Enhancing software defined radio (SDR) security using variations in gray coded mapping scheme in rectangular QAM

Secure communication includes means by which people can share information with varying degrees of certainty that third party cannot know what was said. This paper proposes a novel method for adding security in digital communication system through variations in Gray coded mapping scheme in typical rectangular QAM. This will be achieved by varying the constellation mappings where by each symbol will be mapped to a different amplitude and phase level in a constellation plot at each variation (hop) without breaking the rules of Gray coding i.e. only a single bit difference between adjacent symbols is maintained. The main focus will be on reconfigurable software defined radio (SDR) because the reconfiguration of mappings is very difficult to be achieved in hardware. It will be shown that there exist numerous mapping schemes similar in function to Gray coded mapping scheme. An important fact is that the application of security does not cause undesirable effects. That is to say that the Bit Error Rate (BER) vs. Signal to Noise Ratio (SNR) curves are not deteriorated and remain constant for each variation as explained in Section V. Secondly the use of bandwidth also stays constant unlike the Frequency Hopping Spread Spectrum (FHSS) technique where the security is achieved at the expense of increased bandwidth. In addition further security can be achieved if these mapping schemes are varied pseudo randomly after a specified period of time. The ideas of hopping period and the number of hopping frequencies as used in FHSS can also be applied in the proposed method with a little alteration in the terminology. For example the time interval during which the communication system will work on any one of possible mapping schemes is still termed as hopping period, however we are not dealing with hopping frequencies here, instead we are hopping between various constellation mappings. It remains obvious that the shorter the hopping period and the higher the number of hopped mappings, the more secure is the communication system.