Chapter 8: Existing Wireless Systems: 1G, AMPS System

1. Chapter 8:Existing Wireless Systems:1G, AMPS System Associate Prof. Yuh-Shyan Chen Dept. of Computer Science and Information Engineering National Chung-Cheng University

2. Introduction • A wireless system needs to take many factors into account • Call rate, call duration, distribution of MSs, and traffic in an adjacent cell. • It is important to study various characteristics of existing cellular systems • How they support seamless mobile communication • AMPS (Advanced Mobile Phone System) • As the first representative of wireless system

3. Advanced Mobile Phone System (AMPS) • AMPS is the first-generation cellular system in USA • It transmits speed signals employing FM and important control information is transmitted in digital form using FSK (Frequency shift keying) • Is used for modulating a digital signal over a carrier (or two carriers) by using a different frequency for a “1” or a “0”. • AMPS is the first cellular phone technology created by AT&T Bell labs. • With the idea of dividing the entire service area into logical division called cells • Each cell is allocated one specific band in the frequency spectrum

4. Cont. • To explore a reuse pattern • The frequency spectrum is divided among seven cells • Improving the voice quality as each user is given a higher bandwidth • AMPS uses a cell radius of 1 to 16 miles • Larger cells tend to have more thermal noise and less interference • Smaller cells have more interference and less termal noise • One important aspect of AMPS is that it allows both cell sectoring and splitting

5. Cont. • It is also sufficient to have a lower-power MS (4 Watts or less) and a medium-power BS (about 100 Watts) • AMPS is capable of supporting about 100,000 customers per city • The system is aimed to reduce blocking probability to about 2% during busy hours

6. Characteristics of AMPS • AMPS uses frequency band from 824 MHz to 849 MHz for transmissions from MSs to the BS • Reverse link or uplink • Frequency band between 869 MHz to 894 MHz from the BS to MS • Forward link or downlink • The 3-KHz analog voice signal is modulated onto 30-KHz channels • In transmitting data • The system uses Manchester frequency modulation at the rate of 10 kbps

7. Band Allocation in AMPS

8. Cont. • Separate channels are used for transmitting control information and data • In AMPS, there are one control transreceiver for every eight voice transreceivers • Frequency allocation in AMPS is done by dividing the entire frequency spectrum into two bands – Band A and Band B • The non-wireline providers are given Band A • Bell wireline providers are given Band B • A total of 666 channels is divided among these two bands • A cluster of seven cells allows many users to employ the same frequency spectrum simultaneously

9. Cont. • AMPS’s use of directional radio propagation enables different frequencies to be transmitted in different directions • Thereby, reducing radio interference considerably.

10. Operations of AMPS • A general state diagram of how an AMPS system handles and various other responsibilities is shown in Fig. 10.1 • Three identification numbers are included in the AMPS system to perform various functions • Electronic serial number (ESN): • A 32-bit binary number uniquely identifies a cellular unit of an MS and is established by the manufacturer at the factory • System identification number (SID): • A unique 15-bit binary number assigned to a cellular system • The SID serves as a check and can be used in determining if a particular MS is registered in the same system or if it is just roaming • Mobile identification number (MIN): • A digital representation of MS’s 10-digit directory telephone number

11. General operation of AMPS

12. General working of AMPS phone system • When a BS powers up (enter idle task), it has to know its surroundings before providing any service to the MSs • It scans all the control channels and tunes itself to the strongest channel • It sends its system parameters to all the MSs present in the service area • Each MS updates its SID and establishes its paging channels only if its SID matches the one transmitted by the BS • Then, the MS goes into the idle state, responding only to the beacon and page signals

13. Cont. • If a call is placed to an MS, the BS locates the MS through the IS-41 message exchange • Then the BS pages the MS with an order • If the MS is active, it responds to the page with its MIN, ESN, and so on • The BS then sends the control information necessary for the call • For which the MS has to confirm with a supervisory audio tone (SAT), Indicating completion of a call • If a call is to be placed from an MS, the MS first sends the origination message to the BS on the control channel

14. Cont. • The BS passes this to IS-41 and sends the necessary control signal and orders to the MS • Thereafter, both MS and BS shift to the voice channel • An FVC and RVC control message exchange follows to confirms the channel allocation • Thus the actual conversation starts

15. How an MS know when it receives a call ? • The answer lies in the messages passed on the control channels • Whenever the MS is not in service, it tunes to the strongest channels to find out useful control information • The same happens at the BS as well • The various channels used by the AMPS are as follows • FOCC • RECC • FVC • RVC

16. Forward and reverse channels

17. Forward control channel (FOCC) • FOCC is primarily used by the BS to page and locate the MSs using the control information in three way time division multiplexing mode • The busy/idle status shows if the RECC is busy, and stream A and stream B allow all the MSs to listen to the BS

18. Format of FOCC

19. Reverse control channel (RECC) • One or more MSs using the RECC channel • This could be in response to the pages by the BS • There could be several MSs responding to quesies • A simple mechanism to indicate whether RECC is busy or idle is to model it after the slotted ALOHA packet radio channel • The seizure precursor fields are used for synchronization ad identification • For a multiple-word transmission following the seizure precursor, the first RECC message word repeats itself fie times; then the second RECC message word is repeated five times

20. Format of RECC

21. Forward voice channel (FVC) • FVC is used for one-to-one communication from the BS to each individual MS • A limited number of messages can be sent on this channel • A 101-bit dotting pattern represents the beginning of the frame • The forward channel supports two different tones • Continuous supervisory audio • The BS transmits beacon signal to check for the live MSs in the service area • The discontinuous data stream • BS sends orders or new voice channel assignment to the MS

22. Reverse voice channel (RVC) • Reverse voice channel is used for one-to-one communication form MS to the BS during calls in progress and is assigned by the BS to an MS for its exclusive use

23. IS-41 • IS-41 is an interim standard that allows handoff between BSs under control of different MSCs and allows roaming of a MS outside its home system • In order to facilitate this, the following services need to provided • Registering for the MS with a visiting MSC • Allowing for call origination in a foreign MSC • Allowing the MS to roam from one foreign system to another

24. IS-41 architecture

25. Key terms and concepts

26. Relationship between IS-41 and OSI protocol stack

27. Various Operations Supported by IS-41 • Registration in a new MSC • Calling an idle MS in a new system • Call with unconditional call forwarding • Call with no answer • Calling a busy MS • Handoff measurement request • Recovery from failure at the HLR

28. Internetworking of IS-41 and AMPS