Applications

Q. What is 802.11n?

In response to growing market demand for higher-speed wireless local area networks (WLANs), the Institute of Electrical and Electronics Engineers - Standards Association (IEEE-SA) approved the creation of the IEEE 802.11n. The objective is to define modifications to the Physical Layer and Medium Access Control Layer (PHY/MAC) that deliver a minimum of 100 megabit-per-second (Mbps) throughput at the MAC SAP (top of the MAC; see Table 1 below).

This minimum throughput requirement represents an approximate 4x leap in WLAN throughput performance compared to today's 802.11a/g networks. The purpose for this next step in WLAN performance is to improve the user experience with existing WLAN applications while enabling new applications and market segments.

Table 1. Comparison of different 802.11 transfer rates.

Q. What is Silvus' position with respect to 802.11n?

Silvus believes that simply demonstrating 100 Mbps under some conditions will not be enough to ensure a robust user experience with emerging applications and is developing proprietary techniques that will enhance the throughput and reliability for applications such as HD video. Silvus will support two offerings, a compliant 802.11n offering and an enhanced broadband wireless offering which delivers greater throughput and resistance to interference.

Q. How does Silvus help increase the effective throughput for broadband wireless communicaitons?

There are three key areas that need to be considered when addressing increases in wireless broadband performance. First, improvements in radio technology will be needed to increase the physical transfer rate - enhanced PHY performance modes must be developed. This will also include the improvements in data transfer efficiency are needed to reduce the performance impacts of PHY headers and radio turnaround delays that would otherwise reduce the improvements achieved with increases in physical transfer rate. Second, is the development of new approaches to MIMO configurations to leverage the advanced PHY performances. And third is the investigation and development of interference mitigation techniques that can be applied in the ever increasingly crowded ISM bands.

Q. How does Silvus increase the physical transfer rate of broadband wireless systems?

One of Silvus' approaches for increasing the physical transfer rate of wireless systems employs multiple antenna systems for both the transmitter and the receiver. This technology is referred to as multiple-input multiple-output (MIMO). MIMO exploits spatially unique propagation paths between multiple transmit and receive antennas to improve wireless performance. MIMO is part of 802.11n but at Silvus we are aggressively implementing advanced MIMO technologies such as our current 4x4 decoder.

MIMO can provide many benefits, all derived from the ability to process spatially different signals simultaneously. Using multiple antennas, MIMO technology offers the ability to resolve information from multiple signal paths using spatially separated antennas. Multipath signals are the reflected signals arriving at the receiver some time after the original or line of sight (LOS) signal has been received. Multipath is typically perceived as interference degrading a receiver's ability to recover the information. MIMO actually thrives on multipath. It exploits multipath to provide improved reliability through diversity and improved throughput through spatial multiplexing.

Silvus expects MIMO technology to play an important role in achieving high speed wireless broadband links.

Figure 1. Basic two-antenna MIMO system with two-stream SDM example.

Q. How does the performance improve moving to 40MHz?

These results compare the performance of 20MHz and 40MHz implementations. We illustrate each system configuration using the following convention. A two-antenna transmitter communicating with a two-antenna receiver over a 40MHz channel is represented by 2x2 40 MHz where 2 data streams are transferred. Also presented in these results are:

• 4x4-20 MHz transferring 4 data streams
• 2x3-20 MHz transferring 3 data streams
• 2x2-20 MHz transferring 2 data streams

The primary advantage provided by a 2x3-20 MHz implementation over the 2x2-20 MHz implementation is improved signal-to-noise ratio (SNR.) This is noted with improved range for given throughput capability.

It is easy to see the advantage of a 2x2-40 MHz implementation in these results. Notice that even doubling the number of RF chains using a 20MHz implementation to transmit four data streams does not achieve the performance possible with only two RF chains using a 40-MHz channel transmitting two data streams. Using 40MHz channels allows for reduced complexity, keeping costs down while delivering throughput for a robust user experience.

Figure 3. Over-the-air (OTA) throughput with different bandwidth channels.

Q. How does Silvus manage the PHY performance modes?

When maximizing data throughput, intelligent IP will be required to manage the selection of PHY Layer performance modes. Although the MAC Layer does not contribute directly toward increasing the physical transfer rate, it will play a key role in effectively optimizing selection of the PHY Layer performance modes. We will strive to incorporate as many of the optional modes in the standard that will provide higher throughput or improved reliability. We will then couple this with advanced receiver algorithms implemented efficiently in hardware. The resulting PHY will thus provide the MAC layer with the richest set of modes to choose from.

Q. What is Silvus Source Code Methodology to migrate its IP to silicon?

The Silvus 802.11n MAC/PHY was designed as a silicon-ready IP core. It is a 100% synchronous design. Activity based clock gating of the major components provides automatic power management. Careful attention to signal processing design provides best in class SNR performance.

The relatively low clock speed of the two clocks used by the design (80 and 120 MHz) made it possible to timing close the design at speed in the slowest version of a Virtex 5 FPGA. This provides assurance the design will perform from both a timing and functional perspective. A constrained, pseudo random, self checking test environment is provided to make it as easy as possible for the customer to verify proper instantiation of the 802.11n IP core.

Customer integration requires the editing of only two VHDL files, one for all SRAM instantiations and a top-level wrapper which includes all clock and reset treatments. There are no synthesis, or scan test, unfriendly structures such as negative edge flops, latches, or asynchronous delay elements in the 802.11n IP core. The clean design and registered 1/0's allow the IP core to synthesize and timing close in STA using only general constraints on the clocks and cross-clock domains. Asserting a single scan_en input will give scan test tools and logic complete control of all clocks and resets, allowing the customer to achieve very high test coverage.

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