Increased discrete point processing in an OFDM communication system

Methods and apparatus provide increased symbol length with more subcarriers in a fixed-bandwidth system. The subcarriers spacing may be reduced to provide increased symbol length and enable higher throughput. In one implementation, a system compatible with the IEEE P802.11n proposal can use 128 subcarriers in 20 MHz operation to provide increased throughput in lower-bandwidth channel operation.

FIELD

Embodiments of the invention relate to wireless communication systems, and more particularly to wireless, multi-carrier communication systems.

BACKGROUND

An OFDM (orthogonal frequency division multiplexing) system provides enhanced operation over existing Wi-Fi (Wireless Fidelity) wireless channels while maintaining a certain level of compatibility with existing Wi-Fi (e.g., IEEE (Institute of Electrical and Electronics Engineers) Std. 802.11a, 1999 (the “802.11a standard”), IEEE Std. 802.11g, Jun. 27, 2003 (the “802.11g standard”), which may collectively be referred to herein as “existing Wi-Fi”) systems. Next generation Wi-Fi developments seek to increase the capabilities of wireless local area networks (WLANs), and may be based on, or otherwise compatible with the proposal IEEE P802.11n, May 2005 (referred to herein as “next-generation Wi-Fi”).

The development of both existing and next-generation Wi-Fi networking aims to increase data throughput. The main enabling technologies for next-generation Wi-Fi systems to achieve increase data throughput are the use of several existing Wi-Fi channels in parallel, and the use of MIMO (multiple-input, multiple-output) techniques. However, another goal of next-generation Wi-Fi networking is backward compatibility at the physical layer (PHY) with existing Wi-Fi systems. Backward compatibility reduces or prevents the destruction of operation of existing (legacy) Wi-Fi networks by the introduction of MIMO OFDM. Using the MIMO technique is not in conflict with existing Wi-Fi PHY protocol, enabling both existing and next-generation Wi-Fi systems to coexist.

One problem with exploiting several existing Wi-Fi channels in parallel is that backward compatibility traditionally requires that the use of multiple frequency channels only be allowed in a “pure” scenario, where the network consists only of next-generation Wi-Fi devices. In a “mixed” scenario, where the network includes both existing and next-generation Wi-Fi devices, only the use of single frequency channels is allowed, significantly reducing the potential throughput of next-generation devices in mixed networks. Specifically, existing Wi-Fi systems assume the use of 64 subcarriers (52 of which are used for data transmission) in 20 MHz channels, whereas next-generation devices can use 128 subcarriers (108 of which are used to transmit data) in 40 MHz channels. The next-generation devices are thus required to significantly reduce potential throughput for purposes of backward compatibility.

DETAILED DESCRIPTION

As used herein, references to one or more “embodiments” are to be understood as describing a particular feature, structure, or characteristic included in at least one implementation of the invention. Thus, phrases such as “in one embodiment” or “in an alternate embodiment” appearing herein describe various embodiments and implementations of the invention, and do not necessarily all refer to the same embodiment. However, they are also not necessarily mutually exclusive. Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.

An OFDM (orthogonal frequency division multiplexing) system provides enhanced operation over existing Wi-Fi (Wireless Fidelity) wireless channels while maintaining a certain level of compatibility with existing Wi-Fi (e.g., IEEE (Institute of Electrical and Electronics Engineers) Std. 802.11a, 1999 (the “802.11a standard”), IEEE Std. 802.11g, Jun. 27, 2003 (the “802.11g standard”), which may collectively be referred to herein as “existing Wi-Fi”) systems. An enhanced OFDM system as described herein provides enhanced operation over existing Wi-Fi and next-generation Wi-Fi systems (e.g., proposal IEEE P802.11n, May 2005 (the “next-generation Wi-Fi”)). Because the techniques described herein can be implemented in a manner compatible with next-generation Wi-Fi, a system using the enhancements suggested herein may also be referred to as “next-generation Wi-Fi.” One implementation of embodiments of the invention may include use as a part of or as an enhancement to the 802.11n proposal. Reference is made herein to 802.11a/g systems and 802.11n systems, which is to be understood as examples of system in which the techniques described herein may be used. Embodiments of the invention could apply to other multi-carrier communication systems. The standards and proposals described as examples herein may be considered to be wireless communication protocols, having definitions or descriptions or suggestions to set forth how components of the system should interact, and how the components may function. The descriptions herein may apply to other wireless communication protocols.

