CY4636 WirelessUSB™ LP Keyboard Mouse Reference Design Kit User Guide, Doc. # 001-70355 Rev. *A
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A.3
WirelessUSB 2-Way HID Protocol Overview
The WirelessUSB 2-way Human Interface Device (HID) protocol is designed for reliable two-way
communication between a wireless bridge and target HID device in 1:1 (one HID and one bridge) or
2:1 (two HIDs and one bridge) systems. The WirelessUSB 2-way HID protocol allows HID applica-
tions to establish a connection to the bridge and receive ACK and DATA packets from the bridge.
The host PC is not aware of the wireless connection, since the interface to the host acts like a nor-
mal wired USB HID connection. Therefore, there is no special software required on the host PC in
order to support WirelessUSB.
Figure A-11. WirelessUSB 2-Way System
A.3.1
Radio Channel Management
WirelessUSB uses the unlicensed 2.4 GHz Industrial, Scientific, and Medical (ISM) band for wireless
connectivity. WirelessUSB uses 78 of the available channels and splits the 78 channels into 6 chan-
nel subsets consisting of 13 channels each. The channel subsets are used by each network to mini-
mize the probability of interference from other WirelessUSB systems (see the Channel Selection
Algorithm section for more details). A designated channel subset is used during Bind Mode (along
with an associated pseudo-noise code) in order to enable all WirelessUSB devices to effectively
communicate during this procedure.
A.3.2
Pseudo-Noise Codes
Pseudo-noise codes (PN codes) are the codes used to achieve the special matched filter character-
istics of DSSS communication. Certain codes referred to as 'multiplicative codes' are used for Wire-
lessUSB 2-way communication. These codes have minimal cross-correlation properties, meaning
they are less susceptible to interference caused by overlapping transmissions on the same channel.
The length of the PN code results in different communication characteristics. Higher data rates are
achieved with 32-chips/bit PN codes, while 64-chips/bit PN codes allow a longer range. The number
of frequency/code pairs is large enough to comfortably accommodate hundreds of WirelessUSB
devices in the same space. Each bridge/HID pair must use the same PN code and channel in order
to communicate.
A.3.3
Chip Error Correction
In the presence of interference (or near the limits of range), the transmitted PN code are often
received with some PN-code chips corrupted. DSSS receivers use a data correlator to decode the
incoming data stream. WirelessUSB LP supports a separate SOP and Data threshold. The RDK
uses an SOP threshold of '4'. The data threshold is set to the default value of '4'.
