RFIC’s Challenges for Third Generation Wireless Systems Olga Boric-Lubecke*a, Jenshan Lin**b, Penny Gouldc, and Munawar Kermallid a Bell Labs, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974 bAgere Systems, 600 Mountain Ave., Murray Hill, NJ 07974 cBell Labs, Lucent Technologies, Swindon, UK dBell Labs, Lucent Technologies, Whippany, NJ ABSTRACT Third generation (3G) cellular wireless systems are envisioned to offer low cost, high-capacity mobile communications with data rates of up to 2 Mbit/s, with global roaming and advanced data services. Besides adding mobility to the internet, 3G systems will provide location-based services, as well as personalized information and entertainment. Low cost, high dynamic-range radios, both for base stations (BS) and for mobile stations (MS) are required to enable worldwide deployment of such networks. A receiver’s reference sensitivity, intermodulation characteristics, and blocking characteristics, set by a wireless standard, define performance requirements of individual components of a receiver front end. Since base station handles multiple signals from various distances simultaneously, its radio specifications are significantly more demanding than those for mobile devices. While high level of integration has already been achieved for second generation hand-sets using low-cost silicon technologies, the cost and size reduction of base stations still remains a challenge and necessity. While silicon RFIC technology is steadily improving, it is still difficult to achieve noise figure (NF), linearity, and phase noise requirements with presently available devices. This paper will discuss base station specification for 2G (GSM) and 3G (UMTS) systems, as well as the feasibility of implementing base station radios in low-cost silicon processes. Keywords: 3G, wireless, base stations, silicon RFIC 1. INTRODUCTION Second generation (2G) digital wireless systems, such as Global System for Mobile Communications (GSM) and cdmaOne (IS-95), have been widely deployed over the last decade, resulting in over 800 million users worldwide, with about additional 60 million of analog (first generation) users. Number of mobile phones exceeds the number of landline phones and the mobile phone penetration exceeds 70% in countries with the most advanced wireless markets. Even though this growth has recently somewhat slowed down, demand for more data handling capability is still there. Third generation (3G) standards, envisioned to provide enhanced capacity, quality and data rates, are currently being developed across the industry and by global groups such as the Third Generation Partnership Project (3GPP). The enhancements offered in 3G services will enable high speed multimedia and internet access. Universal Mobile Telecommunication System (UMTS), to replace GSM, has emerged as the most widely adopted third generation air interface. Its specifications has been created in the 3rd Generation Partnership Project (3GPP), which is the joint project of the standard bodies from Europe, Japan, Korea, USA, and China. Deployment of the first UMTS network is well underway in Japan by NTT DoCoMo, and will also become available in Europe in 2002. *[email protected]; phone +1 908 582 1889; fax +1 908 582 4941; Bell Labs, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974, USA; **[email protected]; phone +1 908 582 5182; fax +1 908 582 4941; Agere Systems, 600 Mountain Ave., Murray Hill, NJ 07974, USA Base transceiver station (BTS) radio requirements are more stringent that those for user interface units for all cellular wirelles standards. While handsets handle only one channel at a time, a base station receiver handles multiple channels and thus requires a higher dynamic range [1]. Mobile receivers that meet GSM and other standards requirements have already been implemented in low cost silicon technology [2], while base station receivers are typically still realized in GaAs technology [3]. Even though silicon RFIC technology is steadily improving, it is still a challenge to achieve noise figure (NF), phase noise, and linearity requirements for BTS applications with presently available devices. However, with careful system specification partitioning and choice of radio architecture, it was demonstrated that it is possible to meet GSM900 and DCS1800 BTS receiver requirements in BiCMOS technology [4,5]. While 3G systems promise a substantially enhanced services, radio specifications will be similar to those of 2G systems, and it should be possible to meet them in silicon technologies as well. A receiver’s reference sensitivity, intermodulation characteristics, and blocking characteristics define performance requirements of individual components of a radio front end. While direct downconversion architecture has been explored for handsets [6], base stations typically use heterodyne receiver front-end architecture, as shown in Figure 1. It would be desirable to have as many of these radio components as possible integrated in silicon, to reduce both the cost and size of the base