Obviousness

Obviousness Analysis of US Patent 8,374,096 under 35 U.S.C. § 103

This analysis identifies combinations of prior art references disclosed within the patent document that would render the claims of US patent 8,374,096 obvious to a person having ordinary skill in the art (POSA) as of the filing date (2005-11-21). The primary focus is on Independent Claim 1, which defines the core method.

Independent Claim 1: Key Elements

Independent Claim 1 describes a method for selecting antennas in a MIMO WLAN, comprising:

1. Receiving multiple transmitted sounding packets: Each packet corresponds to a different subset of antennas.
2. Estimating a channel matrix: For each subset of antennas, performed in the station.
3. Sending a frame including an HT control field to initiate selection: Sent after estimating the channel matrix.
4. Selecting a subset of antennas: According to the channel matrices.
5. Specific HT control field mechanism: The HT control field includes an MCS selection feedback (MFB) field. If an Antenna Selection Indicator (ASI) field is set to "1" or a Modulation Coding Scheme (MCS) Request Sequence Number (MRS) field is set to "111", then the MFB field is used for antenna selection, beam selection, or as a transmitter beam forming control (ASBFC) field. This ASBFC field includes a command subfield and a data subfield, where the data subfield indicates the number of the multiple sounding packets.

Identified Prior Art and Background Knowledge

The patent document itself provides crucial insights into the state of the art at the time of filing:

• IEEE 802.11n Standard (WLAN): This standard, incorporated by reference, proposes a fast link adaptation control (LAC) mechanism at the MAC layer for supporting MIMO training requests and exchange of link adaptation information. It describes that LAC functionality can be realized by an HT Control frame or an HT Control Field within any MAC layer frame. The LAC frame includes a MAC header, a LAC mask (with RTS, CTS, TRQ, MRQ, MFB, and reserved bits 126), and an MCS feedback field 130. Similarly, the prior art HT Control Field includes a LAC field with MRQ, MRS (MRQ sequence number), MFS (MFB sequence number), and an MFB field functioning as MCS feedback. The patent explicitly notes that the prior art HT control field "does not define the bit combination of '111' in MRS".
• General MIMO Antenna/Beam Selection: The patent's background clearly states that antenna/beam selection is a known technique to increase system capacity and diversity while reducing hardware complexity and cost by selecting a submatrix from a complete channel matrix. This process conventionally requires estimating the complete channel matrix by sending "training (sounding) frames".
• U.S. patent application Ser. No. 11/127,006 “Training Frames for MIMO Stations,” filed by Andreas Molisch, Jianxuan Du and Daqing Gu on May 11, 2005: This reference (by some of the same inventors) describes "a long sequence of training frames... transmitted from a receive station to a transmit station, and in response the transmit station transmits a short sequence of training frames so that both the transmit and receive station can perform channel estimation and antenna selection". This explicitly teaches using multiple training frames for channel estimation and antenna selection.
• Problem of Overhead: The patent identifies that conventional explicit signaling for antenna selection in the physical (PHY) or media access (MAC) layer using training frames introduces "additional overheads" that are "undesirable due to practical limitations." It suggests that a "slowly varying WLAN channel environment can advocate a more efficient antenna/beam selection training scheme which requires little or no changes in the MAC and PHY layers."

Obviousness Combination and Rationale

A person of ordinary skill in the art (POSA) in MIMO wireless LANs, faced with the known problems of overhead in antenna selection training and the desire for more efficient MAC-layer control, would have been motivated to combine the teachings of US patent application Ser. No. 11/127,006 (Molisch et al.) with the IEEE 802.11n standard (including its LAC and HT Control Field specifications).

Rationale for Combination:

1. Multiple Sounding Packets for Channel Estimation and Selection (Claim Element 1 & 2): Molisch et al. explicitly teaches the use of "a long sequence of training frames" for performing "channel estimation and antenna selection." A POSA, seeking to implement this concept within a WLAN, would naturally consider using standard "sounding packets" (as known in 802.11n for MCS adaptation and TXBF training). To enable selection among different antenna subsets, it would be an obvious design choice to associate each of these multiple sounding packets with a different subset of antennas. This builds directly on the Molisch et al. teaching of multiple training frames for selection, extended to the known practice of evaluating individual antenna configurations. The patent itself notes a prior art method that uses a "single PHY layer training frame... and different antenna subsets are subsequently connected to the RF chains for this single training frame," implying that the concept of different antenna subsets is known in the context of training. The estimation of a channel matrix for each subset is a fundamental and well-known part of any antenna selection scheme.

2. MAC Layer Signaling via HT Control Field (Claim Element 3): The IEEE 802.11n standard provides the HT Control Field as a MAC layer mechanism specifically for "fast link adaptation control" and "MIMO training requests and exchange of link adaptation information." Given the stated problem of high overhead with conventional PHY/MAC layer methods and the goal of an efficient scheme requiring "little or no changes in the MAC and PHY layers", a POSA would be strongly motivated to utilize this existing MAC layer signaling framework to initiate and control the antenna selection training process taught by Molisch et al.

3. Repurposing MFB with ASI/MRS=111 for ASBFC (Claim Element 5): The 802.11n standard's LAC mask field (part of a LAC/HT frame) includes "three bits 126 are reserved", and the HT Control Field has "two other unused fields 1202 - 1203 which may be dedicated for other HT control features". The standard also defines the MRS field, where the bit combination "111" is not defined in the prior art. A POSA, needing to convey antenna selection information without significantly increasing frame size or creating new MAC frame types, would find it obvious to repurpose an existing feedback field (like MFB, used for MCS feedback) by using an available reserved bit (acting as an ASI) or an undefined bit combination (such as MRS=111) as a flag. This is a common and logical protocol extension technique to multiplex different types of control information within an existing frame structure, particularly when the MFB field might not be simultaneously needed for MCS feedback.

4. ASBFC with Command and Data Subfields (Claim Element 5): Once the MFB field is repurposed as an ASBFC field, subdividing it into command and data subfields is a fundamental and obvious aspect of control message design. This allows for conveying specific instructions (commands) related to antenna/beam selection (e.g., "initiate transmit selection," "feedback selection result") along with associated parameters (data).

5. Data Subfield Indicating Number of Sounding Packets (Claim Element 5): For a training sequence involving multiple sounding packets (as taught by Molisch et al.), the recipient needs to know how many packets to expect to correctly process the entire channel estimation. Including this crucial piece of information in the data subfield of the ASBFC message is a straightforward and obvious implementation detail for a POSA designing a robust and coordinated training protocol.

Conclusion:

The combination of the Molisch et al. '006 patent application, which teaches the use of multiple training frames for antenna selection and channel estimation, with the established IEEE 802.11n standard's MAC layer signaling mechanisms (HT Control Fields, LAC frames, and their extensible nature via reserved bits and undefined bit combinations), would have rendered the method of Claim 1 obvious to a POSA. The motivation to combine these references would be driven by the recognized need for more efficient, low-overhead antenna selection training processes that operate primarily at the MAC layer, without requiring modifications to the PHY layer. The specific signaling details within the HT Control Field, such as repurposing the MFB field and using command/data subfields to convey the number of sounding packets, represent obvious engineering choices for implementing such a combined system.