Shaped load modulation in a near field communications (NFC) device
A method and apparatus is disclosed to compensate for overshoot and/or undershoot in a transmission sequence by shaping the transmission sequence according to a shaping envelope to lengthen its rise time and/or fall time to provide a modified transmission sequence. The shaping envelope may represent a trigonometric function, a polynomial function, a piecewise function or any other function that lengthens the rise time and/or the fall time of the transmission sequence. The modified transmission sequence adjusts an impedance of an antenna to load modulate a carrier wave that is inductively coupled to it.
1. A near field communications (NFC) device, comprising:
a controller configured to provide a transmission sequence to be transmitted to another communications device; and
an envelope shaping module configured to shape the transmission sequence according to a shaping envelope to provide a modified transmission sequence and to load modulate an antenna with the modified transmission sequence.
2. The NFC device of claim 1 , wherein the shaping envelope is a mathematical function that is applied to the transmission sequence.
3. The NFC device of claim 2 , wherein the mathematical function is a polynomial function.
4. The NFC device of claim 3 , wherein the polynomial function is represented as:
y
(
n
)
=
∑
n
=
0
K
a
n
x
(
n
)
n
,
where y(n) represents the modified transmission sequence, x(n) represents the transmission sequence, and a n represents a gain coefficient.
5. The NFC device of claim 2 , wherein the mathematical function is an inverse square function.
6. The NFC device of claim 5 , wherein the inverse square function is represented as:
y
(
n
)
=
1
x
(
n
)
2
,
where y(n) represents the modified transmission sequence and x(n) represents the transmission sequence.
7. The NFC device of claim 1 , wherein the envelope shaping module comprises:
an inverter configured to invert the transmission sequence to provide the modified transmission sequence; and
a multiplier configured to multiply the modified transmission sequence with itself to provide a squared transmission sequence,
wherein the squared transmission sequence is fed back to the inverter to provide an inverse square function.
8. The NFC device of claim 1 , wherein the envelope shaping module is configured to shape the transmission sequence such that no substantial overshoot and undershoot are present in the modified transmission sequence.
9. The NFC device of claim 1 , wherein the antenna includes an impedance, the impedance being configured to be adjusted by the modified transmission sequence to load modulate a carrier wave from another NFC device that is inductively coupled onto the antenna.
10. The NFC device of claim 1 , wherein the antenna includes a capacitor and an inductor,
wherein the inductor or the capacitor are completely discharged before the modified transmission sequence transitions from a binary zero to a binary one, and
wherein the inductor or the capacitor are completely charged before the modified transmission sequence transitions from the binary one to the binary zero.
11. The NFC device of claims 1 , wherein the envelope shaping module is further configured to lengthen a rise time or a fall time of the transmission sequence.
12. A method of communication in a near field communications (NFC) device, comprising:
generating, by the NFC device, a transmission sequence to be transmitted to another communications device;
shaping, by the NFC device, the transmission sequence according to a shaping envelope to provide a modified transmission sequence; and
modulating, by the NFC device, a carrier wave from another NFC device with the modified transmission sequence.
13. The method of claim 12 , wherein shaping comprises:
shaping the transmission sequence according to a mathematical function that is applied to the transmission sequence.
14. The method of claim 13 , wherein the mathematical function is a polynomial function.
15. The method of claim 14 , wherein the polynomial function is:
y
(
n
)
=
∑
n
=
0
K
a
n
x
(
n
)
n
,
where y(n) represents the modified transmission sequence, x(n) represents the transmission sequence, and a n represents a gain coefficient.
16. The method of claim 13 , wherein the mathematical function is an inverse square function.
17. The method of claim 16 , wherein the inverse square function is represented as:
y
(
n
)
=
1
x
(
n
)
2
,
where y(n) represents the modified transmission sequence and x(n) represents the transmission sequence.
18. The method of claim 12 , wherein shaping comprises:
inverting the transmission sequence to provide the modified transmission sequence;
multiplying the modified transmission sequence with itself to provide a squared transmission sequence; and
inverting the squared transmission sequence to obtain the inverse square function.
19. The method of claim 12 , wherein shaping comprises:
shaping the transmission sequence such that no substantial overshoot and undershoot are present in the modified transmission sequence.
20. The method of claim 12 , wherein modulating comprises:
adjusting an impedance with the modified transmission sequence to load modulate the carrier wave.
21. The method of claim 12 , wherein generating comprises:
encoding an amplitude of an information sequence to provide the transmission sequence.
22. The method of claim. 12 , wherein shaping comprises:
lengthening a rise time or a fall time of the transmission sequence.