There is an ever-increasing demand on mobile wireless operators to provide voice and high-speed data services. At the same time, these operators want to support more users per basestation to reduce overall network costs and make the services affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities are a pressing need.
Interference Limited Systems
Unfortunately, because the available broadcast spectrum is limited, attempts to increase traffic within a fixed bandwidth create more interference in the system and degrade the signal quality.
In particular, when omni-directional antennas (see Figure 1) are used at the basestation, the transmission/reception of each user's signal becomes a source of interference to other users located in the same cell, making the overall system interference limited. An effective way to reduce this type of interference is to split up the cell into multiple sectors and use sectorized antennas, as shown in Figure 1.
Figure 1. Non-Smart Antennas System
Smart Antenna Technology—Beamforming
Smart antenna technology offers a significantly improved solution to reduce interference levels and improve the system capacity. With this technology, each user's signal is transmitted and received by the basestation only in the direction of that particular user. This drastically reduces the overall interference in the system. A smart antenna system, as shown in Figure 2, consists of an array of antennas that together direct different transmission/reception beams toward each user in the system. This method of transmission and reception is called beamforming and is made possible through smart (advanced) signal processing at the baseband.
Figure 2. Smart Antenna System—Beamforming
In beamforming, each user's signal is multiplied with complex weights that adjust the magnitude and phase of the signal to and from each antenna. This causes the output from the array of antennas to form a transmit/receive beam in the desired direction and minimizes the output in other directions.
Switched and Adaptive Beamforming
If the complex weights are selected from a library of weights that form beams in specific, predetermined directions, the process is called switched beamforming. Here, the basestation basically switches between the different beams based on the received signal strength measurements. On the other hand, if the weights are computed and adaptively updated in real time, the process is called adaptive beamforming. Through adaptive beamforming, the basestation can form narrower beams towards the desired user and nulls towards interfering users, considerably improving the signal-to-interference-plus-noise ratio.
Altera Adaptive Beamforming Implementation
The high-performance digital signal processing (DSP) blocks, embedded Nios® II processors, and logic elements (LEs) of Altera's Stratix® series FPGAs make them ideal for adaptive beamforming applications. In addition, the inclusion of the dual-core ARM® CortexTM-A9 MPCoreTM processor with the NEON acceleration unit can be used for beamforming adaptations. This section describes the Altera implementation of a Rake-Beamformer (also known as two-dimensional Rake) structure that performs joint space-time processing. As illustrated in Figure 3, the signal from each receive antenna is first down converted to baseband, processed by the matched filter-multipath estimator, and accordingly assigned to different Rake fingers.
Figure 3. Adaptive Beamforming with Altera's Stratix Series FPGAs
- DDC: digital down converter
- MRC: maximal ratio combining
- CORDIC: coordinate rotation digital computer
- QRD: QR decomposition
The beamforming unit on each Rake finger then calculates the corresponding beamformer weights and channel estimate using the pilot symbols that have been transmitted through the dedicated physical control channel (DPCCH). The QRD-based recursive least squares (RLS) algorithm is selected as the weight update algorithm for its fast convergence and good numerical properties. The updated beamformer weights are then used for multiplication with the data that has been transmitted through the dedicated physical data channel (DPDCH). Maximal ratio combining (MRC) of the signals from all fingers is then performed to yield the final soft estimate of the DPDCH data.
The beamforming unit implementation on each Rake finger is further elaborated below.
DSP Blocks for Complex Weight Multiplication
The application of complex weights to the signals from different antennas involves complex multiplications that map well onto the embedded DSP blocks available in Stratix series FPGAs. Each DSP block has a number of multipliers, followed by adder/subtractor/accumulators, in addition to registers for pipelining. With these features, Stratix series FPGAs can efficiently implement complex multiplications and reduce the amount of overall logic and routing required in beamforming designs.
CORDIC-Based QR Decomposition
The QRD-RLS weights update algorithm involves decomposing the input signal matrix Y into QR, where Q is a unitary matrix and R is an upper triangular matrix. This is achieved using a triangular systolic array of CORDIC blocks, as shown in Figure 4. Each CORDIC block operates in either vectoring or rotating modes and performs a series of micro rotations through simple shift and add/subtract operations and can run at speeds of 300 MHz.
Figure 4. Triangular Systolic Array Example for CORDIC-Based QRD-RLS
The R matrix and u vector (transformed reference signal vector d) are recursively updated for every new row of inputs entering the triangular array. The triangular systolic array can be further mapped into a linear array with reduced number of time-shared CORDIC blocks—as illustrated in Figure 4—providing a tradeoff between resource consumption and throughput.
Back Substitution for Weights Using Nios II and ARM Processors
The final beamformer weights vector w is related to the R and u outputs of the triangular array as Rw=u. Because R is an upper triangular matrix, w can be solved using a procedure called back substitution that can be implemented in software on the flexible embedded Nios II processor or on a dual-core ARM Cortex-A9 MPCore processor. The Nios II soft processor can also utilize custom instructions for hardware acceleration of program code. For an eight antenna system, the beamformer weights for a Rake finger can be solved via back substitution in approximately 0.2 ms using the Nios II processor operating at 100 MHz. The computation time can be lowered to 3 µs by implementing the back substitution on a hardware peripheral controlled by the Nios II processor. Moreover, the Nios II processor offers a flexible platform to implement other adaptive weight update algorithms such as least mean squares (LMS) and normalized LMS. The ARM Cortex-A9 processor subsystem includes a NEONTM media processing engine for floating point operations and single-instruction, multiple-data (SIMD) processing.
The Altera Advantage for Beamforming
Smart antenna technology requires high processing bandwidth, with computational speeds approaching several billion multiply and accumulate (MAC) operations per second. Such computationally demanding applications quickly exhaust the processing capabilities of digital signal processors. Altera FPGAs, with enhanced DSP blocks and TriMatrix memory, provide throughputs in excess of 50 GMACs, offering a high-performance platform for beamforming applications.
There are a number of beamforming architectures and adaptive algorithms that provide good performance under different scenarios such as transmit-receive adaptive beamforming and transmit-receive switched beamforming. With embedded processors and easy-to-use development tools such as DSP Builder and the Qsys system integration tool. Altera® FPGAs offer a high degree of flexibility in implementing various adaptive signal processing algorithms.
The standards for next-generation networks are continuously evolving, and this creates an element of risk for beamforming ASIC implementation. Transmit beamforming, for example, utilizes the feedback from the mobile terminals. The number of bits provided for feedback in the standards can determine the beamforming algorithm that is used at the basestation. Moreover, future basestations are likely to support transmit diversity including space-time coding and multiple-input multiple-output (MIMO) technology. Because Altera FPGAs are remotely upgradeable, they reduce the risk involved with designing for evolving industry standards while providing the option for the gradual deployment of additional transmit diversity schemes.