Robust Detection of Packet Types

• US 20070253499A1
• Filed: 12/29/2006
• Published: 11/01/2007
• Est. Priority Date: 01/26/2006
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Batra, Anuj, Hosur, Srinath, Waters, Deric Wayne
• Granted Patent: US 7,957,474 B2
• Time in Patent Office: Days
• US Class Current: 375/260
• CPC Class Codes: H04L 27/0012 arrangements for identifying the type of modulation
• CPC Class Codes: H04L 27/22 Demodulator circuits; Receiver circuits
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

Disclosed herein is a system and method for determining the presence of rotated-BPSK modulation. In addition, disclosed herein is a system and method for determining if a received packet is a Legacy, Mixed-Mode, or Green-Field packet in accordance with the determination of the presence of rotated-BPSK modulation. The presence of a Green-Field packet may be determined by detecting if additional tones are being excited in an LTF symbol of the received packet and/or if a SIG field symbol following the LTF symbol is modulated by rotated-BPSK. The presence of a Mixed-Mode packet may be determined by detecting if the first four bits of the SIG field symbol following the LTF symbol are [1 1 0 1] and/or detecting if a symbol following an L-SIG symbol is modulated by rotated-BPSK. The presence of a Legacy packet may be determined by detecting if the symbol following the L-SIG symbol is modulated by BPSK.

Claims

1. A method for determining a modulation of an orthogonal frequency-division multiplexing (OFDM) symbol, the method comprising:

receiving the symbol;

multiplying the received symbol by the conjugate transpose of an overall channel response for the symbol;

applying a decision function to the result of the multiplication for a set of sub-carriers;

determining the modulation of the symbol in accordance with the result of the decision function; and

processing a packet in accordance with the determination of the modulation.

2. The method of claim 1, wherein the decision function is to sum a result of the multiplication and the determination is made based on a location of a result of the sum in the real-imaginary plane, wherein location is determined relative to boundary lines in the real-imaginary plane, where the boundary lines are set in accordance with one of:

arctangent of Im(s)/Re(s),

|Im(s)|ⁿ−

|Re(s)|ⁿ, or

|Re(s)|ⁿ−

|Im(s)|ⁿ,

wherein s is the result of the sum, and n is a variable.

3. The method of claim 2, wherein the modulation is determined to be BPSK if the location of the result is within the boundary lines and closer to the real axis than the imaginary axis in the real-imaginary plane, and the modulation is determined to be rotated-BPSK if the location of the result is within the boundary lines and closer to the imaginary axis than the real axis in the real-imaginary plane.

4. The method of claim 1, further comprising:

determining whether the packet is a Legacy, Mixed-Mode, or Green-Field signal in accordance with the determination of the modulation of the symbol, wherein the packet includes a plurality of OFDM symbols including the symbol.

5. The method of claim 4, wherein the packet is determined to be a Green-Field packet when the symbol is a SIG field symbol following an LTF symbol in the packet and the modulation of the symbol is determined to be rotated-BPSK.

6. The method of claim 4, wherein the packet is determined to be a Mixed-Mode packet when the symbol follows an L-SIG symbol in the packet and the modulation of the symbol is determined to be rotated-BPSK.

7. The method of claim 4, wherein the packet is determined to be a Legacy packet when the symbol follows an L-SIG symbol in the packet and the modulation of the symbol is determined to be BPSK.

8. The method of claim 1, wherein the set of sub-carriers are a subset of all of the sub-carriers in a channel and wherein the set of sub-carriers is determined based on dividing all of the sub-carriers in the channel into a plurality of groups, a first criteria is applied to each group of sub-carriers to produce a first set of sub-carriers that includes all of the sub-carriers that meet the first criteria for all of the groups, and a second criteria is applied to the first set of sub-carriers to produce the set of sub-carriers that includes all of the sub-carriers from the first set of sub-carriers that meet the second criteria.

