Design Considerations and Impact of 802.11n Networks on 2.4 GHz Wireless PC
Peripheral Applications
By (Tony Xia, Senior Product Marketing Engineer, Cypress Semiconductor Corp.)
Executive Summary
The adaptation of 802.11 Pre-N brings both joys and concerns to the consumer electronics world. While users enjoy the
increased data rate and range of the 802.11n network, the technology causes interference issues for many consumer
electronic devices and PC peripheral applications. In particular, the performances of today’s high-end 2.4GHz based wireless
mice and keyboards will be affected if they operate near an 802.11 Pre-N device.
Product managers who design wireless mice, keyboards, and other PC peripheral applications are faced with the challenges
of understanding this new technology and of selecting the best available wireless solution from 27MHz or 49MHz, Bluetooth,
and other proprietary 2.4GHz solutions. While no single solution is perfect, it is clear that best-in-class interference immunity
performance is key, in addition to the usual concerns of low power consumption and BOM cost.
This article helps readers understand the basics of the 802.11 Pre-N standard. It looka at the challenges that product
managers face when selecting a solution for their products, the most popular choices available in the market, and the
advantages of adapting 2.4GHz technology solutions for PC Peripheral applications.
Finally, the article makes
recommendations on what to look for in identifying the best design choice for mid-tier and high-end wireless HID solutions.
Introduction
Today’s leading PC OEMs and PC Peripheral Retail OEMs are moving towards proprietary 2.4 GHz wireless solutions for their
wireless PC Peripheral Systems, which have several advantages.
First, 2.4 GHz is an internationally reserved frequency band for Industrial, Scientific and Medical studies (“ISM” Band).
Products designed with 2.4 GHz wireless technologies do not require a licensing fee. Some proprietary 2.4 GHz wireless
technologies allow multiple devices, such as a wireless mouse, keyboard, presenter tool, and remote controller, to
communicate with a central receiving device that operate at more than 10 meters away. In addition, proprietary 2.4 GHz
wireless technologies are relatively cheaper and consume less power than Bluetooth based solutions, which also operates in
2.4 GHz band.
Although there are many advantages associated with proprietary 2.4 GHz wireless solutions, the 2.4 GHz frequency band is
actually shared by many applications operating in overlapping frequencies. It is a challenge to product managers to select a
proprietary 2.4 GHz solution for their PC Peripheral applications that can co-exist with other 2.4 GHz based electronics
devices and still perform well. In addition to RF interference signals from Bluetooth based devices, Wi-Fi networks, cordless
phones, microwave ovens, and other 2.4 GHz based sources, a new networking standard, 802.11n, will pose a “killer”
interference threat to proprietary 2.4 GHz wireless PC Peripheral applications.
Today’s 802.11 Pre-N consumer electronics products typically operate in the 2.4 GHz frequency band. 802.11n has a
theoretical maximum data rate of 540 Mbit per second (200 Mbit/s typical), and a range of up to 50 meters. An 802.11n signal
takes a much wider bandwidth (40 MHz) to achieve the higher data throughput, and it also requires higher RF power to
achieve the longer transmitting range. The approach used to achieve the higher data rate is to employ multiple antennas at
the transmitting and receiving applications. This is referred to as multiple-input-multiple-output (MIMO) system. As its name
suggests, a MIMO system is implemented by sending multiple signals into the air and using multiple antennas to receive the
data. This also means a bigger impact on interference to other consumer electronics devices. The impact comes from two
design challenges. The first challenge is increased bandwidth causing increased sideband effects that reduce the Signal to
Noise Ratio (SNR). Second, the wider bandwidth also means a reduced number of “clean” channels for other devices to
operate in the same 2.4 GHz frequency band, which is known as a co-location issue. A good 2.4 GHz radio product will need
to address both issues in order to provide a reliable RF link for the PC peripheral systems.
