Non-real time data interfaces such as 3G DigRF interfaces provide direct communication between a base band (BB) IC and a radio frequency (RF) IC used in wireless communication devices. The RF IC is responsible for translating transmit data (digital baseband signals) supplied by the BB IC into analog RF signals for transmission through an antenna. The RF IC also converts incoming RF signals received by the antenna into a digital form suitable for processing by the BB IC. The BB IC is responsible for transmit data generation (such as voice), receive data demodulation, and its processing. A 3G DigRF interface, for example, is responsible for the transfer of signals between RF IC and BB IC.
Prior art FIG. 1 is a block diagram 100 of an RF IC in communication with a BB IC using a 3G DigRF interface. The RF IC 110 includes a dual-mode transmitter (2G/3G TX) 112, a dual-mode receiver (2G/3G RX) 114, a digital to analog converter (DAC) 120, an analog to digital converter (ADC) 122, a buffer 125 including a first RF RX buffer 124 for a first receive mode, a second RF RX buffer 126 for a second receive mode, and a 3G DigRF interface 130. The BB IC 150 includes a 3G DigRF interface 132, TX symbols I&Q signals 152, configuration and control signals 156, RX symbols I&Q signals 154, RF IC responses 158, a first BB RX buffer 155 for the first receive mode, a second BB RX buffer 157 for the second receive mode, a first modem 163, and a second modem 166. Additional buffers may be included, such as a BB TX buffer in the TX symbols I&Q signal path or an RF TX buffer preceding the DAC 120, but they are omitted here for the sake of simplicity.
In the example of FIG. 1, the RF IC 110 is capable of receiving and transmitting 3G (third generation, such as WCDMA) signals and 2G (second generation, such as GSM) signals.
The 2G/3G dual-mode RX 114 is responsible for receiving 3G signals from an external source such as an antenna (not shown) when configured in 3G mode and is responsible for receiving 2G signals from the same or different external source when configured in 2G mode. The ADC 122 converts these incoming analog RF signals from the 2G/3G dual-mode RX 114 at a sampling rate dependent on the active receiver's radio access technology (RAT) into a stream of digital data that can be transmitted to the BB IC 150. This stream of digital data samples is stored in an RF RX buffer 125 allowing samples to arrive at the RF sampling rate dictated by the ADC 122 and then be transmitted to the BB IC 150 from the RF RX buffers 124 or 126 at a different rate dictated by the 3G DigRF interface 130, 132 bit rate.
The RF IC 110 may route the digital data samples to one or more RF RX buffers based on the data type of the samples. In the example of FIG. 1, the first RF RX buffer 124 collects the stream of data samples associated with a 3G RAT (for example, WCDMA) and the second RF RX buffer 126 is used to collect the stream of data associated with a 2G RAT (for example, GSM RAT). Given that only a single receive path exists, only one of these buffers is utilized at a time to support the active receiver.
This stream of digital data samples is transmitted from the RF IC 110 to the BB IC 150 using the 3G DigRF interface 130, 132 and deposited in the active BB RX buffer 155 or 157 at a rate dictated by the 3G DigRF interface 130, 132 bit rate as each packet is received by the BB IC 150. The active BB RX buffer 155 or 157 is allowed to fill to a certain level before beginning to be read out at the RF IC 110 sample rate. After the active BB RX buffer 155 or 157 begins clocking out the received digital data samples to its respective modem 163 or 166, its output is clocked continuously which allows it to stay in synch with the sample stream being collected in the ADC 122.
On the transmit side, the same 3G DigRF interface 130, 132 is used by the RF IC 110 to receive digital signals from the BB IC 150. The DAC 120 converts the digital signals received from the BB IC 150 so that they can be transmitted by the 2G/3G dual-mode TX 112 of the RF IC 150. The digital signals received from the BB IC for transmission are generally modulated using an analog carrier signal (such as, a high-frequency sinusoid waveform). The 2G/3G dual-mode TX 112 is responsible for transmitting 2G or 3G signals using one or more antennas (not shown).
The BB IC 150 creates TX symbols using quadrature I&Q signals 152 and configuration and control signals 156 and receives RX symbols using quadrature I&Q signals 154 and RF IC responses 158 from the RF IC 110 using a 3G DigRF interface 130, 132. The TX symbol I&Q signals 152 contain the transmit data that has been processed by the BB IC 150 and will be transmitted to the RF IC 110. Conversely, the RX symbol I&Q signals 154 contain data that is received by the BB IC 150 from the RF IC 110.
