Chapter 3 Analog Cellular Communications AMPS System This chapter covers the first-generation cellular technology focusing on AMPS (Advanced Mobile Phone System). AMPS delivers basic telephony and supplement services (voice mail and call forwarding). 2019/2/25 Peter Yeh

Network Elements The AMPS specification refers to terminals as mobile stations and to base station as land stations. The common terminology for an AMPS switch is mobile telephone switching office (small and large MTSO). The communication links between the base stations and switch are labeled land lines (copper wires, optical fibers or microwave systems) 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

AMPS Identification Codes Mobile Identification Number (MIN) Area code (3 digits), Exchange number (3 digits) and subscriber number (4 digits) Electronic Serial Number (ESN) System Identifier (SID) Station Class Mark (SCM) Indicates capabilities of a mobile station Supervisory Audio Tone (SAT) Digital Color Code (DCC) Help mobile stations distinguish neighboring base stations from one another 2019/2/25 Peter Yeh

Frequency Bands and Physical Channels The band for forward transmissions, from cell site to mobile station, is 870-890 MHz. The reverse band, for transmissions by mobiles, is 45 MHz lower. An AMPS physical channel occupies two 30 KHz frequency bands, one for each direction. 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

Radiated Power An AMPS terminal is capable of radiating signals at 6 or 8 different power levels (6 mW to 4W). 10 log 4000 = 36 dBm The radiated power at a a base station is typically 25 W. Discontinuous transmission (DTX) Speech activity detector ON-OFF state Power saving and Interference reducing 2019/2/25 Peter Yeh

Analog Signal Processing Compression and pre-emphasis are established techniques for audio signal transmission. An amplitude limiter confines the maximum excursions of the frequency modulated signal to 12 KHz. Low pass filter Attenuates signal components at frequencies above 3 KHz, refer to Fig. 3.5. The notch (at 6KHz) removes signal energy at the frequencies associated with the 3 SAT of the AMPS system. 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

SAT and ST The SAT (Supervisory Audio Tone) transmitted with user information serves to identify the base station assigned to a call. Each base station has its own SAT- at 5970 Hz, 6000 Hz, or 6030 Hz. An analog signals from AMPS terminals can also contain a 10 KHz sine wave referred to as a ST (Supervisory Tone). On-hook and Off-hook indications signaling The channel reuse principles (Section 9.3.2) 2019/2/25 Peter Yeh

Digital Signals AMPS also transmits important network control information in digital form. AMPS digital signal are sine waves either 8 KHz above or 8 KHz below the carrier. The signal format is Manchester coded binary frequency shift keying at a rate of 10 Kbps 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

Spectrum Efficiency Frequency modulation in 30 KHz physical channels Signal-to-Interference ratio (SIR) SIR >= (SIR)req = 18 dB Reuse factor N = 7 (Figure 9.9) Spectrum efficiency E=395 /7*25 = 2.26 conversations/cell/MHz 395 traffic channels, 25 MHz/system, 7 cells in a cluster 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

Logical Channel Categories FOCC: Forward (Downlink) Control Channel Carries the same information from one base station to all of the mobile terminals (Broadcast) RECC: Reverse (Uplink) Control Channel Carries information from many mobile terminals that do not have voice channel (Random access) FVC: Forward Voice Channel (Dedicated) RVC: Reverse Voice channel (Dedicated) Forward and reverse traffic channel User information (Dedicated) 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

Tasks Performed by Terminals Initialization mode The terminal turns the power on A conversation ends Loses contact with the current base station Idle mode Access mode (from Idle mode) The terminal presses the SEND button An incoming call request detected (MIN) A registration event stimulated Conversation mode 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

Capacity There are 3 ways to increase the capacity Operate with smaller cells Obtain additional spectrum allocations Improve spectrum efficiency NAMPS (Narrowband-AMPS) Messages similar to AMPS Synchronization sequences Digital versions of the SAT and ST 2019/2/25 Peter Yeh

2019/2/25 Peter Yeh

Review Exercises What is the purpose of the busy / idle bits in the FOCC? Why are they not used in the other control channel formats? Explain how the AMPS system users supervisory audio tones (SAT) and a digital color code (DCC). Why are both required? Explain why it is sometimes desirable for the AMPS system to set up a call through a base station that is not the nearest base station to the terminal. How does the AMPS system achieve this effect? 2019/2/25 Peter Yeh