Multi-carrier communications systems based on OFDM, including 3GPP Long Term Evolution, utilize quadrature sampling and rely on the preservation of precise signal phase information in the transmitter and receiver. Indeed, OFDM systems must preserve phase coherency at the sample level for the digital signal processing algorithms to be valid. In the past, communication system engineers had to use proprietary synchronization techniques (typically involving shallow FIFOs and programmable logic based state machines) at the board-level to guarantee quadrature sample synchronization. The JEDEC JESD204A specification is intended to address this commonly found technical requirement, and foster interoperability among data converters and commonly used logic devices such as FPGAs.

LTE Downlink Air Interface Basics

The 3GPP Long Term Evolution (LTE) air interface is called High Speed OFDM Packet Access (HSOPA). This standard defines a downlink (i.e., base station to handset) physical layer based on Orthogonal Frequency Division Modulation (OFDM), with QPSK (Quadrature Phase-Shift Keying), 16-QAM or 64-QAM (Quadrature Amplitude Modulation) used for the downlink PHY.

OFDM QAM uses multiple subcarriers, tightly-spaced in the frequency domain (15 KHz between subcarriers), to optimize spectral efficiency, among other things. LTE standard defines twelve QAM subcarriers together as a "resource block", with 100 resource blocks occupying a 20 MHz slice of spectrum.

In the LTE downlink protocol, the radio frame is 10 ms long, comprising 10 sub-frames of 1 msec each. Every sub-frame consists of 2 slots where each slot is 0.5 ms (or 2000 slots/sec). 64-QAM encodes 6 bits per symbol, and LTE fits seven symbols in each slot. When utilizing 20 MHz of transmission spectrum, the theoretical maximum downlink bandwidth is approximately 100 Mbits/sec. [100 resource blocks/20 MHz of spectrum ∗ 12 64-QAM subcarriers / resource block ∗ 6 bits / subcarrier symbol ∗ 2000 slots /sec ∗ 7 symbols / slot = 100.8 Mbits/sec per 20 MHz of transmission spectrum].

In OFDM QAM, adjacent subcarriers are orthogonal, or 90 degrees phase shifted, enabling this tight frequency division multiplexing with minimal interference between channels. In a conventional LTE base station OFDM downlink transmitter, a DSP or FPGA outputs two digital streams: an "in-phase" I channel and a "quadrature" Q channel, phase-shifted by 90 degrees. These digital signals are typically converted into analog signals by a high-speed dual-channel DAC, modulated (i.e., frequency multiplication "up conversion") to the RF carrier frequency, then summed. This signal is then sent to the RF amplifiers for tower transmission.

In order for the OFDM system to function properly, the analog signal reconstruction step in the transmitter signal chain must preserve the phase modulation fundamental to orthogonal frequency division multiplexing (i.e., quadrature amplitude modulation, which is the amplitude modulation of two signals precisely 90 degrees phase shifted). This means the I channel DAC and the Q channel DAC must be perfectly synchronous at the sampling rate level.