itself. It must identify itself by its own directory number and power class as well as its electronic serial number. Page response messages are sent in reply to an incoming message from the cell site via the FOCC data stream. The mobile station recognizes its own MIN in the paging message stream, and responds to the message. The mobile station notifies the cell site via the RECC that it is located in that cell site’s coverage area. This informs the cell site that a call can be set up on a voice channel assigned to that cell site. The cell site then transmits the voice channel and other information to the mobile station via the FOCC data stream.
- The previous slides discussed uses of the control channel. While the control channel is being used, the transceiver at the mobile station is transmitting on the uplink frequency of the control channel frequency pair and receiving on the downlink frequency of the control channel pair. After the various functions (e. g., registration, call setup) have been performed using the control channel, the mobile station receives a transmission from the base station notifying it of the voice channel (that is, the frequency pair used to transmit and receive the voice conversation) to be used for the rest of the call. The transceiver at the mobile station then changes the frequencies that it is using, and begins to transmit on the uplink frequency of the voice channel instead. The voice channel is typically used to pass user information such as voice conversations or data between the mobile and base station. However, after a voice channel has been assigned to a mobile station and the control channel is no longer available for signaling between the base station and the mobile station, additional signaling information must still be sent for control of an AMPS call. In that case, the signaling must take place on the voice channel. There are two methods of transmitting signals or tones on the voice channel: In-band signaling involves sending control information along with, or replacing voice information. It uses the frequency range of 300 to 3000 Hz, that is, the same frequency range that is used to transmit voice conversations. Out-of-band signaling may be sent without alteration to the voice information. It uses a frequency range either above or below the 300 to 3000 Hz range that in-band signaling uses. Either of these methods are used to transmit each of the different signals or tones.
- Just as the AMPS control channel was used for different purposes, several types of signals or tones are sent on the voice channel for different purposes, depending on the function being performed at the time by either the base station or the mobile station. Signals and tones sent on the voice channel include the following: Supervisory audio tone (SAT) Signaling tone (ST) Dual tone multi-frequency (DTMF) Blank and burst signals A description of each of these, including the type of information conveyed, the format of the signal or tone, and the purpose of that signal or tone in controlling an AMPS call are discussed in the following slides. For signaling messages that contain information (as opposed to just a tone sent at a particular frequency), it is necessary to encode that information in binary format (1’s and 0’s). In order to send this “digital” data across the analog air interface used by AMPS, a method is used known as frequency shift keying (FSK). This is a form of frequency modulation (FM) similar to that used to transmit voice, but with variations that allow the receiver to detect the transmission as a series of 1’s and 0’s instead.
- The supervisory audio tone (SAT) is an out-of-band signal that ensures maintenance of a reliable transmission path between the mobile and base station on the voice channel. It may be one of three frequencies: 5970, 6000, or 6030 Hz. Each base station is assigned one of these SATs. The base station indicates initially to the mobile station which SAT frequency is being used via a 2-bit field in the FOCC data stream. After a voice channel has been assigned, the SAT frequency is identified again by the base station to the mobile station by populating the two-bit SCC field in a mobile station control message on the forward voice path. Transmitting an SAT on the voice channel while a call is in progress provides an indication of a closed loop . This tone is transmitted by the base station and is echoed back from the mobile station. A loss of SAT implies the channel conditions have become impaired and if the loss is long enough (approximately 5 seconds), the call is terminated. SAT is also used to identify co-channel interference . If an interfering signal is sufficient enough to interfere with the mobile station, the received SAT frequency will be different from that designated by the two-bit SAT code SCC. If the signal received at the mobile station has an incorrect SAT code, the fade timer at the mobile station is started. If the correct SAT code is not received at the mobile station within five seconds, the call from that mobile station is terminated. This prevents a mobile station that may “see” several cell sites from interfering with a call already in progress in a nearby cell. The diagram above is an example of how the SAT is used to eliminate co-channel interference. Subscriber 1 has a call in progress at Base Station 1, using channel 432 and SAT 0. Subscriber 2, who is located on high ground, originates a call that is initially received by Base Station 2 and assigned channel 432 using SAT 1. Because Subscriber 2 has a line-of-site transmission path to multiple base stations, his mobile station may tune in to the same frequency (channel 432) on Base Station 1, potentially interfering with Subscriber 1’s call in progress. However, when Subscriber 2’s mobile station receives SAT 0 instead of SAT 1 (which the mobile station was told to expect at the beginning of the call), the fade timer at the mobile station is activated. If Subscriber 2’s mobile station continues to receive SAT 0 instead of SAT 1, Subscriber 2’s call will be terminated after 5 seconds.
