# High Performance Receiver Design for RX Carrier Aggregation
## 2 Receiver Architecture  
Figure 1 shows the concurrent transceiver architecture supporting N carriers. When RF carriers belong to the same band, it is called intra-band aggregation. If they come from different bands, then it is named inter-band aggregation. In the former case, they are defined as contiguous or non-contiguous when the channels are adjacent to each other or not, respectively. Support of N carriers does not necessarily mean that N transceivers are required. For instance, ideal software defined radio envisioned by [PERSON] [4] can support concurrent operation of N carriers with only a single transceiver. However, existing transceivers--specifically receivers in this paper--are limited in their RF operation frequency (RF bandwidth) as well as intermediate frequency (IF) (baseband bandwidth). Therefore, we consider a generic case where N transceivers are supporting N carriers simultaneously. CA allows for the full utilization of fragmented spectrum, and different types of CA come from the location of the fragmented spectrum to be combined. The design of the digital baseband is almost invariant to the different CA types. On the other hand, design complexity of radio-frequency  
Figure 1: Concurrent transceiver architecture supporting N carriers. transceiver depends heavily on the CA types [3]. Without losing generality, Figure 2 shows two different implementations of the receiver supporting two CA carriers. Figure 2a supports two inter-band carrier signals, and parallel signal paths are optimized for different frequency bands. For instance, RF1 and RF2 denotes the carriers at band 2 (1930-1990 MHz) and band 12 (729-746 MHz), respectively. Due to the limited RF and IF bandwidth of the circuitry, a single signal path receiver that supports both band 2 and band 12 simultaneously is not easily feasible, and we thus have to adopt separate signal paths. Figure 2b is the embodiment of a two CA receiver with the support of intra-band CA. Due to the limited IF spectrum coverage with the baseband circuits (e.g., variable-gain amplifier, filter, etc. ), N (= 2 in Figure 2) receiver signal paths are required to support both contiguous and non-contiguous intra-band CA. The RF bandwidth that each signal path can support is mainly dictated by the input matching network (IMN) bandwidth. All the low-noise trans-conductance amplifiers (LNTAs) discussed here are based on the inductor-degenerated common-source stage. The matching network utilizing the \(C_{gs}\) of the main transistor is based on the series RLC resonance circuit, where its quality (\(Q\)) factor is typically larger than 1. Passive voltage amplification due to the matching network is beneficial for the receiver with improved signal-to-noise ratio. On the other hand, the bandwidth is inversely proportional to the \(Q\)-factor (\(BW=\frac{\omega_{o}}{Q}\)). For instance, RF1 in Figure 2a has to support 729-960 MHz to practically receive the signals from band 12 (729-746 MHz) to band 8 (925-960 MHz) with the same receiver path. Then, maximum \(Q\) for RF1 LNTA can be derived as follows. \[Q<\frac{\omega_{o}}{BW}=\frac{\sqrt{729\cdot 960}}{960-729}=3.62 \tag{1}\]  
The limitation in the \(Q\)-factor applies to RF2 path as well. Practically, IMN does not possess high \(Q\)-factor larger than 5, and this value is acceptable with various 3 GPP (3 rd Generation Partnership Project) bands support. The realization in Figure 2a is no different from the conventional receiver. Recent advances in receiver architecture opt for the single-\(g_{m}\) receiver implementation [5; 6; 7; 8]. With single-gm approach, low-noise active block interfacing the antenna is called a low-noise trans-conductance amplifier, denoted as LNTA in Figure 2. As the name indicates, LNTA is driving the subsequent stage with a small signal current. Inter-band CA implementation does not alter the characteristics of the LNTA, but the intra-band CA requires the LNTA to deliver multiple outputs. Section 3 discusses the implementation issue of the LNTA in order to support intra-band CA.