The frame starts with the portion of legacy (802.11a/g) frame necessary for legacy devices to detect the packet transmission and for both legacy and 802.11n devices to perform necessary acquisition and estimations such as frequency offset, timing, signal strength, etc.

Next-generation Wi-Fi systems assume the use of a total of 64 subcarriers (52 of which are used for data transmission) in 20 MHz channels, and a total of 128 subcarriers (108 of which are used to transmit data) in 40 MHz channels. Next-generation Wi-Fi systems further assume the use of 40 MHz channels in a “pure” scenario, where a network includes only of next-generation Wi-Fi devices, or devices compatible with the 802.11n standard. In a “mixed” scenario where a network includes both next-generation and existing Wi-Fi devices, existing OFDM systems assume that devices compatible with 802.11n operate in 20 MHz channels using 64 subcarriers, rather than the 40 MHz channels defined in the 802.11n proposal.

The lower channel widths (20 MHz) used for compatibility with existing Wi-Fi devices in mixed systems were thought to render next-generation Wi-Fi devices incapable of utilizing the higher subcarrier count (128), higher throughput mechanisms available when used in higher channel widths (e.g., 40 MHz). Modifications of the 802.11n standard allow the use of 128 subcarriers in 20 MHz channels. The subcarrier spacing can be reduced, and more subcarriers introduced to increase the length of a communication symbol. In one embodiment subcarrier spacing is reduced by half, and the number of subcarriers is doubled, to effectively double an OFDM symbol length. In one embodiment, an OFDM guard interval can be reduced in length to further improve spectrum use and increase throughput.

FIG. 1is a block diagram of an embodiment of a MIMO-OFDM transmitter architecture. System100provides an architecture in which an orthogonal frequency division multiplexed (OFDM) system as described herein can operate. With the exception of differences mentioned herein, the architecture of system100may be identical or similar to that described in IEEE P802.11n, section 11 “MIMO-OFDM HT PHY Specification.” A forward error correction (FEC) encoder102can provide initial encoding of a signal to be transmitted. For convolutional code (CC) architectures, the mother code rate is ½. Encoded bits may optionally be punctured at puncture104to produce code rates of ⅔, ¾, or ⅚. For low-density parity-check (LDPC) architectures, the different code rates are produced within the encoder and external puncturing may not be necessary. Following the encoding/puncturing of bits, the signal is forwarded to spatial stream parser110to separate bits of an output signal stream among multiple transmit paths, which could be for multiple separate channels over which the signal will be sent.

The separate portions of the output stream are processed through frequency interleavers112and122. For each spatial stream, blocks of bits that map to a single OFDM symbol are interleaved by frequency interleavers112and122. The interleaving may be referred to as “frequency interleaving” as bits are interleaved across subcarriers. In addition to frequency interleaving, frequency interleavers112and122may also interleave subcarrier QAM constellation points (for example, to provide increased robustness against channel errors. The number of subcarriers may be different than a standard number of subcarriers expected, or defined by a standard or proposal. For example, 128 subcarriers may be used in a scenario defined to use 64 subcarriers. Interleaving may be across all available subcarriers. The combination of spatial stream parser110and frequency interleaver112results in space-time interleaving.

The elements/components ofFIG. 1can be considered to be coupled with or to each other as depicted in the figure. The coupling of components may include physical coupling, communicative coupling, and/or electrical coupling. Thus, coupling may refer to direct physical interconnection, as well as indirect interconnection.

Note that multiple frequency interleavers and QAM mappers are possible in system100. Two are shown, but the number could be greater. Thus, system100supports multiple-input multiple-output (MIMO). MIMO encoder130provides a first operation in an antenna map transformation. Multiple inputs can come into MIMO encoder130, which sends streams out multiple paths to multiple antennas. The two paths may have a 128 point Inverse Discrete Fourier Transform (IDFT)132,142, an insert guard interval (GI)134,144, a digital to analog converter (DAC)136,146, and an analog and radio frequency (RF) 20 MHz converter138,148. In one embodiment the IDFT includes a number of points other than 128 (e.g., 256) that may be a multiple of 128 (the number of subcarriers). These respective paths provide information to transmit out antennas152and154.

Transmit antennas152-154may be the same antennas used for receiving incoming signals. Transmit antennas152-154may be any type of antenna (e.g., dipole, dish, omnidirectional, etc.), and may be an array of antenna elements. After processing by MIMO encoder130, antenna streams can be transformed from the frequency domain to the time domain by 128 point IFFTs132and142. The increase in transform points allows for higher throughput in system100. The signal can be further formed by the addition of a guard interval by insert GI134and144. Digital conversion to analog signals may be performed by DACs136and146, and the analog signals are then processed by analog and RF processors138and148to generate signals of legacy spectrum width of 20 MHz to be applied to transmit antennas152-154.