- The signaling tone (ST) is an out-of-band signaling 10 kHz tone burst that is used to indicate a status change. It is transmitted only from the mobile station to the base station. The ST signal can be used to notify the base station (as in on- and off-hook indication, or when a hookflash occurs at the mobile station) or to confirm messages sent from the base station (such as alert confirmation, and handoff order acknowledgment). As an example, when an incoming call is received at the MSC, an alert message is sent to the base station (cell site), which in turn sends a message to the mobile station that causes it to ring. The mobile station then begins transmitting the 10 kHz signaling tone back to the cell site. When the user goes off-hook to answer the call, the 10 kHz tone stops, indicating to the cell site that the connection should be completed. This process is known as the alert confirmation . The signaling tone can convey different types of information depending on the length of the burst sent and the current state of the call. For example, an ST burst from a mobile station to the cell site while a call is in progress may be in response to different events, and require a different response from the cell site and the MSC. A subscriber going on-hook or terminating a call causes the mobile station to transmit a 10 kHz burst to the cell site for 1.8 seconds, while a hookflash (used by a subscriber to interrupt a call in progress and initiate a custom calling feature, for example) will cause the mobile to transmit a 10 kHz burst for only 0.4 seconds. Subsequent events in the latter example are discussed under blank and burst signaling.
- Dual tone multi-frequency (DTMF) signals are sent over the voice channel, using in-band signaling. They are not recognized by AMPS, but are used in applications between “end users” (where one end user is the subscriber and the other is commonly a machine). For example, DTMF signals are used to retrieve answering machine messages, direct automated private branch exchange (PBX) systems to an extension, and a variety of other control functions. Parameters such as frequency, amplitude, and minimum tone duration for recognition of DTMF tones have been standardized to allow interworking between various types of subscriber devices (including mobile stations) and the devices that were designed to respond to DTMF signals. While transmission of DTMF tones can be accomplished on the voice channel, varying channel conditions can alter the expected results. In poor radio conditions and a fading environment, the radio path may be interrupted for short periods of time. This results in the possibility of multiple digits being recognized only when one key is depressed.
- Blank and burst signaling is used when more information must be sent across the voice channel than can be conveyed by just a tone. Because the mobile station is transmitting and receiving only on one pair of frequencies at a time, the cell site (base station) needs a way to insert a message into that voice transmission when urgent information must be conveyed while a call is in progress. By muting the SAT being sent to the mobile station, the base station causes the mobile station to mute the audio path (this is the “blank” part). The base station then sends a “burst” of data in the frequency range normally used for voice transmission (in-band signaling), using FSK to encode the message digitally for transmission on the analog voice channel. Blank and burst signaling occurs so quickly that it is almost imperceptible to the subscriber. Blank and burst signaling is used for a number of different control messages while an AMPS call is in progress. It can be used to control the power transmitted by the mobile station, using the dynamic power control feature of AMPS. The cell site sends a command to the mobile station and the mobile station changes its transmission power to a specified level. The mobile station then confirms that it received the message. Blank and burst signaling is also used in the forward direction (cell site to mobile station) in the handoff process. When it is necessary to hand off a call from one cell site to another, the serving cell site sends a blank and burst message to the mobile station commanding it to retune to a new channel. The number of the new channel is transmitted to the mobile station in the message. An example of blank and burst in the reverse direction is the three-way calling custom calling feature. Using this feature during an established conversation, the mobile station subscriber dials the directory number of the third party that they would like to add to the conversation, and then presses the “send” button. Mobile station initially generates a flash signal to cell site by turning on signaling tone for 0.4 seconds. Cell site responds to the flash with a blank and burst “send dialed digits” message. Mobile station then sends a blank and burst sequence on the reverse voice channel to transmit the dialed directory number to the cell site. Cell site relays the data to the MSC where the third party is switched into the conversation.
