The adaptive duplexer architecture requires a low isolation device to give some initial isolation (at least 20dB). The low isolation device should cover the required RF range (800-2200MHz) for software radio. As stated earlier in the chapter some of the components that can be used for this purpose are wideband circulators, directional couplers, low selectivity stripline or microstrip line filters and separate antennas (or crossed polarised antennas).
5.3.1 Wideband Circulator
The circulator is a passive device with 3 or more ports, where power is transferred from one port to the next in a prescribed order. In a 3-port circulator (Figure 5-3 (a)) power entering port 1 leaves port 2 while port 3 is decoupled, power entering port 2 leaves port 3 while port 1 is decoupled, and power entering port 3 leaves port 1 while port 2 is decoupled [135]. It is possible to use circulators to isolate the Rx from the Tx when Tx and Rx use the same antenna (Figure 5-3 (b)).
Figure 5-3 (a) A 3-port circulator (b) A circulator used as an isolation device in a transceiver. Port1 Port2 Port3 (a) (b) Tx Rx fRx fTx Circulator
5.3.2 Directional Coupler
A directional coupler is a 4 port device in which two transmission lines pass close enough to each other for energy propagating on one line to couple to the other line [136]. When one port is terminated internally, power coupled to the port is absorbed and not available to the user. This is called a single-directional coupler. If forward and reflected signals are allowed to be sampled simultaneously, i.e. no internal termination, it is called a bi-directional coupler. One advantage of this type of coupler is that a higher power termination can be selected to suit higher input power requirements [137]. The dual-directional coupler is constructed with two single-directional couplers. They can either be connected back-to-back in series, with the main line output of the forward coupler connected to the output of the second coupler, or integrated into one device with a single main line and two secondary lines [138]. For this project only single-directional couplers are used and hence directional couplers mean single-directional couplers.
Directional couplers are commonly used to both divide and combine signals. The basic function of a directional coupler is to operate on an input so that two output signals are available. A directional coupler separates signals based on the direction of signal propagation. These devices are used to unequally split the signal flowing in the mainline and to fully pass the signal flowing in the opposite direction [139]. Directional couplers can also be used to provide low isolation between the Tx and Rx.
5.3.3 Low Selectivity Stripline or Microstrip Filters
Stripline and microstrip duplexing filters have been discussed in Section 4.2.6. While it is difficult achieve a high level of isolation (40dB or over) over a larger frequency band, similar isolation over a smaller frequency band is not difficult. Therefore, it is possible to implement a low isolation (20dB) duplexer using low selectivity microstrip filters for multi-band applications to give an initial isolation for this adaptive duplexer architecture. It might be possible to make these tuneable to cater for wideband operation.
5.3.4 Separate Antennas
Separate antenna schemes use two element antennas; one for transmission only and the other for reception only. The duplex isolation in this case is due to the mutual coupling loss between the two antennas. High isolation is obtained by minimising the mutual coupling effect. Increasing the separation between two antennas, or locating them in a natural transmission null, can minimise the mutual coupling effect. Two co-linear dipoles are examples of transmission null locations. The amount of separation is limited by the physical size of the mobile handset. Omni directional radiation pattern in the horizontal plane is suitable for this design and the antennas should have a wide bandwidth. The dual antenna design in [122] achieved 29dB isolation in one handset.
Dual antenna systems can be more agile in their response to different frequencies. As a result they can be used to access multi-band systems based on different standards. Future mobile handsets should be compact in size and therefore separate antennas are less likely to be an attractive solution. Dual polarised antennas might be the solution for this case.
Since it is very flexible and inexpensive to design microstrip duplexers, they are a good candidate for the low isolation device. However the implementation of this low-isolated microstrip duplexer for multi-band software radio needs theoretical and experimental investigation and is outside the scope of this Ph.D. study.
In a similar vein, LC devices directly implemented on silicon provide a low footprint low isolation device. They can also be made tuneable by varicap and switched element designs. The Q of these devices is not high, <<20, and their design is outside the scope of this thesis.
The experimental prototype described in this thesis uses a commercially available wideband circulator as the low isolation device.
5.4
Conclusion
A novel duplexer architecture is proposed and described in this chapter. Adaptive duplexing eliminates many external components in the multi-band transceivers. This solution handles the desensitisation problem and receiver sensitivity problem due to Tx leakage in multi-band software radio front end.
The proposed duplexer architecture is based on a low isolation device and an adaptive cancellation unit. Some low isolation devices have been considered and wideband circulators or dual antenna systems are good low design effort choices. In the future, tuneable filters or microstrip devices might provide a better solution since they provide some additional level of selectivity against other interfering signals.
The proposed new algorithm uses a single cost function based on the superposition of squared errors. The algorithm is simple; uses low frequency error signals (non RF) and is of low computational complexity. The scheme is suitable for low power integrated design.