FIG. 2is a block diagram of an embodiment of a MIMO-OFDM receiver architecture. In one embodiment, the operation of the elements ofFIG. 2is the reverse of the operations ofFIG. 1. A signal is received at one or more antennas, and processed by analog and RF processors202and212operating in legacy spectrum width of 20 MHz. The received streams are converted with analog to digital converters (ADCs)204and214to produce a digital signal that includes discrete points representing the received signal. Remove GI blocks206and216represent the removal of guard intervals or other signal formatting to be removed prior to processing the received signals.

DFTs208and218(for example, a Fast Fourier Transform (FFT)) transform the received signals from the time domain to the frequency domain to extract information relative to processing the received signals. DFTs208and218can be 128 point DFTs, even though the operation occurs in a 20 MHz channel. The extension of the symbols of the received streams allows better spectrum use than traditional64point DFTs in 20 MHz channels. 128 point DFTs208and218provide the transformed signal streams to MIMO decoder220to de-map the signals from the antennas and allow QAM de-mapping blocks222and232to determine symbols from the stream. Frequency de-interleavers224and234to extract the individual subcarriers of the streams, which are sent to spatial stream combiner240for construction of spatial streams.

If puncturing is used in system200, de-puncture242may be applied to produce a signal with a mother code rate for decoding by FEC decoder244. From FEC decoder244, the signal may be forwarded to upper layers of software for processing or other operation on the received signals. As with system100ofFIG. 1, system200ofFIG. 2may function the same or similar to a system defined in the IEEE P802.11n proposal, and operating with a 128-point DFT (FIG. 2) or IDFT (FIG. 1) in 20 MHz.

FIG. 3is a block diagram of an embodiment of a communication frame. Frame300represents one example of a frame that can be used in a system with extended symbol length according to any embodiment described herein. Frame300can be a PLCP (PHY (Physical) Layer Convergence Protocol) Protocol Data Unit (PPDU). Frame300includes a legacy portion to provide compatibility with legacy systems, or systems that operate at a lower bandwidth channel and a lower throughput (e.g., 802.11a/g systems). L-STF (Legacy Short Training Field)302, L-LTF (Legacy Long Training Field)304, and L-SIG (Legacy SIGnal field)306provide a frame portion necessary to legacy devices to detect packet transmission. These legacy fields also enable both legacy and other devices to perform acquisition and estimation operations (e.g., frequency offset, timing, signal strength, etc.), for participating in communication in a system.

128-subcarriers HT (High Throughput) frame portion310provides a high-throughput portion with 128 subcarriers in 20 MHz. HT frame portion310can include HT-STF312and HT-LTFs314-316for training to enable a receiving device to obtain the signal. HT frame portion310also includes HT-DATA fields322-324, which represent the information of frame300, with 128 subcarriers. HT-SIG (HT SIGnal field)308, enables an HT-compatible device to know that an HT signal is being transmitted and can indicate that a 128-subcarrier symbol is being transmitted. HT-SIG308can contain a “long symbol” bit indicating 128 subcarriers in 20 MHz. In the IEEE P802.11n proposal, no such bit exists in the HT-SIG field of the proposal, but one could be added by shortening the CRC (Cyclic Redundancy Check) field of the HT-SIG field to obtain an extra bit for use as a “long symbol” indicator. Other fields within the HT-SIG field could be modified to obtain an extra bit.

The following table represents one example embodiment of a possible system implementation according to the 128-subcarrier signal in 20 MHz as described herein:

FIG. 4is a block diagram of an embodiment of an addition circuit and an example of an operation of the addition circuit. Circuit400represents an additional circuit added to a system that could be used in devices that are enabled to operate with 2 different channel bandwidths, for example, 20 MHz and 40 MHz. Received signal412could be either a 20 MHz signal or a 40 MHz signal according to one system implementation. When mixed systems are employed and a system including circuit400operates in 20 MHz using a 128-point DFT, circuit400is activated. Received signal412is mixed in multiplier416with frequency shift signal414, which may be an offset of 10 MHz in the system described. Other frequency shifts are possible.

