IS-54 and IS-136 (Digital AMPS)
IS-54 is the standard for the digital version of the US AMPS
system. Recently IS-54 has been replaced by the IS-136 standard. The system
uses and hybrid Frequency Division Multiple
Access and Time Division Multiple Access
concept as it accepts 3 users per carrier. The carrier spacing 30 kHz, similar
to the analog AMPS. Thus, assuming that the analog and digital system use the
same frequency reuse pattern, the digital version can accommodate three times
more users. In terms frequency planning the digital system behaves similar to
analog AMPS. Carrier frequencies are in the 800 and 1900 MHz bands.
Although the digital system has advantages
for the operator, marketers found it difficult to convince subscribers to go
digital, i.e., to buy a new phone. The advantages of improved security in a
digital system were intentionally not used, to avoid that subscribers would
be alerted that the analog AMPS system had less security.
Radio Aspects
- Channel bit rate 48.6 kbit/s
- Frame duration 40 ms, divided into six 6.67 ms slots. If follows that for
usual carrier frequencies, on the average one fade per bursts occur. See Spreadsheet
v = 27 m/s, fc = 1900 MHz, fade margin 6 .. 10 dB)
- Each slot: 324 bits, 260 user data
- Full rate and half rate codebook excited linear predictive coding:
Vector Sum Excited Linear Prediction (VSELP)
Source rate 7.95 kbit/s, transmitted at 13 kbit/s
- Differential QPSK (not constant envelope: power penalty) pi/2 shifted,
root cosine roll-off filtering, roll-off factor 0.35
- 1.62 bit/s/Hz
- Idle voice channels can be used for CDPD
packet data transmission.
- Equalization was initially not included in the standard, but appears useful
for delay spreads larger than T/8, with T the symbol time. This
corresponds to delay spreads of a few microseconds.
EXTENDED AMPS
- Proposed by Hughes
- Digital Speech Interpolation (DSI):
Speech pauses are exploited to enhance user capacity. Typically speech activity is about 0.4. Compare with PRMA.
- Frame of six slots; dynamically assigned to different users
Example of DSI performance:
- No DSI: E-AMPS has 6 times capacity of analogue AMPS
- DSI on 1 RF carrier: 6 times analogue
- 3 RF carriers: 7 times analogue
- 8 RF carriers: 9 times analogue
- 19 RF carriers: 11 time analogue
- Many RF carriers: Law of large numbers:
- ultimately 6 / 0.4 = 15 times analogue AMPS
- DSI in forward link: multiplexing
DSI in reverse link: random access: throughput loss because of collisions
- Slow frequency hopping: mitigates effect of fading
