Since the dawn of cellular communications in the mid-seventies, the industry has advanced through three generations of radio standards and is about to launch 4G. New wireless technologies continue to emerge while the old ones linger, forcing wireless equipment, both phones and base stations, to support multiple radio interfaces where interoperability is required.
But as the number of radios grows, implementing them all in hardware becomes increasingly challenging. Multimode radios today must support GSM, CDMA, WCDMA, HSPA+, LTE and other protocols.
|Fig. 1: Wireless technology evolution " the 'G's|
With the advent of parallel processor architectures, programmable radios are gaining on hardware implementations both in terms of throughput and power consumption. According to John Glossner, CTO and EVP of Sandbridge Technologies, "LTE and WiMAX, are completely different from WCDMA and GSM. When taking a hardware approach you have to replicate the hardware for all of these systems, so the chips get big, expensive and power hungry."
Software defined radio (SDR) has been touted since the early nineties to support the growing diversity of radio interfaces around the world. But, according to Willie Anderson, VP of engineering at Qualcomm, software has been evolving slower than hardware. Though studying programmable radio architectures, Qualcomm is taking a hybrid hardware/software approach with its new HSPA+/LTE MDM9000 silicon.
Many in the industry agree that SDR has been held back by the software. "Programming has been hard and compilers struggle to do a good job optimizing over multiple threads.", states Steve Muir, CTO of Vanu, the pioneer of commercial SDR base stations.
According to Phil Moorby, CTO of Sigmatix, a new entrant in the programmable radio space, "SDR implies a traditional software development approach, but this is completely inadequate for baseband applications. To take maximum advantage of the new baseband processors, we have to move away from the traditional C language semantics and from compilers, which are inherently sequential and limited. The focus needs to be on optimizing the 4G algorithms to the parallel machine operations and memory hierarchies."
Dissecting the radio
Despite its compact appearance, a new generation cell phone is a data communications terminal with a complex medium access control (MAC) and an even more complex physical layer (PHY). The MAC and the baseband component of the PHY are digital and inherently programmable. The MAC has already evolved from hardware to mostly software. The baseband is just starting its journey along the same route.
|Fig. 2: Radio block diagram. The MAC and the baseband component of the PHY are digital and inherently programmable. The MAC has already evolved from hardware to mostly software. The baseband is just starting its journey along the same route.|
The MAC layer, implementing the access protocol, has already evolved from hardware to software. When Ethernet first emerged, in the early 1980s, its MAC was implemented in dedicated hardware. General-purpose processors were too slow then to run a 10-Mbit/s MAC. Today's 100+ Mbit/s Wi-Fi, WiMAX and LTE MACs are >90% software based, taking advantage of high-speed CPU cores, such as those from ARM.
The PHY consists of a baseband, analog-to-digital (A/D) and digital-to-analog (D/A) converters and the RF front end connecting the device to the antenna(s). The baseband is where modulation and demodulation take place. Modern wireless baseband is a complex DSP-based system implementing multi-carrier FFT/IFFT, forward error correction (FEC) coding and a variety of transmit/receive diversity and beamforming algorithms.
|Fig. 3: A single input, single output (SISO) generic OFDM baseband block diagram. For multiple input, multiple output (MIMO) systems, multiple transmit and receive chains are replicated.