"Microwave point-to-point link is an integral part of wireless mobile network. It provides the return capability between base stations (BTS / nodes) and wireless controllers (BSC / RNCs). This architecture adopts optical fiber link, which has high cost. More than half of the world adopts this microwave link. Driven by the trend of market transfer to smart phones, the recent rapid growth of mobile network traffic, such as video streaming and other data demand, has also put pressure on the capacity of existing microwave return equipment. In order to adapt the data throughput on the return network to the requirements of LTE and LTE advanced, the next generation microwave link needs:
Moving towards higher and higher digital modulation, from today's QAM256 to the future qam4096, so as to increase the capacity allocated to a fixed channel by 50%.
Channel allocation from 56 MHz to 112 MHz is supported in the traditional frequency band of 6 GHz to 42 GHz. If the carrier signal-to-noise ratio (CNR) remains constant, the data throughput capacity will increase proportionally every time the channel bandwidth is doubled.
Such as polarization diversity and channel set, n × N-line multiple input multiple output (MIMO) and other technologies.
The typical characteristic of communication system design is that the improvement of throughput needs to pay a certain price. To support higher QAM and channel bandwidth at the same time, the microwave link must have greater dynamic range capability to ensure the required minimum EVM performance. In particular, each doubling of QAM size or bandwidth will reduce the receiver sensitivity by 3dB. Microwave equipment must remain flexible, so some additional considerations are required when supporting all possible working conditions. At the same time, the requirements of receiving filter and AGC should be simplified to improve performance and reduce cost.
Another industry trend is the emergence of complete outdoor units (Odus), in which integrated radio modems, transceivers, switch / multiplexing units and traffic interfaces are integrated in a separate box and installed on signal towers or other similar buildings. The new base station capex / OPEX and the space constraints of existing base stations promote the development of this trend. The traditional separated indoor (IDU) / outdoor (ODU) system places the microwave / RF components in the ODU and connects them with another part of the system in the equipment room (IDU) through coaxial cable. The maximum length of the coaxial cable can reach 300m and conduct two-way communication. The receiver IF signal with a center frequency of 140 MHz is separated from the transmitter if signal with a frequency center of 340 mhz-400 MHz through a duplexer.
The ad6676 provides the advantage of high dynamic range for IDU design, which is also beneficial to the design of ODU receiver. Fig. 5 shows how the ad6676 is applied to an ODU receiver link from 18 GHz to 23 GH, which consists of an equalized RF mixer such as adl5801, a microwave image suppression mixer such as hmc966, and an RF VGA such as adl5246. Note: different microwave image rejection mixers, microwave PLLs and possible first if frequencies can be selected for the remaining microwave bandwidth in the range of 6 GHz to 43 GHz. When there is no full UDU with cable limiting if selection, the ad6676 can be set to a higher if frequency, such as 300 MHz, to further simplify the RF filter requirements for image suppression. If the stray component of any mixer is larger, additional suppression is required. The ad6676 can interface directly with the RF mixer or through a simple cubic low-pass smoothing filter. The 1960 MHz RF filter is designed to support channel bandwidth up to 112 MHz. If the attenuator of ad6676 is set to 0 dB, the synthetic noise bottom of adl5801 and ad6676 will be lower than – 157 dBfs / Hz and the equivalent NF is 17 dB on the 56 MHz bandwidth channel. The default total conversion gain of adl5246 and hmc966 can be initially optimized with the real-time dynamic range of adl5801 / ad6676, so the digital demodulator tracks the initial attenuation (at the nominal receiver input power supply). The threshold of adl5246 can be set so that the gain begins to increase when the BER of the demodulator receiver is below the preset level of a specific QAM signal. This hybrid method will only activate RF AGC when the input signal is very low, so as to improve the minimum sensitivity of the receiver.
