Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2002-05-10
2004-06-22
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S338000, C370S342000
Reexamination Certificate
active
06754195
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communications, and more particularly to a wireless communication system configured to communicate using a single-carrier to multi-carrier mixed waveform configuration.
BACKGROUND OF THE INVENTION
The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 standard is a family of standards for wireless local area networks (WLAN) in the unlicensed 2.4 and 5 Gigahertz (GHz) bands. The current 802.11 b standard defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Megabits per second (Mbps). The 802.11b standard uses direct sequence spread spectrum (DSSS) with a chip rate of 11 Megahertz (MHz), which is a serial modulation technique. The 802.11a standard defines different and higher data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band. It is noted that systems implemented according to the 802.11 a and 802.11b standards are incompatible and will not work together.
A new standard is being proposed, referred to as 802.11 g (the “802.11 g proposal”), which is a high data rate extension of the 802.11b standard at 2.4 GHz. It is noted that, at the present time, the 802.11 g proposal is only a proposal and is not yet a completely defined standard. Several significant technical challenges are presented for the new 802.11 g proposal. It is desired that the 802.11 g devices be able to communicate at data rates higher than the standard 802.11b rates in the 2.4 GHz band. In some configurations, it is desired that the 802.11b and 802.11 g devices be able to coexist in the same WLAN environment or area without significant interference or interruption from each other, regardless of whether the 802.11b and 802.1 g devices are able to communicate with each other. It may further be desired that the 802.11 g and 802.11b devices be able to communicate with each other, such as at any of the standard 802.11b rates.
A dual packet configuration for wireless communications has been previously disclosed in U.S. patent application entitled, “A Dual Packet Configuration for Wireless Communications”, Ser. No. 09/586,571 filed on Jun. 2, 2000, which is hereby incorporated by reference in its entirety. This previous system allowed a single-carrier portion and an orthogonal frequency division multiplexing (OFDM) portion to be loosely coupled. Loosely coupled meant that strict control of the transition was not made to make implementations simple by allowing both an existing single-carrier modem and an OFDM modem together with a simple switch between them with a minor conveyance of information between them (e.g., data rate and packet length). In particular, it was not necessary to maintain strict phase, frequency, timing, spectrum (frequency response) and power continuity at the point of transition (although the power step would be reasonably bounded). Consequently, the OFDM system needed to perform an acquisition of its own, separate from the single-carrier acquisition, including re-acquisition of phase, frequency, timing, spectrum (including multi-path) and power (Automatic Gain Control [AGC]). A short OFDM preamble following the single carrier was used in one embodiment to provide reacquisition.
An impairment to wireless communications, including WLANs, is multi-path distortion where multiple echoes (reflections) of a signal arrive at the receiver. Both the single-carrier systems and OFDM systems must include equalizers that are designed to combat this distortion. The single-carrier system designs the equalizer on its preamble and header. In the dual packet configuration, this equalizer information was not reused by the OFDM receiver. Thus, the OFDM portion employed a preamble or header so that the OFDM receiver could reacquire the signal. In particular, the OFDM receiver had to reacquire the power (AGC), carrier frequency, carrier phase, equalizer and timing parameters of the signal.
Interference is a serious problem with WLANs. Many different signal types are starting to proliferate. Systems implemented according to the Bluetooth standard present a major source of interference for 802.11-based systems. The Bluetooth standard defines a low-cost, short-range, frequency-hopping WLAN. Preambles are important for good receiver acquisition. Hence, losing all information when transitioning from single-carrier to multi-carrier is not desirable in the presence of interference.
There are several potential problems with the signal transition, particularly with legacy equipment. The transmitter may experience analog transients (e.g., power, phase, filter delta), power amplifier back-off (e.g. power delta) and power amplifier power feedback change. The receiver may experience AGC perturbation due to power change, AGC perturbation due to spectral change, AGC perturbation due to multi-path effects, loss of channel impulse response (CIR) (multi-path) estimate, loss of carrier phase, loss of carrier frequency, and loss of timing alignment.
SUMMARY OF THE INVENTION
A wireless communication system configured to communicate using a mixed waveform configuration is disclosed and includes a transmitter configured to transmit according to a mixed waveform configuration and a receiver configured to acquire and receive packets with a mixed waveform configuration. The mixed waveform includes a first portion modulated according to a single-carrier scheme with a preamble and header and a second portion modulated according to a multi-carrier scheme. The waveform is specified so that a channel impulse response (CIR) estimate obtainable from the first portion is reusable for acquisition of the second portion.
In one configuration, the transmitter maintains power, carrier phase, carrier frequency, timing, and multi-path spectrum between the first and second portions of the waveform. The transmitter may include first and second kernels and a switch. The first kernel modulates the first portion according to the single-carrier modulation scheme and the second kernel generates the second portion according to the multi-carrier modulation scheme. The switch selects the first kernel for the first portion and the second kernel for the second portion to develop a transmit waveform. In one embodiment, the first kernel operates at a first sample rate and the second kernel operates at a second sample rate. The first kernel may employ a single-carrier spectrum that resembles a multi-carrier spectrum of the multi-carrier modulation scheme.
The first kernel may employ a time shaping pulse that is specified in continuous time. The time shaping pulse may be derived by employing an infinite impulse response of a brick wall approximation that is truncated using a continuous-time window that is sufficiently long to achieve desired spectral characteristics and sufficiently short to minimize complexity. The first kernel may sample the time shaping pulse according to a Nyquist criterion. The average output signal power of the first kernel and the average output signal power of the second kernel may be maintained substantially equal. The first kernel may employ a first sample rate clock while the second kernel employs a second sample rate clock. In this latter case, the first and second sample rate clocks are aligned at predetermined timing intervals. Also, a first full sample of the multi-carrier modulation scheme begins one timing interval after the beginning of a last sample of the single-carrier modulation scheme.
The single-carrier signal from the first kernel may be terminated according to a windowing function specified for OFDM signal shaping defined in the 802.11a standard. The carrier frequency may be coherent between the first and second kernels. The carrier phase may be coherent between the first and second kernels. In one embodiment to achieve coherent phase, carrier phase of the second kernel multi-carrier signal is determined by carrier phase of a last portion of the second kernel single-carrier signal. The carrier phase of the second kernel multi-carrier signal may further be rotated by a corresponding one of a plural
Seals Michael J.
Webster Mark A.
Chin Wellington
Intersil America's Inc.
Schultz William
Stanford Gary R
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