- Previously, we talked about how signaling is performed on the AMPS control channel and the AMPS voice channel. This slide summarizes the steps required to convert a voice conversation (referred to as an “audio” signal) into a format that can be transmitted from the mobile station to the cell site via the AMPS air interface. AMPS voice processing at the mobile station transmitter consists of baseband processing and modulation. Baseband processing - In AMPS, baseband processing does very little conditioning to the raw audio signal (the baseband signal) before sending it to modulation. Baseband processing is used to improve signal to noise ratio (S/N), improve fidelity, reduce occupied bandwidth on the air interface, and reduce spectral splatter (spreading of a signal into adjacent frequencies). Baseband processing consists of the following steps. Compression - A form of noise reduction based on amplifying the original signal into one with a higher average level and a smaller dynamic range; “compression” at the transmitter is coupled with “expansion” at the receiver (at the cell site) to restore the original signal. Emphasis - “Pre-emphasis” at the transmitter increases the higher frequency components of an FM signal in order to reduce overall noise levels, improving the S/N; it is coupled with “de-emphasis” at the receiver to restore the original signal. Limiting - Uses filters to reduce transmitted bandwidth and interference between adjacent channels in an FDMA system. Modulation - Modulation takes the modified baseband signal and turns it into an FM modulated signal. The transmitter at the mobile station can then transmit this signal across the air interface to the cell site; the corresponding function at the receiver is called demodulation .
- As discussed previously, AMPS was one of the first commercial mobile telephone services to deploy lower power cell sites in order to cover a geographic area with multiple, relatively small cells rather than a few large cells. The significance of this is that it made it possible to reuse frequencies, allowing an AMPS network to serve more simultaneous calls in the same amount of spectrum than was possible with earlier commercial mobile telephone services. This section discusses the concept of cell structure and frequency reuse in more detail. Based on the use of 30 kHz for the uplink and 30 kHz for the downlink, each active call using AMPS requires a total of 60 kHz of spectrum. In the United States, an AMPS service provider has been allocated generally a total of approximately 25 MHz, which is sufficient for 416 channels (frequency pairs) or 416 simultaneous conversations. Allocation of 21 channels as control channels reduces the number of channels available for voice conversations to 395. It is clear that in order to support a subscriber base of any reasonable size, the frequency pairs must be used more than once in an AMPS network. Using the same frequencies in adjacent cells, however, would result in large-scale occurrence of what is known as co-channel interference , that is, subscribers whose mobile stations are transmitting on the same frequency to multiple cell sites. A solution developed as a standard part of AMPS network design was to reuse frequencies, but only in a pattern that would allow cells using the same set of frequencies to be separated by a sufficient number of intermediate cells. This solution is known as frequency reuse . Although AMPS transmission results in approximately a circular area of coverage (depending on local conditions), cellular network design generally requires that, for complete coverage, cells must overlap. This results in a coverage area per cell that is usually represented as a hexagon. This hexagonal cell shape is used to illustrate the AMPS cell structure in the following slides.
- AMPS uses a number of channels in each cell. However, the same channels cannot be used in adjacent cells due to co-channel interference. The distance between frequency reuse cells is determined by a ratio that describes the distance (D) between base stations and the radius (R) of a given cell (D/R). Cells that are alphabetically adjacent are also adjacent in frequency. In the diagram above, the pairs D,C and D,E are still adjacent. The most common frequency reuse plans involve splitting the spectrum into N frequency cells , each using a hexagonal grid to define the relative positions of the base stations. In N=3 or N=4 cell reuse, immediate market sectorization is required and provides about one-cell distance between co-channel cells (i. e., cells using the same set of frequencies). In N=7 cell reuse, the configuration can start off as an omni-configuration (that is, a configuration using only omnidirectional antennas at cell sites) and grow to an exclusively sectorized configuration ,or a “mix’ of sectored and omni sites. The N=7 pattern shown in the diagram is dominant in the industry because it can further be subdivided to form a 21-cell plan as capacity requirements grow. The less common N=12 cell pattern provides good carrier to interference ratio (C/I) (about three-cell radii between frequency reuse cells) even when sites are omnidirectional. The individual sites in this configuration are very small, however, and traffic efficiency is low. It is possible to devise a workable system that is virtually “non-cellular.” This is a system that uses a large number of channels at one or two sites (up to four) or that is cellular but has very irregular channel configurations. Some designers plan a “non-cellular” system to meet short-term objectives (such as lower cost) but these systems are not recommended in areas where even a moderate level of frequency reuse is contemplated. This technique should be reserved for small, remote areas. (For more information on frequency reuse plans, refer to the GWEC module FRP-Cellular Coverage Concepts.)