In order to overcome the first challenge, the 2.4 GHz PC Peripheral device may simply stay farther away from the interference
source. However, sometimes this is not an option because the wireless receiving dongle is connected to the USB host, which
Design Considerations and Impact of 802.11n Networks on 2.4 GHz Wireless PC Peripheral Applications
Published in Embedded Systems Engineering (http://www.esemagazine.com)
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May 2007
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is likely to be in close proximity to the 802.11n transmitter of a desktop or notebook computer. Reducing the power output of
the 802.11n network is an alternative solution, but the data throughput will be reduced, which defeats the purpose of using
802.11n technology. Thus, the PC Peripheral device needs to employ a radio that can transmit at a higher power output
greater than 0 dBm and receive at a higher sensitivity level. These capabilities should be incorporated in the radio hardware.
In addition, the radio also needs to be able to retry transmission automatically and quickly, if the first attempt of the
transmission fails.
The co-location issue is a combination of the fact that the 802.11n signal occupies a wider bandwidth, reducing the number of
available channels in the ISM band, and the fact that “available channels” are also very noisy because of the reduced SNR
from the higher sideband effect. In this case, a robust and agile wireless protocol is needed to manage and process
transmitting and receiving data based on the radio’s hardware capability.
The agility of a protocol is based on the ability to intelligently determine the strength of the interference signal and hop to a less
noisy channel to avoid the interference source even before data transmission. This can be accomplished if the radio has a
hardware block function such as a Received Signal Strength Indicator (RSSI). In addition, should a new interference source
be turned on during the data transmission period, the receiver and the transmitter should be able to synchronize the selection
of another channel and hop to the new channel without interrupting the continuity of the RF link. All of these processes should
also be transparent to the end user. Figure 1 below illustrates the agility of the wireless protocol available from Cypress
Semiconductor’s PRoC™ LP programmable radio-system-on-a-chip family that finds and hops to a clean channel for data
transmission when it detects Wi-Fi interference signals.
Figure 1. Frequency-Agile Hardware & Firmware in Cypress Semiconductor’s PRoC LP programmable radio-systemon-a-chip
The robustness of the protocol comes from the methodology used for data encoding and decoding. One way to encode data
is to use Dynamic Sequence Spread Spectrum (DSSS). In this method, each bit of data in a byte is encoded with multiple bits
using a Pseudo Noise (PN) Code (PN code). Certain codes referred to as Multiplicative Codes have minimal cross-correlation
properties, meaning they are less susceptible to interference caused by overlapping transmissions in the same channel.
Higher data rate can be achieved with a shorter PN Code (e.g. 32-bit), while longer data range can be achieved with a longer
PN code (e.g. 64-bit). The number of available channels and the PN codes create so many permutations that hundreds of
radios can operate in the same working space because the transmitters and receivers must use the same PN Code and
channel to communicate. In the case of an 802.11n enabled environment, even if the 802.11n transmitting channel is not
overlapping with the channels used by the 2.4 GHz PC Peripheral devices, these channels are still relatively noisy due to
reduced SNR, so the PN code encoding greatly improves the robustness of the RF link. Figure 2 below illustrates how a 64-bit
PN Code is used to encode 1-byte of data using DSSS.
Design Considerations and Impact of 802.11n Networks on 2.4 GHz Wireless PC Peripheral Applications
Published in Embedded Systems Engineering (http://www.esemagazine.com)
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May 2007
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Figure 2. Direct Sequence Spread Spectrum using PN Code
A 2.4 GHz radio with higher output power and receiving sensitivity level combined with a robust and agile protocol increases
the chance of survival for a 2.4 GHz wireless device in an 802.11n enabled environment. However, when targeting wireless
HID applications, other considerations for protocol logic are important, such as radio binding/pairing, data encryption, radio
payload, power consumption, and receiving acknowledgement. Program Managers who select a solution that addresses all of
the above design considerations will deliver 2.4 GHz wireless PC peripheral products with good user-satisfaction.
Design Considerations and Impact of 802.11n Networks on 2.4 GHz Wireless PC Peripheral Applications
Published in Embedded Systems Engineering (http://www.esemagazine.com)
Page 3 of 4
May 2007
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References
Cypress Semiconductor
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San Jose, CA 95134-1709
Phone: 408-943-2600
Fax: 408-943-4730
http://www.cypress.com
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Design Considerations and Impact of 802.11n Networks on 2.4 GHz Wireless PC Peripheral Applications
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May 2007
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