Although this trend deserves attention, most of the microwave equipment shipped are still existing separate IDU / ODU systems at present and in the foreseeable future. This will help promote the back-end modem architecture for design reuse, which supports the original system and the next generation ODU platform. The progress of high-speed DAC and ADC technology working at clock rate above 1.5 GSPS makes it possible to support the synthesis and digitization of high and medium frequency QAM signals of 4096 QAM and above. With high dynamic range and high oversampling rate, not only the quadrature error correction required by traditional analog I / Q is not required, but also most filters in the digital domain are realized, thus reducing the number of analog filters and digital equalizers required for compensation. At the signal path end of the transmitter, in order to synthesize broadband QAM signals, ad9142 and ad9136ad6676 are released.
Ad6676 is the industry's first bandwidth if receiver subsystem based on bandpass ADC (Figure 1). It supports IF signal bandwidth up to 160 MHz and internal clock frequency up to 3.2 GHz. Σ-Δ The high oversampling capability of ADC simplifies the requirements of if analog filtering, and these filters need to be used to suppress adjacent channels (and interference / blocking) in ADC with low sampling rate, otherwise these signals will be mixed back to the IF signal, thus reducing the sensitivity performance of the receiver. In addition, the high dynamic range of ADC with NSD bottom (narrow bandwidth QAM channel) of – 160 dBfs / Hz will reduce the isolation requirements of duplex transmitter and receiver or the analog AGC range of attenuation compensation. The ad6676 includes an on-chip 27 DB digital attenuator with an accuracy of 1 dB to correct the static gain error caused by the initial device tolerance and the change of coaxial cable loss.
Let's take a look at how the ad6676 if receiver subsystem cooperates with a high-speed DAC such as ad9136 to greatly simplify the traditional IDU transceiver and improve its performance at the same time. Figure 2 upper receiver link shows a direct conversion mode to support typical low if receivers and transceivers at 140 MHz and 400 MHz. The challenges of direct conversion transceiver architecture are documented, but can be overcome by I / Q balance correction, DC bias correction, adjustable baseband I / Q filtering, and duplexer design to suppress transmitter leakage. However, the traditional IDU receiver supporting the maximum 56 MHz channel bandwidth and 256 QAM has been mass produced. If more capacity is required, doubling the channel bandwidth and increasing the QAM level by 8 times are major challenges to the direct conversion architecture. The latest progress of high-speed ADC / DAC technology is expected to replace the traditional method by digital IDU, as shown in Figure 2. The transceiver scheme in the lower half of Figure 2 requires only four ICs, has significantly loose filtering requirements, and achieves almost perfect performance.
On the transmitter side, a high-speed DAC such as ad9136 operating at 1.6 GSPS clock rate can synthesize a 112 MHz, 1024 QAM signal with excellent EVM performance and intermediate frequency as the center frequency. In this way, most transmitter link error budgets can be reserved for ODU (in which the cumulative effect of phase noise and linearity will reduce most EVMS). At the same time, a low-pass filter is required to suppress the drop of the first DAC image at 1.2 GHz. Compared with the harmonic suppression filter that needs to filter the third Lo image of the I / Q modulator at 1.2 GHz, it can be relaxed to 12 dB. The transmission power control used to overcome cable loss has been realized in ad9136. The EVM performance of QAM signal exceeds the range of 15 dB, and the attenuation can be ignored.
On the receiver side, 112 MHz, 1024 QAM channels are digitized by ad6676 with excellent dynamic range and accuracy, even if there are a large number of transmission leakage signals caused by the wide duplex filter, as shown in Fig. 3. In this example, the configuration of ad6676 supports 112 MHz bandwidth, and its attenuator is set to 3 dB, so that the effective RTI NF entering the hmc740 preamplifier is maintained at about 10 dB. The left curve in Figure 3 is ad6676 Σ-Δ Fast Fourier change results of ADC data output (for demonstration purposes only), in which the transmitter – 26 DBM leaky signal center is 400 MHz, represented by a continuous single tone signal of – 17.2dbm at 143 MHz. Note that the adjustable bandpass Σ-Δ The inherent noise shaping of type a ADC is obvious in the high dynamic range region centered on the desired intermediate frequency (up to – 160 dBfs / Hz). The right curve in Figure 3 is 16 bits centered on the IF signal. The 200 MSPs I / Q data is digitally converted and 16 bits × Fast Fourier change results after sampling filtering. Note that the + 85 dB suppression provided by the digital filter is used to remove out of band noise and aliased transmitter leakage signals returning to the 112 MHz passband. Residual shaping noise falling outside the 112 MHz passband is removed by the RRC filter of the modem.