If the received signal is 20 MHz, the signal is processed with 20 MHz filter418and down-sampler420. If the received signal is 40 MHz (e.g., a pure system), 20 MHz filter418and down-sampler420are bypassed. Below circuit400, the processing of a received signal is depicted visually. PPDUs432and434represent 20 MHz signals that could be received. The PPDUs are frequency shifted and centered on a reference frequency (e.g., 0 MHz). The resulting shifted PPDU is shown as PPDU442. PPDU442can be filtered by 20 MHz filter450to remove unwanted or unintentional signal elements.

FIGS. 5A-5Care block diagrams of embodiments of OFDM symbols with guard intervals. With reference toFIG. 5A, a prior art embodiment of an OFDM symbol is shown with guard intervals, or symbol-spacing guard intervals. The OFDM symbol is a 64-point symbol with full guard intervals in 20 MHz, as would be used by next-generation Wi-Fi devices in mixed systems.

FIG. 5Brepresents an OFDM symbol that is extended to include twice as many points, thus providing a 128-point symbol. Even maintaining the guard interval duration as the prior art inFIG. 5A, doubling the number of subcarriers can provide an 11% throughput increase. In an embodiment according toFIG. 5C, a 128-point symbol is combined with a half-length OFDM guard interval, which can improve throughput by approximately 17.6%.

FIGS. 6A-6Bare block diagrams of embodiments of OFDM symbols with pilot subcarriers.FIG. 6Arepresents a prior art representation of a 64-point OFDM symbol with full pilot subcarrier overhead.FIG. 6Bprovides an increased 128-point OFDM symbol with reduced pilot subcarrier overhead. Reducing the pilot subcarrier overhead allows increased use of available spectrum for data transmission. When using 128 subcarriers instead of 64 subcarriers, the pilot subcarrier overhead can be reduced because the spectrum occupied by each subcarrier, and in particular by the pilot subcarriers, reduces proportionally to the subcarrier number increase, provided that the number of pilot subcarriers is the same. Thus, the pilot subcarrier spectrum can be reduced and consequently pilot subcarrier overhead reduced. Combining this decrease with a decrease of guard interval overhead provides increased spectrum for data throughput.

FIGS. 7A-7Bare block diagrams of embodiments of OFDM symbols with guard bands.FIG. 7Arepresents a prior art embodiment of a 64-point symbol with full guard bands, or frequency-spacing/separating guard bands. Note that guard intervals separate one symbol from the next (temporal separation). A guard band provides a frequency buffer zone to reduce the effects of out-of-band emission (frequency separation of channels). As shown with reference toFIG. 7B, using 128 subcarriers instead of 64 subcarriers allows a system to use part of the spectral resources occupied in traditional systems by the guard bands. When using 128 subcarriers, the guard band spectrum can be reduced proportionally to the increase in the subcarrier number to provide the same level of protection against the effects of out-of-band emission. Therefore the required guard band spectrum part reduces proportionally to the increase of the subcarrier number, and provides additional spectrum resources for data transmission.

In one embodiment108subcarriers of the 128 subcarriers are used for data transmission, which is more than twice the 48 data subcarriers of the 64 total subcarriers of existing Wi-Fi systems. Because the overhead of both pilot subcarriers and the guard frequency bands are reduced, the increase to 108 data subcarriers can be achieved, which can improve throughput by approximately 12.5%. With the use of 128 subcarriers, 108 of which are data subcarriers, the reduction in guard intervals, guard bands, and pilot subcarrier overhead, a system as described herein can provide approximately 32% throughput improvement over traditional next-generation Wi-Fi.

Techniques described herein can be performed on components that may include hardware, software, and/or a combination of these. Software to instruct a machine to implement the techniques herein, or software to use to manufacture a device to perform the techniques described herein may be provided via an article of manufacture by a machine/electronic device/hardware. An article of manufacture may include a machine accessible/readable medium having content to provide instructions, data, etc. The content may result in an electronic device performing various operations or executions described. A machine accessible medium includes any mechanism that provides (i.e., stores and/or transmits) information/content/instructions in a form accessible by a machine (e.g., computing device, electronic device, electronic system/subsystem, etc.). For example, a machine accessible medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.), as well as electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), etc. The machine accessible medium may further include an electronic device having code loaded on a storage that may be executed when the electronic device is in operation. Thus, delivering an electronic device with such code may be understood as providing the article of manufacture with such content described above. Furthermore, storing code on a database or other memory location and offering the code for download over a communication medium via a propagated signal may be understood as providing the article of manufacture with such content described above.