Tuesday, July 12, 2016

RF Power Gating: A Low-Power Technique for Adaptive Radios

RF Power Gating: A Low-Power Technique for Adaptive Radios
In this paper, we propose a low-power technique, called RF power gating, which consists in varying the active time ratio (ATR) of the RF front end at a symbol time scale. This technique is especially well suited to adapt the power consumption of the receiver to the performance needs without changing its architecture. The effect of this technique on the bit error rate (BER) performances is studied for a basic estimator in the specific case of minimum-shift keying signaling. A system-level energy model is also derived and discussed to estimate precisely the power reduction based on the characteristics and the power consumption of each block. This model allows highlighting the different contributors of the power reduction. The BER results and the energy model are finally merged to determine the best ATR meeting the design constraints. Applying this technique to the IEEE 802.15.4 standard, this paper shows that an ATR of 20% is a good tradeoff to meet the packet error rate constraint while maximizing the energy reduction ratio. Using typical block power consumptions, an energy reduction ratio around 20% can be reached. Even better energy reduction ratios (60%) are also achievable when most of the blocks are power-gated. The proposed architecture of this paper analysis the logic size, area and power consumption using Xilinx 14.2.
Some techniques have been proposed toward the reduction of the RX power consumption, the main solution proposed by adaptive radios is to leverage the emitted power of the transmitter (TX) to adapt the TX/RX system to the quality of the communication channel and save a substantial amount of power at the TX level. However, saving power at the RX level is of major concern for some wireless sensor network or Internet of Things applications. For example, in a star network topology, a main node is usually used to manage the network and is required to broadcast some updates to all the end nodes. In those cases, the main node is usually supplied by the grid power, while the end nodes are battery powered (see agriculture field monitoring or smart meters applications), and it is more efficient to increase its TX power while reducing the RX power of the multiple end nodes. To do so, with narrow-band RXs, recent works mainly leverage the tradeoff between the power consumption of RX circuits and their linearity or noise factor (NF). While a few works use this tradeoff to design block circuits, extending it at the system level requires scalable devices and still belong to the simulation domain. Although the narrow-band RXs do not benefit from duty-cycle modulation as impulse-radio ultra wide band (IR-UWB) do, some recent works consider duty-cycling some parts of the narrow-band RXs. The baseband circuits are duty-cycled, both LNA and mixer are intermittently shut down. More recently, Pons et al. proposes to reduce the RX power consumption by leveraging the analog-to-digital converter (ADC) sampling frequency and by allowing sampling points as close as possible, enabling the use of power-gated ADC. Similarly, we propose to extend this power adaptability to the whole RF RX.

·        Power consumption is high

The RFPG technique is a power management technique based on shutting down RF modules inside a symbol time. This principle is widely used for digital circuits by enabling clock-gating or by disabling the parts of the design. The RFPG extends this principle to the RF front end.
Basically, as shown in Fig. 1 for the MSK modulation, inside a time symbol, the direction of the transmitted signal phase remains the same. Consequently, for a given noise environment, it should be possible to decode the data from samples of the received signal taken during a fraction of the symbol time at the expense of degraded performances. Assuming it is possible, the parts of the RF RX could be shut down outside this portion of the symbol time called the sampling window.
Fig. 1. Phase direction of a binary CPFSK modulation, where m is the modulation index. In the case of an MSK modulation, m = 0.5.
the shorter the sampling window is, the longer the RF components are turned OFF and then the less power is consumed. However, as explained in the rest of Section II, choosing a smaller sampling window degrades the sensitivity of the RX. In other words, considering the noise provided by the communication channel and the analog parts, the sampling window width must be bounded to ensure a certain BER under some specific signal-to-noise ratio (SNR) conditions.
RF Front End
This is deals with all the RF front-end components of the RX (see Fig. 2) placed before the ADC, including the LNA, the mixer, the filter, and the phase-locked loop (PLL). In the rest of this paper, these components are called analog parts. Each analog part, indexed by i, is represented by four parameters: the active and inactive currents drawn ION,i and IOFF,i , and the settling and shutting down times t↑,i and t↓,i . These times are defined in the following.
1)    t↑,i is the necessary time for the supplying current of an analog part i starting from IOFF,i to remain in a 2ρ(ION,i − IOFF,i)-wide interval centered in ION,i .
2)    t↓,i is the necessary time for the sinking current of the analog part i starting from ION,i to remain in a 2ρ(ION,i − IOFF,i)-wide interval centered in IOFF,i .
Fig. 2. Example of an RFPG zero-IF front-end architecture.
Analog-to-Digital Converter (ADC)
In this section, the relationship between the sampling time width tw and the consumption of the ADC is given. Using the work provided in that collects thousands of ADC characteristics since 1974, a model of the active power consumption of an ADC can be bounded by the Walden slope and the thermal slope expressed by
·        Power consumption is reduced

·        Modelsim
·        Xilinx ISE