The in band noise under the continuous large signal test condition of – 2 dBfs is – 68.6 dBfs. If a continuous tone is replaced by a full-scale 1024 QAM RX signal with a peak to root mean square value of 10 dB, an additional fallback of 7 dB is required to prevent ADC distortion. In this case, the input power of the receiver IDU will be – 9 dBfs (or – 24.2 DBM), and a CNR close to 60 dB is recommended. For the simplified design of duplex filter, the current suppression from duplex transmitter to receiver is about 20 dB, so as to suppress the receiver signal of – 6 DBM, so the input of preamplifier will appear – 26 DBM. In the case of short cable deployment between IDU and ODU, the attenuator of ad6676 can be enhanced to allow ODU to have higher QAM.
When unexpected adjacent signals appear, the IDU receiver ability to recover QAM signals at low sensitivity (BER < 10-6, with FFC enable) is a very important index. Perhaps the most demanding test (according to etsien 301 390 V1.2.1) is that a continuous interfering tone (blocking) with 30 dB energy higher than the QAM signal is placed at 2.5% of the desired QAM signal × Channel offset. Note: most of the tunable or switchpack filters used in today's receivers are driven by this specification because the modem must support a channel bandwidth of 3.5 MHz to 56 MHz. The previous example represents the next generation 112 MHz channel bandwidth. We can assume that adjacent continuous interference is effectively suppressed by fixed channel filters above 112 MHz, and image suppression is realized before the last conversion in the ODU RF link. In fact, this filter still provides effective blocking suppression of 70 MHz to 140 MHz bias for 28 MHz to 56 MHz channel bandwidth. If the channel bandwidth is 14 MHz or lower, the continuous single tone will fall within the passband range of the filter. Therefore, an additional bandpass filter needs to be added at 140 MHz for suppression, or digitized by ADC and then digitally filtered.
The IDU receiver architecture based on ad6676 has real-time dynamic range and supports the scheme without additional filters. Figure 4 shows the frequency response of the ad6676 fast Fourier change of the same receiver link as Figure 3. The only difference is that the adjustable bandwidth of the ADC is reduced to 56 MHz. In this example, the – 32 DBM continuous tone (or – 32 DBM offset) at 175 MHz (or 35 MHz offset) will increase to the – 26 DBM transmission drain signal present at 400 MHz. The continuous tone response is at the input level level of – 17 dBfs visible to the ad6676 and is set 30 dB higher than – 47 dBfs and 1024 QAM at the lowest sensitivity (CNR = 36 dB). If the continuous interference single tone is increased by an additional 15 dB, the prominent over design margin may contribute to the noise distribution of microwave / RF circuits. Without a noise barrier, the desired 1024 QAM signal may increase by 38 dB, providing an additional dynamic range for the IDU receiver to process signal attenuation.
Summary:
The next generation of microwave point-to-point receivers need to support 3.5 MHz to 112 MHz channel bandwidth and have a high dynamic range, so as to support higher and higher M-QAM within a wider attenuation boundary. The ad6676 if receiver subsystem enables the ordinary microwave point-to-point platform to support the existing IDU / ODU separated system and the new ODU platform. For IDU / ODU separated system, its outstanding high dynamic range can ensure excellent modulation accuracy (EVM) without complex adjustable or huge switch bank filter when adjacent interference signals appear in the system. For the complete ODU system, the high instantaneous dynamic range (with mixer direct interface) reduces the RF AGC range required for tracking attenuation and simplifies the RF filtering requirements. Ad6676 is 4.3 × 5.0 mm, 80 pin WLCSP package, which can operate at 2.5 V or 1.1 v“
Our other product:
