## 4 Characterization of Real-Life Signals
### Signal Selection  
Real-life wireless technologies can be divided into two categories based on continuity in the time domain, namely the streaming and non-streaming technologies. Streaming technologies are mostly used in the licensed spectrum, representing technologies including DVB-T and the downlink signal of the Long-Term Evolution (LTE) technology used by the 4G cellular network; while non-streaming technologies are more often seen in the unlicensed spectrum, such as Wi-Fi, ZigBee and Bluetooth. For each of these technologies, its medium access control and carrier allocation method are briefly introduced.  
In terms of the medium access scheme, Wi-Fi and ZigBee belong to the same category, because they both use carrier-sense multiple access (CSMA) to avoid collision [34], leading to random bursty signals in the time domain. Instead of CSMA, Bluetooth uses the frequency hopping spread spectrum, and different Bluetooth piconets rely on different frequency hopping sequences to share the medium, causing random bursty transmission in both the time and frequency domain. Streaming technologies such as DVB-T and LTE do not pause for medium access control.  
Carrier allocation of single-carrier-based technologies is by definition constant. Hence, this paragraph only discusses OFDM-based technologies, i.e., IEEE 802.11a/g-compliant Wi-Fi, DVB-T and LTE (ZigBee uses Offset QPSK and Bluetooth uses GFSK, both being single-carrier modulation). Wi-Fi performs padding at the end of a packet to ensure that the payload size is multiples of the amount of data carriers. Similar to Wi-Fi, all carriers of OFDM symbols in DVB-T are constantly occupied; hence, there is no variation in carrier allocation for Wi-Fi and DVB-T. The LTE standard fragments the frequency-time plain into very fine resource blocks and resource elements (each resource block is 180 kHz wide and 0.5 ms long, consisting of 12 subcarriers and seven OFDM symbols, where a single carrier in an OFDM symbol is referred to as a resource element) [35]. Some resource blocks are periodically used for control and broadcasting purposes, whereas others are available to carry user data. There are 16 resource elements in a resource block used to transmit cell-specific reference signals, which are always present no matter if the resource block is allocated for data transmission or not. Thus, the LTE signal is modulated with a variable amount of carriers and transmitted in very fine and regular intervals.  
Based on the above description, we select Wi-Fi, LTE and DVB-T as the target technologies to be classified in this work. The motivations are as follows: (i) the three technologies are either different in carrier allocation or medium access control, which are suitable to be identified with RSSI distribution; (ii) these technologies are relatively wideband, which are more challenging to recognize for narrow-band devices, hence more meaningful to investigate. For differentiating multiple non-streaming technologies, such as Bluetooth, Wi-Fi, and ZigBee, information in the time domain should be involved, which is briefly illustrated by a case study in Section 4.3.2. Note that identification of technologies in ISM bands with time domain RSSI-based statistics are already proposed in [27; 28; 29]; therefore, it is not the focus of this work. The characteristics of the three selected signals are summarized below:  
* Wi-Fi (IEEE 802.11a): signal transmitted in random bursts, modulated with a constant amount of carriers;
* LTE: signal transmitted in very fine and regular intervals, modulated with a variable amount of carriers;
* DVB-T: signal transmitted continuously and modulated with a constant amount of carriers.