Reconfigurable Baseband Processing for Wireless Communications - Adaptive Signal Processing in Wireless Communications - page 448

Within wireless communications, most traffic is in the GT category. Besides the
mainstream of GT communication, a minor part (assumed to be less than 5%) of BE
communication is foreseen, e.g., control, interrupts, and configuration data.
Several NoC solutions have been proposed that use different techniques to provide
guarantees. The Æthereal NoC [23] combines a global time division multiplexing (TDM)
schedule to provide contention free (i.e., guaranteed throughput) routes to network
streams. The best-effort streams are handled in the slack time of the schedule via worm-
hole routing. The Nostrum NoC [38] uses a TDM-related technique called temporally
disjoint networks. Containers are used to route the GT traffic. A third packet-switched
solution uses virtual channels and wormhole routing to provide guarantees [33]. Using
deterministic local arbitration mechanisms (e.g., round-robin) results in a guaranteed
throughput per virtual channel. Assigning a single stream of traffic per virtual channel
will give network-wide guarantees.
The performance of this virtual channel network is illustrated by a HiperLAN/2 case
study. For a 6 × 6 network running at 333 MHz, the processes of multiple HiperLAN/2
receivers are placed on the processor tiles of the platform. Each process in the Hiper-
LAN/2 process graph sends GT traffic at a rate of 256 bytes (i.e., one OFDM symbol)
per 4 µs. Extra BE messages are offered to the network to control the receivers and emu-
late other applications running on the system concurrently. Figure 15.4 shows how the
latency of the GT and BE messages depends on the offered BE load.
For the GT traffic, the mean and maximal latency of packets are given. When the
offered BE load is low, the latency of the GT packets is lower than the guaranteed (or

600

Guarantee
GT Mean
GT Max
BE Mean

500

400

300

200

100

0

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

BE load per PE [fraction of channel capacity]

FIgure 15.4 Message latency of the GT and BE traffic versus BE load for a 6-by-6 network
(queue size 2 flits).