- There are two types of cells in an AMPS network: Omnidirectional cells Sectored cells An omnidirectional cell uses an antenna that radiates in all directions and single or multiple channel sets can be assigned to the cell. In some instances, omnidirectional antennas are used in conjunction with reflectors that “pattern-shape” the transmitted energy to allow for contour in border sites where, possibly, roaming agreements could not be obtained or where, due to the “letter” of the agreement, only certain areas may be covered in the adjacent market. For more information on omidirectional antennas, refer to the GWEC module RT - RF Antenna, or Antennas and Propagation for Wireless Communication Systems by Simon R. Saunders.)
- Sectored cells are used when capacity requirements increase in a given cell area. Antennas in a sectored cell do not radiate in all directions but have a directional beam that can vary in width depending on the antenna design. The example shown above has three 120-degree sectors in each cell. Each sector can now be assigned a different channel set for control of capacity and increased reuse. Each sector is considered a face . Sectoring further subdivides channel groups and prevents the worst effects of adjacent- and co-channel interference, since sectors using the same set of frequencies can be made to transmit in different directions (e. g., by rotating the pattern counterclockwise by 120 degrees as the pattern is repeated). Sectoring can be visualized to occur either at the cell edge or at the cell center. (For more information on sectored antennas, refer to the GWEC module RT - RF Antenna, or Antennas and Propagation for Wireless Communication Systems by Simon R. Saunders.)
- Narrowband AMPS (N-AMPS) technology was introduced by Motorola and was formally recognized in the TIA/EIA/IS-88 standard for dual mode analog air interface compatibility published in 1993. N-AMPS took the 30 kHz voice channel used in AMPS for transmitting or receiving, and divided it into three channels of 10 KHz each. An N-AMPS system continues to use 21 control channel sets per service provider, while increasing the 395 voice channel sets available by a factor of 3 (to 1185 channel sets per network provider). As in AMPS, FDMA is used to separate the individual conversations. (N-AMPS should be distinguished from the digital technology TIA/EIA IS-136 TDMA which, while it can also support three subscriber conversations in 30 KHz of bandwidth, does so by assigning each conversation to a separate time slot rather than to a separate frequency pair.) In addition to the capacity gains associated with using 10 kHz channels rather than 30 kHz, Motorola asserted that N-AMPS could achieve an adequate C/I ratio with a frequency reuse of 4 rather than the frequency reuse of 7 needed by AMPS. This improved frequency reuse would allow more frequencies to be deployed at each cell site, increasing the total number of subscribers that could be served by the same number of cell sites. However, practical considerations (that is, the need to implement a frequency reuse plan consistent with that used by the underlying AMPS network) might dictate that a frequency reuse of 7 be used when both N-AMPS and AMPS were used in the same network. While the basic capabilities and operation of N-AMPS are similar to those of AMPS, the way that signaling is done in the voice channel while a call is in progress changed substantially. Rather than using out-of-band tones (SAT or ST) or in-band blank and burst signaling to convey information during a call, N-AMPS uses a continuous data stream that is transmitted at low out-of-band frequencies (that is, below the 300 Hz that constitutes the lower limit of voice frequency transmission.) This digital data stream, which is filtered at the receiving end at the cell site (on the uplink) or at the mobile station (on the downlink) contains the representation (in bits) of the elements such as identification information, on-hook or off-hook status, and alert confirmation that in AMPS are conveyed by SAT or ST tones. In addition, the digital data stream incorporates the messages that are sent digitally, in AMPS, as part of the blank and burst signaling process. Control channel functions in N-AMPS remain the same as in AMPS.
