Through the use of multiple-antenna technologies and orthogonal-frequency-division-multiplexing, fourth generation wireless technologies such as Long-Term Evolution (LTE), WiMAX, and Super3G will offer much greater capacity to mobile customers. However, what represents a tremendous opportunity for mobile customers for faster downloads, video at higher frames rates and resolution, and streaming audio, also poses challenges to the CAPEX structure for the mobile operator, particularly in the fiber optic transport to their radio heads.
In fact, mobile operators can end up spending as much in wireline technology as they do for wireless technologies with 4G. To address these challenges, signal compression technology offers the promise of reducing the bit rates for carrying baseband data to the radio elements, and therefore keeping fiber optic transport costs in line with existing 3G systems.
The typical remote radio head architecture for a 3G base station is illustrated in Fig 1. The radio heads and the baseband processor are different colors to illustrate that they can be provided by different vendors.
1. Remote radio head architecture for 3G wireless.
(Click this image to view a larger, more detailed version)
In the receive direction, the radio head contains all of the electronics to downconvert an RF band (typically two with antenna diversity) to an intermediate frequency, convert to digital, and then digitally downconvert individual carriers to baseband in-phase and quadrature (I/Q) pairs of samples.
Current systems utilize intermediate frequencies in the range of 120 to 180 MHz, and employ dual 14-bit ADCs sampling at 122.88 MHz. Likewise, in the transmit direction, I/Q baseband samples are modulated onto a digital IF carrier which then passes through processing to reduce the crest factor, and to digitally predistort the signal to compensate for sidelobe generation in the power amplifier. Since this digital processing occurs primarily in FPGAs, this market has become one of the most important for the FPGA vendors.
The baseband samples (I/Q) in both the receive and transmit directions are multiplexed onto a high speed SERDES interface using either the CPRI or OBSAI standards. CPRI intellectual property is available off the shelf for leading FPGA families. The radio head sends the serialized data stream to the baseband processor over a fiber optic cable.
If the baseband processor is located at the tower site, the length of the cable may be modest, a couple of hundred feet. However, often times for residential coverage or coverage along highways, the radio heads may be daisy-chained and thus be remote from the baseband processor. The bit rate over this fiber optic connection directly affects installation costs as well as operating costs if the mobile operator must lease fiber optic facilities.
If the raw data from the ADC were transported over the fiber optic cable, the data rate would be 4300.8 Mbps = (2 antennas × 122.88 Msample/sec × 14 bits/sample / (8B/10B) = 4300.8 Mbps), which exceeds the capacity of low cost fiber optic transceiver modules developed for the storage area networking market, particularly 4G Fibrechannel.
Consequently, channelization and decimation of the data is required. For wideband CDMA, the received carriers are typically decimated to twice the chip rate of 3.84 MHz to a sample rate of 7.68 Msample/sec. Since W-CDMA receptions from handsets are power controlled to low signal to noise ratios, very few bits per carrier need to be transported across the fiber optic interface with narrowband digital AGC applied at the output of the channel filters.
Thus, for a 20 MHz radio with four W-CDMA carriers, the aggregate bit rate is a modest 614.4 Mbps = (2 antennas × 4 carriers/antenna × 7.68 Msample/sec/carrier × 4 bits/sample × 2 (I/Q) / (8B/10B)). This is the predominate bit rate for 3G remote radio heads and can easily utilize low cost SFP fiber optic transceiver modules developed for the OC-12 and gigabit networking market. Three such radio heads can also be daisy-chained within the capacity of an OC-48 (2.5 Gbps) link, enabling a single fiber to be used for all three sectors on a tower, or for coverage along highways or in rural areas.
Fourth generation wireless systems such as LTE (Long-Term Evolution) will require four antennas per sector in both the transmit and receive directions to support multiple-input-multiple-output (MIMO) baseband technology. In addition, WiMAX systems can support smart antenna beamforming technology with up to eight antennas per base station.
Furthermore, for MIMO and beamforming technologies, narrowband AGC cannot be applied in the radio head without interfering with the antenna-combining function in the baseband processor, so often the full dynamic range of the signal must be preserved. Utilizing the same ADC configuration, but now allowing for bit growth through the digital downconverter and channel filter, the resulting signal can contain 16 bits of dynamic range.
With OFDM signals, the channel filter must allow for decimation to the FFT sample rate which is defined as the channel bandwidth. With the same 20 MHz of bandwidth divided into perhaps two 10 MHz channels, the bit rate over the fiber optic cable now becomes 3.2 Gbps = (4 antennas × 2 carriers/antenna × 10 Msample/sec/carrier × 16 bits/sample × 2 (I/Q) / (8B/10B)).
At 3.2 Gbps, the bit rate exceeds OC-48 capacity driving the cost for long haul fiber transceivers. This bit rate also requires the high-speed SERDES interfaces of high-end FPGAs. Moreover, for an 8 antenna beamforming system, the fiber bit rate is doubled to 6.4 Gbps, which now requires very expensive 10 gigabit networking transceivers and FPGA SERDES interfaces.
By employing signal compression, the fiber optic bit rates can be reduced to enable the continued use of low cost fiber optic transceivers. For example, with a compression ratio of 1.28:1, the 3.2 Gbps of data from a 4×4 MIMO LTE can now fit into an OC-48 link at 2.5 Gbps.
One such signal compression algorithm is Samplify's Prism, which provides lossless and near lossless compression for a wide range of wireless signals including W-CDMA and orthogonal-frequency-division-multiplexing (OFDM) waveforms. Prism has a mode called RateTrak, which adapts the dynamic range in the signal to maintain a user-specified compressed bit rate.
Fig 2 shows the results of applying Samplify's Prism compression algorithm in RateTrak mode at a compression ratio of 1.35:1. The decompressed signal is indistinguishable from the original waveform, and the distortion introduced by compression is 64 dB below the signal peak, and spectrally flat, except around the unused subcarriers at DC.
2. 10 MHz channel spacing OFDM signal compressed at 1:35:1; Full-scale (top) and 12 dB back-off (bottom).
RateTrak will adapt the dynamic range of the signal to meet the output bit rate requirements. In the case where the input signal level is below full scale, RateTrak will automatically preserve more of the least significant bits, as illustrated by the lower plot in Fig 2, where the signal level has been lowered by 12 dB, yet the distortion level introduced by compression is still 60.4 dB below the signal.
Note that RateTrak does not affect the sample rate of the signal during compression and decompression, so the FTT function of the OFDM demodulator can operate directly on the decompressed samples.
