Wireless systems operators must adapt their networks to increasing data rates to meet customer demands. Next-generation wireless systems are adopting such techniques as receive diversity and multicarrier architectures to cope with the increasing data needs. In China, the use of time-division synchronous code-division-multiple-access (TD-SCDMA)1 as an alternative to wideband CDMA (WCDMA) attempts to provide improved coverage in a wide range of environments, compared to WCDMA, which is optimized for symmetric traffic and macrocell sites. In support of the technology, a compact multichannel TD-SCDMA receiver with digital intermediate-frequency (IF) stage and multiple active antenna modules, has been developed. The flexible design allows for a wide range of applications in multicarrier, diversity-receiver systems. Simulated and experimental results show excellent linearity and phase-noise performance for the receiver in a compact footprint.

TD-SCDMA technology as proposed by the China Academy of Telecommunications Technology (CATT) was accepted by the International Telecommunications Union (ITU) as one of the third-generation (3G) mobile communications standards in May 2000. TD-SCDMA technology features advanced access techniques integrated with time-division-multiple-access (TDMA), frequency- division-multiple-access (FDMA), code-division-multiple-access (CDMA), and spatial-division-multiple-access (SDMA) approaches. The uplink and downlink traffic in a TD-SCDMA system share the same frequency band, but with different time slots. As a result, TD-SCDMA is well suited for asymmetric data services and provides high spectral efficiency. Key techniques employed in TD-SCDMA systems include multiplexing, smart antennas, and joint detection techniques.2

The receiver is designed with a digital IF section for simplicity and flexibility. In contrast to the analog-to-digital-converter (ADC) block in a standard superheterodyne receiver, the ADC block is shifted to the IF output ports. By replacing analog components with digital components a digital IF receiver provides the flexibility to handle broadband frequency ranges and multiple wireless communications standards.

Receiver diversity techniques are commonly used to reduce the effects of multipath and Rayleigh fading on wireless communications performance. The main diversity techniques are frequency diversity, time diversity, antenna diversity, angular diversity, and polarization diversity.3

Antenna receive diversity has been applied to the TD-SCDMA receiver to enhance link gain. With this approach, multiple uncorrelated RF signals are collected at the receiver and then combined in such a way that the impact of fading and multipath effects can be reduced or removed. Typical linear diversity combination methods are selective combining (SC), maximal- ratio combining (MRC), and equal gain combining (EGC), each with its respective advantages and disadvantages.4,5

Multicarrier techniques have been used with TD-SCDMA to increase the format's data capacity and transmission rates in support of high-datarate wireless services. A multicarrier TD-SCDMA system employs three different frequencies as the carrier frequency for a given cell. One of these frequencies is designated the primary carrier while the others are called secondary carriers. The difference between the primary carrier and the secondary carriers is whether pilot and broadcast channel (BCH) information is carried. The primary carrier handles pilot and BCH information while the secondary carriers do not. The paging indicator channel (PICH) and secondary common control physical channel (SCCPCH) can only be configured in the primary carrier.

Figure 1 shows a typical TD-SCDMA radio channel. The channel includes three carriers using a low chip rate at 1.28 Mchip/s corresponding to a carrier bandwidth of 1.6 MHz. TDSCDMA helps provide high flexibility in spectrum usage and network design, especially in densely populated areas. In addition, each TDMA frame during a 5-ms duration is divided into 7 time slots, which can be flexibly assigned to either multiple users or a single user requiring multiple time slots.7

Figure 2 shows the system architecture for a multichannel TD-SCDMA RF receiver with digital IF section and multiple active antenna modules. The system includes three active antenna modules and one RF receiver module comprised of three individual RF receive channels. The active antenna module is made up of an omnidirectional antenna with 6-dBi gain, an RF bandpass filter, and a lownoise amplifier (LNA). Each channel is comprised of an RF amplifier, frequency downconverter, local oscillator (LO), IF surface-acoustic-wave (SAW) filter, variable gain amplifier (VGA) controlled by a baseband processing unit, and an IF amplifier.

The receiver supports several connection schemes. In the first, only one active antenna module is connected to all three channels of the receiver, which is used as a multicarrier TD-SCDMA receiver. In the second scheme, three active antenna modules are connected to the three channels of the receiver for use as a receive diversity TD-SCDMA receiver (shown by the dashed arrowhead line in Fig. 2). In this case, the active antenna modules must be spaced far enough apart to receive different propagation delays for the received signal. Usually a spacing of at least five wavelengths is required between two antennas to experience received signals with significantly different fading characteristics. In the third connection scheme, the three active antenna modules are connected to the three multichannel RF receiver modules for use as a diversity receiver as well as a multicarrier TD-SCDMA RF receiver.

In order to evaluate the performance of the TD-SCDMA receiver, its reference sensitivity and capabilities of its fast automatic-gain-control (AGC) circuitry must be better understood. Reference sensitivity is the most important specification for a receiver. Generally, it refers to the minimum input power level at the antenna port while the system for a required bit-error rate (BER). The specification is also impacted by the noise figure of the receiver, the noise floor of the transmitter, the in-phase/quadrature (I/Q) gain imbalance, the I/Q quadrature phase imbalance, the phase noise of the local oscillator (LO), the voltage noise of the power supply, the linear phase distortion, and the linear amplitude distortion.