9. The method of claim 1, wherein the decision function is:

10. The method of claim 1, wherein the decision function is:

P—

N,

where P is the number of elements in the set {k|kε

S and |x_k|>

β

}, N is the number of elements in the set {k|kε

S and |x_k|<

|}, β

is any of |S|/2, −

|S|/2,

11. An apparatus comprising:

at least one receiver configured to receive a packet of data and to determine whether the packet is a Legacy, Mixed-Mode, or Green-Field packet and to process the packet in accordance with the determined type of packet.

12. The apparatus of claim 11, wherein the receiver determines the packet is a Green-Field packet by detecting that additional tones are being excited in an LTF symbol of the received packet and/or by detecting that a SIG field symbol following the LTF symbol is an HT-SIG symbol.

13. The apparatus of claim 11, wherein the receiver determines the packet is a Mixed-Mode packet by detecting that the first four decoded bits of a SIG field symbol following an LTF symbol of the received packet are [1 1 0 1] and/or detecting that a symbol following an L-SIG symbol of the received packet is an HT-SIG symbol.

14. The apparatus of claim 11, wherein the receiver determines the packet is a Legacy packet by detecting that a symbol following an L-SIG symbol of the received packet is not an HT-SIG symbol.

15. The apparatus of claim 11, wherein the receiver determines whether the packet is a Legacy, Mixed-Mode, or Green-Field packet in accordance with a type of modulation by applying a decision function to a symbol on a subset of sub-carriers in a channel, the decision function is:

16. A system comprising:

a programmable processor configured to determine whether a received packet is a Legacy, Mixed-Mode, or Green-Field packet and further configured to process the packet in accordance with the determination, the determination comprising;

applying a decision function to a SIG field symbol following an LTF symbol of the packet for a subset of all of the sub-carriers in a channel to determine a type of modulation for the SIG field symbol following the LTF symbol;

determining that the packet is a Green-Field packet if the type of modulation for the SIG field symbol following the LTF symbol is rotated-BPSK;

if the packet is not a Green-Field packet, applying a decision function to a symbol following an L-SIG symbol of the packet for a subset of all of the sub-carriers in a channel to determine a type of modulation for the symbol following the L-SIG symbol;

determining that the packet is a Mixed-Mode packet if the type of modulation for the symbol following the L-SIG symbol is rotated-BPSK; and

determining that the packet is a Legacy packet if the type of modulation for the symbol following the L-SIG symbol is BPSK.

17. The system of claim 16, wherein the subset of sub-carriers includes any of the sub-carriers in the channel that meet a criteria |a−

b|/c≧

α

wherein the parameters a, b, c, and α

are chosen to balance performance and complexity of the decision function.

18. The system of claim 17, wherein the parameter a includes any of |x_k|, ∥

h_k∥

²/σ

_k², a, or |x_k|, where a is a constant, the parameter b includes any of h_k*A_kh_k, 0, 0, or mean_{i=1 . . . T}(|x_k|) respectively, the parameter c includes any of h_k*A_kh_kσ

_k, 1, 1, 1 respectively, and the parameter a includes any of g(a_i,b_i,c_i), where g(a_i,b_i,c_i)>

g(a_j,b_j,c_j) is true for at least K values of j, and 0≦

K≦

T, or g(a_i,b_i,c_i), where g(a_j,b_j,c_j)>

g(a_i,b_i,c_i) is true for at least K values of j, and 0≦

K≦

T, and where x_k is one of;

arctangent of Im(y_k,l)/Re(y_k,l),

|Im(y_k,l)|ⁿ−

|Re(y_k,l)|ⁿ, or

|Re(y_k,l)|ⁿ−

|Im(y_k,l)|<sup>n−

,</sup>

where y_k,l is a result of multiplying an l-th symbol that is received on a k-th sub-carrier by a conjugate transpose of the k-th sub-carrier'"'"'s channel response, and were n is a variable.

19. The system of claim 16, wherein the decision function for the SIG field symbol following the LTF symbol or the symbol following the L-SIG symbol is:

P−

N,

where P is the number of elements in the set {k|kε

S and |x_k|>

β

}, N is the number of elements in the set {k|kε

S and |x_k|<

β

}, β

is any of |S|/2, —

|S|/2,

20. The system of claim 16, further comprising:

a computer readable media configured to store instructions, wherein the programmable processor is configured to execute the instructions for implementing the determination.