stations. While there is no compact, integrated solution for filtering components available today, other components could in principle be integrated on a single chip. In this paper, we will derive radio specifications for a 2G (GSM) and a 3G (UMTS) standard, to illustrate that base station requirements are more stringent than for handsets, and to show that 2G and 3G requirements do not differ greatly. Next, we will discuss silicon implementaion of most important receiver circuits: low noise amplifier (LNA), mixer, and voltage-controlled oscillator (VCO). Finally we will discuss the feasibility of implementing a GSM BTS receiver in silicon-MMIC 0.25 µm BiCMOS process. The receiver chips that achieve low noise figure and high third order intercept (IP3) simultaneously without any gain control will be described. These chips were fabricated using a 0.25µm BiCMOS process with inductor quality factor (Q) of approximately 10. With a supply voltage of 3V, current consumption is 132mA for the GSM900 receiver and 117mA for the DCS1800 receiver. This was the first reported silicon-integrated radio front end for base stations [4]. LNA Driver BPF Mixer IF Amp SAW To IF Subsystem (Interchangeable) Buffer Duplexer Amp To Antenna Synthesizer VCO From Transmitter Fig. 1 Typical BS receiver front-end architecture. 2. UMTS AND GSM STANDARDS GSM is currently the most widely used digital cellular radio system. As of July 2001 [7], there were over 560 million GSM users, accounting for two thirds of world users. In contrast, second most popular standard, cdmaOne, has only about 100 million users. GSM provides international roaming capability, which is currently available in over 150 countries. GSM standard is based on a time-division-miltiple access (TDMA) and frequency-division-multiple access (FDMA) scheme, and it uses Gaussian Minimum Shift Keying (GMSK) modulation. The bit rate is 270 kbit/s, and channel bandwidth 200 kHz, with each channel shared by 8 users in a TDMA mode. It uses a full duplex communication, in three frequency bands: 900 MHz, 1800 MHz, and 1900 MHz. Digital cellular system (DCS) 1800 MHz band was introduced in Europe in early 1990’s to provide more bandwidth. Personal communications services (PCS) 1900 MHz band is used in the US. GSM and UMTS bandwidth allocations and other features are summarized in Table I. Table I. GSM and UMTS features GSM900 DCS1800 PCS1900 UMTS MS Tx-BTS Rx 890-915 1710-1785 1850-1910 1920-1980 f [MHz] BTS Tx-MS Rx 935-960 1805-1880 1930-1990 2110-2170 f [MHz] Channel 0.2 0.2 0.2 5 spacing [MHz] Users per cell 992 2992 2383 3060 Modulation GMSK GMSK GMSK QPSK MS Tx 2 1 1 0.25 Power [W] GSM is a second generation cellular standard, which will gradually be replaced by a third generation UMTS standard. UMTS, or WCDMA (wide-band CDMA) standard is based on direct sequence spread spectrum technology, with code division multiple access (CDMA) scheme. User information bits are spread over a wide bandwidth by multiplying the user data with quasi-random bits (called chips) derived from CDMA spreading codes. All users in one cell share same frequencies all the time, and signal is “spread” over the whole bandwidth of a channel. This “spreading” of the signal results in a processing gain, allowing for the detection of signal levels below the noise floor. UMTS uses Quaternary Phase Shift Keying (QPSK) modulation, with the chip rate of 3.84 Mcps, and corresponding channel bandwidth of about 5 MHz. Data rates are variable, depending on the type of service. For example, voice service will operate at 12.2 kbps, while non-real time data service will operate at 384 kbps. NTT DoCoMo’s Freedom of Mobile multimedia Access (FOMA) service, to begin in Tokyo area by the end of 2001, is using a downlink data rate of 384 kbps. One disadvantage of the CDMA type systems is that they require elaborate transmit power control for mobile units. Since all users in the cell use the same bandwidth simultaneously, if the transmit power is not controlled, a single strong user close to the base station can completely mask all the users far from the base station. Receiver power from all users must be within 1dB, and mobile power must be able to vary by as much as 85 dB in a few microseconds. 3. GSM SPECIFICATIONS GSM base station and mobile radio receiver requirements, referenced at the antenna connector, will be derived from the GSM specification 05.05 document [1]. Values obtained for noise figure (NF), linearity and phase noise should be considered as reference values only. Vendors typically design their products to exceed these values by a certain margin to give them a competitive advantage. 3.1 Noise Figure Receiver NF can be determined from reference receiver sensitivity (S) [1], knowing the input noise floor, and signal to noise ratio (SNR) requirement in the absence of interferes. Noise floor (N) can be determined from the Nyquist equation for thermal noise: N = kTB, (1) where k is a Boltzman constant, T is the temperature in Kelvins, and B is the single-sided noise bandwidth of the system.