Ling Tian and Wenhu Xu
Long Term Evolution (LTE) third-generation (3G) cellular systems are designed for faster data transfers than earlier cellular networks, with uplink speed to 50 Mb/s and downlink speeds to 100 Mb/s. LTE systems promise high-speed data services as well as high-capacity voice and media (such as video) capabilities.1,2 The bandwidth of an LTE system is scalable, from 1.25 to 200 MHz per channel, to meet the needs of different network operators with different bandwidth allocations. By exploiting several novel technologies, including multiple-input, multiple-output (MIMO) antenna subsystems and orthogonal frequency division multiple (OFDM) modulation techniques, LTE systems are expected to provide greatly enhanced spectral efficiency compared to existing Third-Generation (3G) cellular networks.1 Specifications for LTE systems include both frequency-division-duplex (FDD) and time-division-duplex (TDD) techniques to separate uplink and downlink traffic.
The TDD variant of LTE will become widely used in China as that nation upgrades its communications networks. There are several unpaired frequency allocations for LTE TDD, unpaired frequency allocations being uplink and downlink bands sharing the same frequency and time multiplexing. For example, in PRC, the frequency-band allocations for LTE are 1880 to 1920 MHz, 2010 to 2025 MHz, and 2320 to 2370 MHz.1-3
A scanning receiver is an important part of any LTE TDD system, since it can indicate signal strength throughout the network coverage area. Such a receiver assists in optimizing and monitoring an LTE communications network, and helps to ensure maximum coverage, capacity, and quality of service (QoS) for that system. This report focuses on the design of a tri-band RF front end for an LTE TDD scanning receiver. The front end was assembled with ambitious goals in mind: fast scanning speed, high sensitivity, high measurement accuracy, small size, and low power consumption. A superheterodyne configuration was adopted for the LTE scanning receiver front end, since this approach was deemed suitable to meet the requirements of the LTE TDD standard. The three common receiver approaches available for LTE TDD use are direct-conversion receivers, digital intermediate-frequency (IF) receivers, and superheterodyne receivers. The superheterodyne receiver is widely used because of its wide dynamic range, excellent overall performance, lack of a DC offset, and minimal local oscillator (LO) leakage.
Figure 1 shows a block diagram for a tri-band RF front end for an LTE TDD scanning receiver, based on a superheterodyne receiver architecture. Key components in the RF front end are the low-noise amplifier (LNA), the RF switch, the RF filter, the RF amplifier, the frequency downconverter, the IF surface-acoustic-wave (SAW) filter, the variable-gain amplifier (VGA), the demodulator, and the dual-LO frequency synthesizer. The single-pole, four-throw (SP4T) switch is employed in the LTE TDD receiver front end to switch among three different frequency-band allocations.
This receiver front end differs from a traditional approach in the position of the RF filter and the LNA. In a conventional receiver scheme, the RF filter precedes the LNA. But in this LTE TDD scanning receiver front end, the LNA is placed in front of the RF filter. The advantages afforded by this approach will be detailed in the next section.
Sensitivity is one of the most significant performance parameters for an RF receiver front-end section. A receiver's sensitivity is generally accepted as a minimum input signal power level at the system's antenna port for which the system can achieve a given bit-error-rate (BER) performance level.4-6 Sensitivity performance is most strong impacted by the noise figure of the RF front end. The receiver's noise figure can be calculated by the numeric values of gain and noise figure, F, for the receiver's different active input stages and components.7 Equation 1 shows how to calculate the cascaded noise figure of a receiver based on the values of noise figure and gain for the different receiver stages, as well as the loss of the passive components:
Because a coaxial cable typically connects a scanning receiver to its antenna, the noise figure of the receiver system includes the loss of the cable, the loss of the switch, the loss of the RF filter, the noise figure and gain of the LNA, and the remaining cascade of components. In a conventional design, the receiver system includes the coaxial cable, switch, RF filter, LNA, and cascade of components in sequence. Based on Eq. 1and using values of 1 dB insertion loss for the coaxial cable, 1 dB insertion loss for the switch, 3 dB insertion loss for the RF filter, and 1 dB noise figure and 12 dB gain for the LNAthe noise figure of the scanning receiver will be more than 6 dB. This noise figure affects the sensitivity that is possible with a receiver.
In the new LTE TDD scanning receiver front end, the LNA is connected with the coaxial cable prior to the switch and RF filter, compensating for the insertion loss of the coaxial cable, switch, and filter. The noise figure for this new LTE TDD scanning receiver configuration is less than 3 dB. The performance of the novel LTE TDD scanning receiver front end was simulated by means of the Advanced Design System (ADS) software from Agilent Technologies, which yielded a simulation value of 2.38 dB for noise figure (Fig. 2). This agrees well with the results of a physical (test) analysis.
In the scanning receiver front end, a balanced-unbalanced (or balun) transformer is used to convert balanced electrical signals to unbalanced electrical signals, and unbalanced electrical signals to balanced electrical signals. Baluns are available as wire-wound components, inductive-capacitive (LC) components, transmission-line components, and microstrip baluns. An LC balun was used before the demodulator in the LTE scanning receiver, selected for its small size, high performance, and low cost.
The electrical contributions of the IF balun to the scanning receiver were simulated with the ADS software. As the simulation results show (Fig. 4), the balun provides a wide bandwidth of about 20 MHz, centered around an IF of 140 MHz. This supports the use of the balun in an LTE TDD scanning receiver.
The LTE TDD tri-band receiver front end was integrated on a four-layer printed-circuit board (PCB) in a modular design (Fig. 5) meant to fit into an LTE TDD scanning receiver chassis (Fig. 6). The power consumption of the LTE TDD scanning receiver front end is only 1 W. The noise figure of the scanning receiver front end was measured with a model N8973A noise figure analyzer from Agilent Technologies, which is a portable, single-box solution for measuring gain and noise figure at frequencies from 10 MHz to 3 GHz (and beyond with optional frequency converters). It offers measurement bandwidths from 100 kHz to 4 MHz and uses a calibrated noise source to make noise figure measurements over a range as wide as 0 to 35 dB and gain measurements over a range as wide as -20 to +40 dB with 17 dB instrument uncertainty. The measurements performed on the LTE TDD scanning receiver front end with the N8973A noise figure analyzer are shown in Fig. 7 for the three different frequency bands. These results show noise figure performance of better than 3 dB, which meets the LTE TDD scanning receiver requirements.
The performance of a scanning receiver can be evaluated in terms of its received-signal-strength-indication (RSSI) capabilities. This function in the LTE TDD scanning receiver was measured and plotted in Fig. 8. To verify the accuracy of these measurements, an input test signal was fixed at 2017.4 MHz while the amplitude of the input test signal was varied. When the input level of this test signal was adjusted to -43 dBm on the test signal source, the measured result on the scanning receiver showed -43.12 dBm. To further evaluate the performance of the LTE TDD scanning receiver, a layer 3 analysis was performed and plotted in Fig. 9, while the analysis of nine different scanned frequencies is shown in Fig. 10.
In summary, a LTE TDD scanning receiver is an essential part of a full-performance LTE TDD communications network and, as has been shown here, it is possible to design a high-performance LTE TDD scanning receiver front end for high scanning speed and overall high performance using a low-cost approach. The final design features high sensitivity and fast scanning speed with good measurement accuracy, while consuming low power and occupying very little volume as part of a four-layer circuit module.
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