Signal modulation for secure communication

1. A method for generating a communication signal that is decodable according to a first mapping of a first data symbol to a first waveform having a leading edge that is indicative of the first waveform, and a second mapping of a second data symbol to a second waveform having a second leading edge that is indicative of the second waveform, the method comprising:

using a circuitry, generating a plurality of data symbols that includes the first data symbol and the second data symbol;

modulating, for the first data symbol, a first portion of the communication signal according to the first waveform;

modulating, for the second data symbol, a second portion of the communication signal according to a modified second waveform, the modified second waveform having a leading edge that is indicative of the first waveform;

transmitting the communication signal over a transmission medium to a remote device; and

authenticating the remote device based on a response from the remote device.

2. The method of claim 1 , further comprising a communications circuit, communicatively coupled with the circuitry, and of generating the modified second waveform by modifying the leading edge of the second waveform to include one or more characteristics indicative of the first waveform.

3. The method of claim 2 , wherein modifying of the leading edge of the second waveform includes modifying the leading edge to have a shape substantially similar to a shape of the leading edge of the first waveform.

4. The method of claim 1 , wherein

the communication signal is generated by modulating the communication signal using a set of waveforms including the first and second waveforms and one or more additional waveforms, each of the set of waveforms being mapped to a respective data symbol; and

the method further comprises, for at least one data symbol of the plurality of data symbols,

pseudo-randomly selecting a leading edge from a plurality of leading edges, and

modulating the communication signal using the waveform mapped to the data symbol modified to exhibit the selected leading edge.

5. The method of claim 1 , wherein the second waveform is an inversion of the first waveform, and wherein transmitting the communication signal over a transmission medium includes transmitting the communication signal, as modulated by the steps of modulating, over a transmission medium.

6. The method of claim 1 , further comprising the step of using circuitry to perform each of the above steps and wherein

the plurality of data symbols includes a set of response data generated as a function of, and in response to, a set of challenge data received from a radio frequency (RF) device; and

the transmitting of the communication signal includes transmitting the communication signal to the RF device.

7. The method of claim 6 , further comprising generating the response data by decrypting the set of challenge data according to a decryption key stored in a memory circuit.

8. The method of claim 7 , further comprising, at the RF device, determining a distance to the RF device based on an a difference between a first time at which the set of challenge data is transmitted by the RF device and a second time at which the communication signal is received by the RF device.

9. The method of claim 1 , wherein

the plurality of data symbols includes a set of challenge data;

the transmitting of the communication signal transmits the communication signal to an RF device over a wireless transmission medium; and

the method further comprises

receiving a second communication signal from the RF device, the second communication signal encoding a set of response data, and

determining a distance to the RF device based on an amount of time between the transmitting of the first-mentioned communication signal and the receiving of the second communication signal.

10. The method of claim 9 , further comprising:

determining whether or not the set for response data is a correct response to the set of challenge data; and

wherein the determining a distance to the RF device is performed in response to determining that the set of response data is a correct response to the set of challenge data.

11. An apparatus comprising:

a first circuit configured and arranged to generate a plurality of data symbols that includes a first data symbol and a second data symbol; and

a second circuit coupled to the first circuit and configured and arranged to generate a communication signal that is decodable according to a mapping of the first data symbol to a first waveform having a leading edge that is indicative of the first waveform, and a mapping of the second data symbol to a second waveform having a leading edge that is indicative of the second waveform, the second circuit including a modulation circuit configured and arranged to

modulate, for the first data symbol, a first portion of the communication signal according to the first waveform; and

modulate, for the second data symbol, a second portion of the communication signal according to a modified second waveform, the modified second waveform having a leading edge that is indicative of the first waveform; and

a third circuit coupled to the second circuit and configured and arranged to transmit the communication signal over a transmission medium to a remote device and to authenticate the remote device based on a response from the remote device.

12. The apparatus of claim 11 , wherein the second circuit includes a waveform generation circuit configured and arranged to generate the first waveform and the modified second waveform.

13. The apparatus of claim 12 , wherein the waveform generation circuit is configured and arranged to

generate the second waveform; and

modify the leading edge of the second waveform to include one or more characteristics indicative of the first waveform to generate the modified second waveform.

14. The apparatus of claim 13 , wherein the waveform generation circuit is configured and arranged to modify the leading edge of the second waveform to have a shape substantially similar to a shape of the leading edge of the first waveform.

15. The apparatus of claim 14 , wherein the waveform generation circuit is configured and arranged to modify the leading edge of the second waveform by replacing the leading edge of the second waveform with the leading edge of the first waveform.

16. The apparatus of claim 11 , wherein

the first circuit is configured to generate a set of response data included in the plurality of data symbols as a function of, and in response to, a set of challenge data received from a radio frequency (RF) device; and

the third circuit is configured and arranged to transmit the communication signal to the RF device.

17. The apparatus of claim 16 , wherein

the apparatus is a remote key fob for an automobile; and

the RF device includes a control system configured to engage one or more systems in the automobile in response to authenticating the communication signal.

18. The apparatus of claim 11 , wherein

the first circuit is configured to generate a set of challenge data included in the plurality of data symbols;

the third circuit is configured and arranged to transmit the communication signal to an RF device; and

the first circuit is further configured to:

receive a second communication signal including a set of response data from the RF device; and

determining a distance to the RF device based on an amount of time between the transmitting of the first-mentioned communication signal and the receiving of the second communication signal.

19. The apparatus of claim 18 , wherein

the apparatus includes a control system in an automobile, the control system being configured to engage one or more systems in an automobile in response to determining that the distance to the RF device is less than a threshold distance stored in a non-volatile memory; and

the RF device is a remote key fob.

20. The apparatus of claim 11 , wherein

the leading edge of the first waveform has a positive slope;

the leading edge of the second waveform has a non-positive slope; and

the leading edge of the modified second waveform has a positive slope.