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Time Division Multiple Access Methods for Wireless Personal Communications
David D. Falconer, Fumiyuki Adachi, and Björn Gudmundson
TDMA and various modulation and detection optimizations are presented.
- Basics: Bitstream broken into frames, frames broken into
slots and slots are assigned to users. Upstream (user to base)
and downstream (base to user) are duplexed with in time domain
(TDD) or frequency domain (FDD). Slots contain data bits,
CRC check bits, guard bits, synchronization training bits, and
- Pros and Cons: Variable bit rate to users, less power
control needed because slots are temporally separated within cell, and
frequency separated amongst cells. Users can use idle slots to
measure neighboring BS power levels for handoff (MAHO - mobile assisted
handoff). Pulsating power envelope is bad - interference and
complexity. High data rates needed as 1/N "duty cycle" for N
users - equalization required to overcome delay spread.
- ACA: adaptive channel allocation is an optimization in which
frequencies are borrowed from less crowded cells to increase a cell's
capacity. ACA also performed to dynamically avoid interference.
- FH: frequency hopping is another optimization in
which all transmitters in a cell cycle through a sequence of frequencies
(offset from each other). Differet cells have random sequences, averaging
out inter-cell interference. FH-TDMA is effectively spread spectrum
CDMA with the sequence as code.
- HCS: hierarchical cell structures are used to increase
coverage - large macrocells fill in "holes" of microcells, and
are used in MAHO. Macro and microcell frequencies are obviously
- Modulation: QPSK (4-level modulation) best for wireless/cellular
environments. GMSK and DQPSK also used, as is QAM. Coherent
versus noncoherent modulation depends on time variability of the channel.
Differential detection is the common form of noncoherent modulation,
and can be improved with decision feedback of the past L symbols.
- More optimizations: Convolutional coding and adaptive equalization
can further combat multipath effects like delay spread.
TDMA is a well-understood, commonly employed, and efficient media access
technique. The enhancements described in this paper are worth understanding,
but for the most part are "magic tricks" used to improve
performance, from the perspective of the network layer and above.
- The main thrust of the paper is clear, but it tended to get
sidetracked and bogged down in poorly explained technical details.
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