Due to the aforementioned aspects, the interest in wireless IBFD communications as a duplexing method in bidirectional data transfer applications has been constantly increasing in the more recent years [62, 188, 204, 266]. Starting from [35], where some of the first experiments regarding bidirectional IBFD communications are reported, the research has been intensive. To first quantify the required amount of SI suppression in these generic IBFD transceivers, let us assume a maximum LTE user equipment (UE) transmit power of 23 dBm, and a sensitivity requirement of −90 dBm [69]. With these example system parameters, the SI should be cancelled in total by 113 dB to attenuate it to the level of the receiver noise floor, where it would still decrease the sensitivity by 3 dB. Although there is usually some physical isolation between the TX and RX chains, several active cancellation stages are clearly needed as it is extremely challenging to achieve such suppression levels by passive isolation mechanisms alone.
Starting from the first IBFD prototype implementations, the prevalent solution has been to utilize two active SI cancellation stages to suppress the SI sufficiently: cancellation in the analog/RF domain and then subsequent cancellation in the digital domain [P3–P5], [3, 19, 28, 29, 44, 59, 61–63, 103, 265]. The analog/RF cancellation is usually performed already before the actual RX chain to limit the total power entering the RX low-noise amplifier (LNA). In principle, the RF canceller subtracts the regenerated SI signal from the overall received signal in the RF domain and thereby ensures that the extremely high SI power does not saturate the LNA or damage any of the delicate components in the RX chain. However, due to the immense SI cancellation requirements, further cancellation is typically required in the digital domain. There, the original baseband transmit data is used to regenerate the residual SI, which is then subtracted from the overall digitized signal to suppress the remaining SI [P3, P4]. Complemented with the physical isolation between the transmitter and the receiver, this type of an overall cancellation solution has been shown to be sufficient for suppressing the SI below the receiver noise floor [P4, P5], [28, 29, 50]. It is also possible to omit either the RF or digital canceller but this requires advanced techniques for significantly increasing the amount of passive isolation [P5], [8]. These different cancellation solutions are covered in more detail in Section 2.3 below, while a more detailed review of the different IBFD prototypes and their corresponding SI cancellation capabilities is given in Chapter 5.
Stemming from these successful demonstrator implementations, there is also a wide body of more theoretical and fundamental research into the data transfer–oriented applications of wireless IBFD communications. For instance, the medium access control (MAC)–related aspects of networks consisting of IBFD devices have been widely analyzed, since the IBFD capability obviously affects the whole channel access procedure [12, 43– 47, 58, 78, 103, 147, 148, 174, 222, 226, 251, 271]. Hence, new solutions for MAC mechanisms are needed when IBFD transceivers are introduced. As a specific example, utilizing IBFD–capable transceivers in a network based on carrier sense multiple access with collision avoidance (CSMA/CA) naturally removes or alleviates the well-known
hidden node problem illustrated in Fig. 2.1a, where a collision occurs when two nodes
outside each other’s transmission range try to transmit data signals to the same receiving node at the same time. In particular, many of the IBFD MAC algorithms solve this
2.2 Modern Take on Full-Duplex: Bidirectional Data Transfer over the Same Time-Frequency Resource Transm ission r ange Collision (a) Transm ission r ange (b)
Figure 2.1: (a) The hidden node problem in a CSMA/CA network with HD devices and (b)
the same scenario with IBFD-capable devices, where no collision occurs.
issue by forcing also the receiving node to transmit either a data signal or a busy tone, facilitated by its IBFD capability [44, 47, 58, 78, 103, 174, 222, 271]. This type of a scenario is illustrated in Fig. 2.1b, where it can be observed that the channel is now reserved around both parties, ensuring that the collision avoidance mechanisms prevent further transmissions.
Furthermore, in a CSMA/CA-based network, the IBFD or STAR capability can also be used to sense the channel while transmitting, which enables much faster detection of a possible collision [147, 148, 226]. This obviously results in a higher average throughput in the network, illustrating that the IBFD technology can also provide performance improvements in the higher layers, in addition to increasing the physical layer spectral efficiency. It should also be noted that STAR is extremely useful also for cognitive radios, where the secondary users must cease their own transmission upon detecting a primary user initiating a transmission [5, 232, 236]. Being capable of continuously sensing the channel obviously decreases the level of interference that the secondary users cause to the primary users, resulting in more efficient and operational cognitive radio networks. Moreover, in order to decrease the overall deployment complexity of the IBFD networks, systems where only a subset of the transceivers are capable of IBFD operation have also been widely studied. A popular example of this is the system illustrated in Fig. 2.2 where the BS or the AN is IBFD capable, while the UEs or the clients are legacy HD devices [46, 60, 70, 73, 167, 171, 185, 217, 229, 254, 255, 272]. The benefit of this solution is that the BS can serve the UL and DL UEs simultaneously on the same time-frequency resource, while the potentially costly SI cancellation must be performed only in the BS. However, as also shown in Fig. 2.2, a downside of this type of a network is that the transmissions of the UL UEs will produce interference to the receiving DL UEs, and therefore this so-called inter-user-interference (IUI) must be mitigated by some means. There are already various solutions for addressing the IUI, such as assigning the UL and the DL UEs so that their mutual path losses are maximized [46, 60] or interference alignment [65, 114, 205], indicating that this issue can be alleviated at least to some extent. Altogether, a network where only the BS is IBFD capable is hence an
INBAND FULL-DUPLEX: BASIC PRINCIPLES AND ESSENTIAL SYSTEM MODELS UE UE UE UE IUI Access node
Figure 2.2: An illustration of a network where an IBFD-capable AN is using the same
frequency band for simultaneous DL and UL.
Access
node Backhaul node
UE UE UE UE UE UE
Figure 2.3: An illustration of a network where an IBFD-capable AN is using the same
time-frequency resource for DL, UL, and backhauling. Note that this type of a network suffers also from IUI between the DL and UL UEs and the backhaul node, but the different interference links are omitted from this figure to improve its readability.
intriguing commercial application for IBFD transceivers, as it will ensure a lower cost for the UEs while still improving the spectral efficiency via the simultaneous UL and DL transmissions.
A particular example of a network with an IBFD-capable AN is discussed, for instance, in [P7, 129, 131], [183, 237] and depicted in Fig. 2.3, where the AN serves the UL and DL UEs simultaneously on the same frequency band while also using the same frequency resources for backhauling the data wirelessly. Such wireless self-
backhauling is greatly beneficial, for instance, in densely deployed cellular networks,
where it would be very expensive to install wired backhaul links for all the individual cells [41, 91, 183, 217, 237, 268]. This type of a situation is a probable scenario in the future 5G networks [67, 98, 110]. As no additional spectral resources are needed for establishing the backhaul connectivity, such a network is spectrally very efficient while requiring no expensive cabling for the backhaul link. However, although omitted from