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U. Glaeser

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FIGURE 42.1 Block diagram of (a) generic DSSS transmitter, (b) successive interference canceller for multiuser<br />

detection, and (c) rake receiver for a system with parallel pilot channel (i.e., IS-95).<br />

The tremendous processing requirements of this architecture will become evident by considering a<br />

modest system operating at a chip rate of say, 4 Mcps using a 32-tap shaping filter, four rake fingers per<br />

user, four complex correlators per rake finger and 10 users in the cell, the number of operations (real<br />

multiply-adds) needed for a 5-stage SIC is upwards of 14 billion operations per second or giga-operations<br />

per second (GOPS). This amount of processing can easily overwhelm even the latest generation of generalpurpose<br />

processors such as the TI TMS320C6x which delivers 1.6 giga-instructions per seconds (GIPS),<br />

but only 400 mega multiply-add operations per second [6]. At an anticipated power dissipation of 850 mW<br />

per processor, the overall power consumption of a SIC circuit based on such units will be quite large.<br />

It is also worth noting that many operands are in the SIC or other MUD receiver that require only<br />

a few number of bits (i.e., multiplication with a 1-bit PN code sequence). This fact can be exploited<br />

in a dedicated ASIC datapath architecture but not in a general-purpose software programmable<br />

architecture.<br />

Broadband Wireless Networks<br />

Emerging broadband fixed wireless access systems provide high-speed connectivity between a cellular<br />

base station and a home or office building at data rates of a few Mbps to a few tens of Mbps. On the<br />

other hand, standardization activities that are currently targeting high-speed wireless mico-cellular (wireless<br />

LAN) systems are looking at delivering 10–20 Mbps over the air data rates in the near future, with<br />

higher rates projected in the long term.<br />

It is generally accepted that in order to achieve such high data rates, beam switching or beamforming<br />

techniques must be integrated into the development of the nodes. In addition, single carrier systems<br />

must include adaptive equalization to overcome time varying channel impairments, while multicarrier<br />

systems based on OFDM will require a large number of subcarriers [7]. The signal processing requirements<br />

for such high data rate systems could easily mount into the tens of GOPS range, thus necessitating<br />

the development of ASICs.<br />

© 2002 by CRC Press LLC<br />

(a)<br />

Received<br />

Sig. NF chip<br />

(b)<br />

(c)<br />

Data, F baud<br />

Shaping<br />

Filter<br />

Variable rate<br />

interpolator<br />

PN, F chip<br />

PN<br />

↓N<br />

↑2<br />

To Next Stage SIC<br />

Pilot Correlator<br />

Data Correlator<br />

Early Correlator<br />

Late Correlator<br />

Rake Finger 1<br />

Rake Finger 2<br />

Rake Finger K<br />

Σ<br />

NCO<br />

Shaping<br />

Filter<br />

user-1<br />

Rake<br />

AFC<br />

Loop<br />

Loop<br />

Filter<br />

user-K<br />

Rake<br />

Remodulator<br />

Choose<br />

Largest<br />

To VCO<br />

Rake<br />

Combiner

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