All the configuration of the system and the control of the system are regulated by the BB IC 150. The BB IC 150 sends the configuration and control signals 156 to the RF IC 110 to initialize the required upcoming mode of operation and timing as required. The RF IC responses 158 are the information sent by the RF IC 110 to the BB IC 150, in response to the BB IC 150 requests for changes in configuration or status of the system.
The 3G DigRF interface 130, 132 helps the BB IC 150 and RF IC 110 to communicate with each other. The 3G DigRF interface 130, 132 combined with the RF RX 124, 126, and BB RX 155, 157 buffers allows asynchronous and dissimilar radio access technologies (for example, 2G and 3G) to share a common physical interface between the BB IC and the RF IC. The interface 130, 132 provides a transmission path (TX path) 160 for the transmission of control, data and timing from the BB IC 150 to the RF IC 110 and a reception path (RX path) 170 for receiving data by the BB IC 150 from the RF IC 110. The BB IC 150 is the keeper of the master clock and the timing between the RF IC 110 and the BB IC 150 clock is maintained by sending a timing strobe 180 from the BB IC 150 to the RF IC 110 over the 3G DigRF interface 130, 132. The timing strobe 180 from the BB IC 150 to the RF IC 110 is sent on the TX path 160. In one example, the timing strobe 180 is a signal embedded into the TX path 160 and sent over the 3G DigRF interface 130, 132.
In an example, where the RF IC 110 transmits a first RAT's data samples to the BB IC 150, the BB IC 150 transmits a timing strobe 180 to the RF IC 110 and the RF IC 110 starts transmitting samples associated with a first RAT to the BB IC 150. The ADC 122 begins converting the incoming analog RF signals from the 2G/3G dual-mode RX 114 into a stream of digital data. Then the ADC 122 fills the first RF RX buffer 124 with the stream of digital data that can be transmitted to the BB IC 150, at a predetermined time interval relative to the timing strobe 180 depending on the active RAT. When a full 3G DigRF packet worth of digital data samples are collected in the first RF RX buffer 124, the 3G DigRF interface 130 on the RF IC 110 transmits the packet to the 3G DigRF interface 132 on the BB IC 150, which then inserts the received digital data samples into the first BB RX buffer 155. After the first BB RX buffer 155 is filled to a programmable threshold, the digital data samples are read out of the first BB RX buffer 155 in a first in-first out manner at the original RF sample rate continuously and are fed to a first modem 163 until the first RAT receive activation is terminated. The programmable threshold is set such that, the bursty, asynchronous, and shared traffic aspects of the 3G DigRF interface would not result in the first BB RX buffer 155 running out of samples to deliver to the modem 163. In one example, the first modem 163 is a 3G modem and the second modem 166 is a 2G modem.
The RF IC 110 does not contain a sense of time and must rely entirely on the BB IC 150 to provide a timing reference in the form of a timing strobe 180. Despite the fact that the 3G DigRF interface 130, 132 is asynchronous with respect to the RF sample rate, the BB IC 150 can tolerate variation in the delay from when a sample is first taken at the ADC 122 to when it is read out of the first or second BB RX buffer 155,157 because the BB IC 150 begins by setting its timer to a known offset between when the RF IC 110 receives the timing strobe from the BB IC 150 and when the ADC 122 takes its first sample. The BB IC 150 holds its timer at this count until the first sample is read out of the first or second BB RX buffer 155, 157 making any variation in delay across the 3G DigRF interface 130, 132 irrelevant. This is true as long as the stream of digital data samples delivered from the RF IC 110 to the BB IC 150 continues uninterrupted at a rate fast enough to ensure no buffer underflows or overflows.
A problem arises when the data received on a particular RAT at the RF IC 110 is discontinuous such as when the RF IC 110 switches receive modes from one RAT to another RAT during a compressed mode gap. During the gap, the timer in the BB IC 150 which maintains timing with the network for the first RAT continues and cannot be stopped (as was required when the RX path 170 was initialized) without requiring a portion of the compressed mode gap to reacquire timing resulting from the time varying delay introduced by the 3G DigRF interface 130, 132. In addition, resending the timing strobe 180 will also contribute timing error, subsequently, increasing the timing uncertainty further. The process of re-establishing timing after the discontinuous activity gaps reduces a window of opportunity that may be used to monitor another RAT. The net effect is that more compressed mode gaps are required to accomplish the same task resulting in reduced call quality, increased time to detect suitable hand off candidates, and increased probability of a dropped call.
Accordingly, there is a need for maintaining timing across discontinuous activity gaps for a non-real time data interface.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.