Carrier aggregation (CA) is introduced in 3GPP LTE to meet the demand for further increased bitrates. While LTE Rel-10 supports simultaneous reception of two carriers either in contiguous intra-band or in inter-band CA configuration, the upcoming LTE Rel-11 will add support for non-contiguous (NC) carriers within bands. Supporting NC CA in handsets is a demanding challenge for several reasons. Foremost, the total bandwidth spanned by the carriers may be several times the bandwidth of the individual carriers, possibly spanning an entire band with interfering signals between desired carriers. Furthermore, the distance between TX and RX carriers will vary and worse, may be much smaller than the fixed duplex distance for LTE Rel-8 and W-CDMA single carrier operation [2][3][4][5].This paper presents a single-chip receiver supporting both NC intra-band and inter-band CA with the capability to receive up to three carriers simultaneously. The receiver block diagram is illustrated in Fig. 19.5.1. Using two configurable RF frontends the receiver spans frequencies from 700 to 1500MHz in low-band and 1800 to 2700MHz in high-band. The baseband section contains three IQ baseband paths. Each contains a pair of channel-select filters and ADCs capable of handling one LTE carrier with a bandwidth of 5, 10, 15, or 20MHz. The RF front-ends are connected to a switch unit that allows the signal from each frontend to be routed directly to one of the baseband paths to operate as a regular homodyne receiver for single-carrier reception. Inter-band CA reception with two carriers is supported by configuring the receiver as two parallel homodyne paths operated at different RF frequencies. Intra-band CA reception is achieved by double conversion where one of the RF front-ends first downconverts both the carriers to an intermediate frequency (IF) equal to half the carrier separation. The RF mixer output signal is then routed to the IF circuit where the two carrier components are separated and downconverted to baseband. Since the two intraband carriers only occupy one RF front-end and two baseband paths, a third carrier can be downconverted using the other RF front-end and the remaining baseband path as a homodyne receiver.

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When a gas mixture containing carbon dioxide (CO2) is inhaled by an individual, alveolar ventilation is increased. This ventilatory response to CO2 highlights the relationship between the increase in exercise ventilation and the increase in alveolar CO2 pressure (paCO2). This response is mediated centrally by brainstem chemoreceptors in the medulla and, to a lesser extent, peripherally by the carotid and aortic bodies. However, the response of increased breathing to rising paCO2 varies markedly among individuals. The responses to CO2 rebreathing have been investigated by a variety of research groups for different reasons. The range of responses by children and adults including: gender differences; responses by athletes; the relationship between age and body size; and whether it is an acquired or inherited response have been studied. The following is a summary of these different aspects of CO2 rebreathing has been complied to assist researchers studying any single or multiple facets of the area.

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