1. Field of the Invention
The present invention relates to a radio receiver, and more particularly, to a radio receiver that is capable of supporting multiple modulation formats with a single pair of analog to digital converters (ADCs).
2. Description of the Prior Art
This is the age of radio communications. Around the world, cell phones are now one of the most common pieces of equipment to be found among the general public. Wireless local area network (WLAN) is an acknowledged trend for business and consumer networking environments. The mobility enabled by wireless connections has dramatically changed and improved the lives of ordinary people. However, there is no universal standard for wireless modulation formats. For example, there are digital enhanced cordless telecommunications (DECT), global system for mobile communications-900 (GSM-900), GSM-1800, wideband code division multiple access (WCDMA), and code division multiple access 2000 (CDMA2000) systems for mobile telephony. Another situation is that there may be multiple modes within a standard. For example, there are orthogonal frequency division multiplexing (OFDM) modes and direct sequence spread-spectrum/complementary code keying (DSSS/CCK) modes within the Institute of Electrical & Electronics Engineers (IEEE) 802.11g standard. For mobile telephony systems base stations supporting multiple formats, signals of different standards need to be dealt with simultaneously. On the other hand, on the mobile station end, only one signal standard needs to be supported at the same time. For example, a base station that supports both GSM-1800 and WCDMA standards needs to deal simultaneously with signals modulated according to these formats. But a cell phone needs to receive or transmit signals of only one standard at one time.
The IEEE 802.11 WLAN standard provides a number of physical layer options in terms of data rates, modulation types and spreading spectrum technologies. Please refer to FIG. 1. FIG. 1 illustrates provided data rates, sample rates, carrier frequencies and modulation types for various modes of IEEE WLAN standard. An extension of the IEEE 802.11 standard, namely IEEE 802.11a, defines requirements for a physical layer operating in the 5 GHz frequency and data rates ranging from 6 Mps to 54 Mps. IEEE 802.11a defines a physical layer based on the orthogonal frequency division multiplexing (OFDM) modulation scheme. A second extension, IEEE 802.11b, defines a set of physical layers' specifications operating in the 2.4 GHz industrial, scientific, and medical (ISM) frequency band up to 11 Mps. The direct sequence spread spectrum/complementary code keying (DSSS/CCK) physical layer is one of the three physical layers supported in the IEEE 802.11 standard and uses the 2.4 GHz frequency band as the RF transmission media.
The IEEE standard committee has created a working group, TGg, with the mission of developing a higher speed PHY extension to the 802.11b standard. The 802.11g standard will be compatible with the IEEE 802.11 MAC and will implement all mandatory portions of the IEEE 802.11b PHY standard. A part of the scope of TGg is to provide a wireless LAN standard where stations communicating in OFDM modulation and legacy stations communicating in DSSS/CCK modulation coexist and communicate with each other.
In a multiple mode standard, like IEEE 802.11g, signals of both the OFDM mode and the DSSS/CCK mode need to be dealt with, but only one mode can be performed at a time. All of these standards have been extensively adopted and have their own features and market niches. As a result, systems supporting multiple modulation formats are highly desirable.
Recently, technological advances in the design of programmable computing devices have made possible the real-time processing of algorithms formerly implemented by ASIC circuits, and this is especially true in the field of telecommunications. Thus, the complexity of transceivers can be partly moved from hardware to software. Moreover, due to the high level of reconfigurability, improved algorithms or newer versions of the transmission standards can be downloaded to the terminal, avoiding the need for hardware upgrades. Under this scenario, the adoption of a single user terminal, capable of interfacing with different transmission standards, opens the door to an enlarged set of services that can be delivered to the end user, with complete independence of the air interface if proper techniques are chosen. For example, there are now available dual-system or even tri-system cell phones. There also exist IEEE 802.11g WLAN cards supporting both the OFDM DSSS/CCK modes.
Please refer to FIG. 2. FIG. 2 is a block diagram of a dual-system receiver R1 according to the prior art. Receiver R1 is a zero-IF radio architectured receiver for a mobile phone supporting both GSM-1800 and FDD WCDMA standards. The two systems operate in different radio frequency bands and at different data rates. Because of the different radio frequency bands, two radio frequency (RF) modules 10 and 20 are required to complete band selection, low noise amplification, down-conversion, and channel selection, as some of these functions are performed by frequency-selective components. Because the data rates of the two standards are different, and the modulations have an I/Q structure, two pairs of analog to digital converters (ADCs) 12 and 22 are adopted to sample the analog signals. The respective sampling rates of the two pairs of ADCs are in accordance with the modulation formats for GSM-1800 and the FDD WCDMA standard. Two baseband processing modules 14 and 24 are respectively electrically connected to the two pairs of ADCs 12, 22, and each takes responsibility for performing detection, demodulation, time and frequency synchronization, decoding, and de-scrambling of signals of the respective predetermined standard.
Please refer to FIG. 3. FIG. 3 is a block diagram of another two-standard receiver R2 according to the prior art. Receiver R2 is a zero-IF radio architectured receiver for a WLAN card supporting IEEE 802.11g standard. As mentioned above, there are OFDM modes and DSSS/CCK modes within the IEEE 802.11g standard. The radio frequency bands of the two modes are the same, but the modulation formats and data rates are different. Since the radio frequency bands are the same, the RF module 30 is shared by the two modes. As I/Q structures are adopted in each of the modes, electrically connected to the RF module 30 are two pairs of ADCs 32 and 42. The two pairs of ADCs are capable of sampling the analog signals, and the sampling rate of each ADC pair corresponds to the data rate of the respective predetermined mode. Similarly, two baseband processing modules 34 and 44 are respectively electrically connected to the two ADCs 32, 42, providing functionality like that of the baseband processing modules 14 and 24 in receiver R1.
It is easily seen that there are duplicated analog circuits, especially the ADC circuits, in a receiver supporting multiple modulation formats of the prior art. The ADC circuits usually have very complicate configuration, and the area occupied by these circuits can be relatively large. Hence, the ADCs present a severe drawback in the design and manufacture of integrated circuits. The larger the area of a circuit is, the higher the total cost becomes. In addition, in a user terminal supporting multiple standards or multi-mode standards, only one signal modulation format needs to be handled at one time. There is no need to waste power on paths other than that which is receiving or transmitting the signals.