- The initial N-AMPS standard originated with Motorola, and Motorola was the only company to develop the technology. Although the TIA/EIA/IS-88 standard (and later, the TIA/EIA/IS-91 standard) defined the interworking of the N-AMPS and AMPS air interfaces, the need to replace AMPS mobile stations with dual mode AMPS/N-AMPS mobile stations created an additional barrier to deployment except in cases where the need for additional capacity was critical. N-AMPS filled a need for expansion of AMPS capacity at a time when digital wireless technologies were in a developmental stage. The primary application for N-AMPS was in urban areas where a high density of cellular subscribers resulted in inadequate capacity of the existing AMPS networks. As the major digital technologies (e. g., TDMA, CDMA, and GSM) became available, the advantages of digital service (error correction, encryption, advanced services, reduced interference, and, in the case of CDMA and GSM, higher capacity) made deployment of digital technologies more attractive than continued use of analog cellular. AMPS continues to exist in low density suburban and rural areas, resulting in many networks with dual mode (AMPS plus digital) operation. Higher density urban areas are served primarily by digital technologies in the 800 MHz band (used for AMPS and N-AMPS), as well as in the personal communications service (PCS) band at 1900 MHz. Outside the United States, a scattering of wireless networks around the world still use the N-AMPS technology. Among these are networks in Argentina, Aruba, El Salvador, Guatemala, Indonesia, Israel, Kazakhstan, the Philippines, Russia (St. Petersburg), Thailand, Venezuela, and Zaire. (Source: http://www.cellular.co.za)
- In the table above, AMPS technology is compared with other wireless techologies: Narrowband AMPS (N-AMPS) TIA/EIA IS-136 Time Division Multiple Access (TDMA) TIA/EIA IS-95 Code Division Multiple Access (CDMA) Global System for Mobile Communication (GSM) The following key technology characteristics are considered: Voice Transmission - AMPS and N-AMPS are analog technologies; others shown are digital. Modulation AMPS and N-AMPS use FM, including FSK to encode digital data. TDMA uses differentially encoded quadrature phase shift keying (DQPSK). CDMA uses direct sequence code modulation. GSM uses Gaussian minimum shift keying (GMSK). Multiple Access - The different technologies separate multiple users either by frequency (FDMA), by timeslots (TDMA and GSM), or by coding (CDMA). Channel bandwidth - Required in each direction for full duplex transmission. Frequency reuse - Frequency reuse of 7 means that, for 3-sector cells, 21 different frequency pairs are repeated for each 7-cell pattern. Frequency reuse of 4 means that, for 3-sector cells, 12 different frequency pairs are repeated for each 4-cell pattern. CDMA reuses the same frequencies in each sector since individual subscriber conversations are separated by codes, not frequencies or time slots. Comparable capacity - Measures number of simultaneous users that can be served in a 3-sector cell in the same amount of spectrum for each technology. Signaling protocol - Indicates whether the mobile application part (MAP) used for signaling between network elements such as the MSC and the HLR is IS-41 based or GSM based. Technologies that use the same MAP permit roaming between networks using those technologies, provided the appropriate carrier agreements and equipment are in place.
- AMPS technology has strengths and weaknesses of relative to other wireless technologies: Strengths - While AMPS does not necessarily have “advantages” over digital wireless technologies, it is unlikely that existing AMPS networks will disappear in the immediate future. Some strengths of AMPS includes the following: Because AMPS was the first widespread deployment of mobile wireless technology, and is primarily deployed (in the United States) in the 850 MHz band, there is substantial coverage by existing AMPS networks in both urban and rural areas, extending to places not yet served by digital technologies A signaling protocol (IS-41) compatible with that used by the major U. S. digital technologies. (TDMA and CDMA) and the availability of dual mode handsets (TDMA/AMPS and CDMA/AMPS) means that, in the U.S. at least, the existing AMPS network will continue being used to provide roaming coverage in areas where the digital networks have not yet been fully built out. Because AMPS mobile stations and base stations have long been standardized, the costs of new customer acquisition, expansion, and upgrades are low relative to newer technologies. Weaknesses - Weaknesses of AMPS relative to newer digital technologies (and some reasons that new network buildouts are using newer digital technologies) include the following: AMPS technology provides low capacity per unit of spectrum used. Frequency reuse is less efficient. Quality of analog transmission is considered inferior to that of newer digital systems. Equipment suppliers are placing emphasis on development of existing (“Second Generation”) digital voice technologies and “Third Generation” technologies that will support both voice and data, so that availability of AMPS equipment may decline over time. (For more information on other wireless technologies, refer to the corresponding GWEC modules AI-TDMA , AI-CDMA , AI-GSM, and AI-Third Generation and Emerging Standards.)