The noise figure of the receiver and the noise floor of the transmitter exhibit the influence of additive white Gaussian noise (AWGN), and the combined noise figure can be used to describe these two items. In timedivision- duplex (TDD) operation, when the receiver is on, the transmitter should be off. As a result, the noise floor of the transmitter is not an issue for a TD-SCDMA receiver. Using a digital IF approach, the I/Q gain and phase imbalance caused mainly by the analog demodulator can be corrected in the digital domain, without impacting the sensitivity specification. The influence of phase noise and voltage noise can be neglected if the performance levels of the LO and power supply are sufficiently high. Linear phase distortion and linear amplitude distortion can be compensated by the baseband processor. From this analysis, it can be seen that the noise figure mainly affects the reference sensitivity of a TD-SCDMA receiver.8

In a traditional receiver-antenna setup, the loss caused by the RF cable connecting the antenna and RF receiver adds to the noise figure of the system. In a TD-SCDMA RF receiver system, the system is divided at the active antenna module directly connecting the antenna with the LNA. Therefore, the loss caused by the RF cable can be compensated for and the receiver diversity performance can be improved.

When a receiver is made up of several blocks, each block has its own insertion gain (Gi) and noise factor (Fi). Each block adds noise to the signal, but the contribution to the overall noise factor from succeeding blocks is reduced when the signal is amplified in previous stages. The noise figure of a receiver can be calculated usingEq. 1. The values in Eq.1 must be calculated by the numeric value of gain and noise factor (F), not as the logarithmic noise figure (in dB). The implications of this simple cascaded noise figure formula can be considerable in system design.9

Based on Eq. 1, accounting for a 3-dB loss in the 5-m-long coaxial cable, a 1-dB loss in the bandpass filter, a 1-dB noise figure and 20-dB gain in the LNA, and with the other parts remaining unchanged, the noise figure can be decreased from 5.1 dB (Eq. 2) to 2.14 dB (Eq. 3), with a notable improvement in the TD-SCDMA receiver reference sensitivity.

An AGC circuit is used in a receiver to supply a constant-level signal to the ADC. PIN-diode-based AGC attenuators are commonly used in many broadband system applications such as WiMAX and 3G cellular systems. In the TD-SCDMA receiver, a digital AGC approach is used instead of analog circuitry due to its flexibility and consistent performance. A PIN diode attenuator is used for analog attenuation in each RF channel, controlled by the digital baseband circuits.

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Figure 3(a) shows a typical pi resistor attenuator, with its attenuation determined by Eq. 4. Parameter K is defined as the input-to-output voltage ratio while the system characteristic impedance is represented by Z0.

For common values of attenuation, the value of the resistors is 50 Ohms. Based on the pi resistor attenuator diagram, four PIN diodes are used in the circuit as shown in Fig. 3(b). In a switch circuit, the resistance characteristics of the PIN diode are exploited at their extreme high and extremely low values. However, in an attenuator, finite values of the PIN diode's resistance are used. The benefit of the circuit is its symmetry, allowing for a simpler bias network and reduction of distortion due to cancellation of harmonic signals in the back-to-back layout of the series diodes. Although there are other methods for providing AGC functions, such as varying the gain of the RF transistor amplifier, the PIN diode approach generally results in low power consumption, broadband constant impedance, wide dynamic range, less frequency pulling, and high linearity.10,11

The PIN diode attenuator used in the AGC circuit was computer simulated with the Advanced Design System (ADS) suite of software tools from Agilent Technologies. The simulated results for dynamic range are shown in Fig. 4. The dynamic range, according to these simulations, can reach 120 dB. Although the attenuation curve is not linear, the AGC control voltage can be corrected by a baseband algorithm to achieve an effectively linear response. Compared with traditional looped AGC circuits, this digital AGC technology is faster and more suitable for a TDD system.

Figures 5(a) and 5(b) show the active antenna module and the multichannel RF receiver module, respectively. The multichannel RF receiver module is integrated on a four-layer printed-circuit board measuring 150 x 200 x 20 mm. Figure 6 shows the whole receiver noise figure as measured by a model N8975A noise figure analyzer from Agilent Technologies, with modulation also measured by a model E4438C signal generator and 89600 software from Agilent Technologies. As Fig. 6 indicates, the receiver noise figure is less than 2 dB with the system tested for reference sensitivity to -115 dBm. Fig. 7 shows the error vector magnitude (EVM) performance.

REFERENCES

1. Jigang Liu, Ronghui Wu, and Qingxin Su, "RF transceiver Requirements and Architectures for TD-SCDMA UE," Third International Conference on Microwave and Millimeter Wave Technology Proceedings, 2002, pp. 1149- 1153

2. Bo Li, Dongliang Xie, and Shiduan Cheng, "Recent Advances on TD-SCDMA in China," IEEE Communications Magazine, January 2005, pp. 30-36.

3. John G. Proakis, Digital communications, 3rd ed., Publishing House of Electronics Industry, 1997, pp. 770-777.

4. F. Adachi, T. Hattori, and K. Hirade, "A Periodic Switching Diversity Technique for a Digital FM Land Mobile Radio," IEEE Transactions on Vehicular Technology, 1978, Vol. 27, No. 2, pp. 211-219.

5. Heather MacLeod and Zhizhang Chen, "A Hybrid Diversity Antenna System," Antennas, Propagation, and Electromagnetic Theory, 2006, ISAPE 2006, 7th International Symposium, 2006, Vol. 10, pp. 1-4.

6. Kambiz C. Zangi, "Impact of wideband ADC's on the performance of multi-carrier radio receivers," VTC'98, 1998, pp. 2155-2159.

7. 3GPP: TS 25. 105 V6. 2. 0 base station (BS) radio transmission and reception. www. 3GPP. org.

8. Phase noise and TD-SCDMA UE receiver, Application Note, Maxim, www.maxim-ic.com/an1842.

9. Joseph F. White, High Frequency TechniquesAn Introduction to RF and Microwave Engineering, JFW. 10. Application Note APN1002, "Design With PIN Diodes," Skyworks Solutions, www.skyworks.com.

11. Application Note APN1003, "A Wideband General Purpose PIN Diode Attenuator," Skyworks Solutions, www.skyworks.com.