Driven by consumer demand and the proliferation of electronic devices, data consumption is rising rapidly. The Cisco Visual Networking Index (VNI) predicts that from 2014 to 2019, networked devices and connections will grow globally from 14.2 to 24.4 billion. The report forecasts that global Internet traffic will grow 3.2 fold from 2014 to 2019 at a compound annual growth rate of 26 percent.

The rapid adoption of higher resolution, video-streaming services has further increased demand for faster connections. In fact, the VNI estimates global Internet video traffic will be 77 percent of all Internet traffic in 2019. It predicts that five million years of video content will cross the Internet each month in 2019. This estimate means nearly a million minutes of video will be streamed or downloaded every second.

The cable industry faces the challenge of supporting this increasing demand for more high-speed home data. The VNI forecasts that 62.8 percent of global Internet traffic in 2019 will be fixed Wi-Fi and 19 percent will be fixed wired.1 Consumers and businesses are demanding faster connections, which is putting enormous strain on the broadband system. From customer premise equipment (CPE) devices to multi-service operators (MSO) head-end infrastructure, the cable industry is in a race with data consumption.

DOCSIS 3.1 Technology

To meet these growing data requirements, the cable industry developed the DOCSIS 3.1 cable industry standard. The Data Over Cable Service Interface Specification (DOCSIS) is an industry-wide collaboration that is issued by research and development consortium CableLabs. The DOCSIS specifications are used by the cable market to define how the cable modem interacts with the overall data delivery infrastructure, from the back office network to the hybrid fiber/coaxial (HFC) system to the cable modem itself, which delivers services to the end-user. DOCSIS 3.1 is the latest generation. The first standard, DOCSIS 1.0, came out in 1997; past specifications include DOCSIS 3.0, DOCSIS 2.0 and DOCSIS 1.1.

DOCSIS 3.1 technology offers many improvements over its precursor, the DOCSIS 3.0 standard. DOCSIS 3.1 increases speeds up to 10 Gbps downstream and up to 1 Gbps upstream. It allows for greater system capacity with the ability to support up to 4,096 quadrature amplitude modulation (QAM), which is 12 bits/symbol. This is a vast improvement over DOCSIS 3.0, which only supported up to 256 QAM (8 bits/symbol). DOCSIS 3.1 reduces network delays and improves responsiveness with Active Queue Management. Additionally, it is more energy efficient and enables companies to improve their current energy metrics.

One of the main benefits of DOCSIS 3.1 is that it does not require upgrades to existing data-delivery infrastructure. The current HFC network does not need to be modified to work with DOCSIS 3.1 technology; therefore, network capacity can be increased without extensive adjustments. Furthermore, DOCSIS 3.1 maintains backward compatibility with DOCSIS 3.0 technology.

DOCSIS 3.1 allows the cable industry to increase network capacity by more than 50 percent over the same spectrum. It enables cable companies to offer consumers and businesses higher download and upload speeds without sacrificing quality in any way. Consumers not only benefit from higher speeds, but they can enjoy the reduction in network delays and receive higher resolution images. It is also designed to support the latest innovations, such as 4K video and ultra-high definition movies.2

Figure 1

Figure 1 Nonlinearities of the cable modem upstream can block the downstream receiver from capturing the desired signal.

Linearity & Harmonics Challenge

The new specification sets an ambitious goal for the cable industry. From an RF perspective, one of the toughest challenges is to comply with the new linearity requirements. For DOCSIS 3.1 technology, high linearity is necessary to support the increased data rates and the upstream/downstream spectral integrity. When transmitting higher data rates, linearity preserves the integrity of the in-band complex modulated waveform. Increased linearity also shows itself in reduced out-of-band harmonic spurious generation—a key factor in assuring that any device that is simultaneously handling both upstream and downstream signals is able to maintain its spectral separation and purity. It is detrimental to have spurious levels from one signal stream overriding the desired signal of the other data stream. Therefore, any switch that is in a position of routing both upstream and downstream signals will have stringent second and third harmonic requirements.   

In DOCSIS 3.1, there is a move to orthogonal frequency division modulation (OFDM) for downstream data transmission and orthogonal frequency division multiple access (OFDMA) for upstream data delivery. The frequency range is increasing from 1.002 GHz to at least 1.218 GHz and maybe even 1.794 GHz. Additionally, the number of possible frequency partitions has increased from three to four and the ability to select between partitions has been added. QAM has increased from a maximum of 1,024 to 4,096, with an interleaved option for 16,384 QAM. In general, greater implementation flexibility has been incorporated in the entire specification.

Regarding channel impairments, as QAM level increases the signal-to-noise ratio (SNR) requirement also increases. Thus, implementation of DOCSIS 3.1 implies ever-increasing signal integrity requirements. To maintain a low symbol error rate the SNR must continue to increase as the modulation scheme grows in complexity. For 4,096 QAM an SNR of 45.4 dB is needed to obtain a 10-6 probability of a symbol error compared to 13.7 dB for the same error probability using a much simpler QPSK scheme.

Further, nonlinearities of the cable modem upstream can block the downstream receiver from capturing the desired signal. Figure 1 shows signal level vs. frequency for the upstream Tx signal, the downstream Rx signal and the second (H2) and third harmonic (H3) of the Tx upstream signal. The Tx signal level is much higher than the minimum Rx downstream signal the cable modem will receive at its F-connector. If the harmonic generation levels are not considered (H2 and H3), they can be greater than the downstream Rx signal and can block the downstream signal from being received.

A single cable must support both downstream and upstream paths, so bandwidth is frequency divided. The challenge is to ensure that spurious out-of-band emissions of the upstream transmitter do not corrupt the signal of the receiver downstream. Prior to DOCSIS 3.1, there had not been a need to perform frequency selectivity during the operation of the cable modem—a fixed frequency partitioning was sufficient. Now there is a need for greater upstream/downstream frequency band flexibility.

Figure 2

Figure 2 Typical harmonic performance of SOI switch. At 17 MHz, the second harmonic is –121 dBc, and the third harmonic is –140 dBc (PIN = 65 dBmV).

RF Switches Enable Dual-band Architecture

In response to the DOCSIS 3.1 linearity and harmonic requirements, many companies set out to create a solution. In October 2014, Peregrine introduced a high linearity RF switch with excellent harmonic performance. Subsequently in March 2016, they set a new record for high linearity and introduced an upgraded version switch that reportedly boasts the highest linearity specifications on the market today.

The switches offer to solve the DOCSIS 3.1 linearity challenge even when supporting a dual upstream/downstream band architecture. They are currently the only RF switches that enable dual upstream/downstream bands to reside in the same CPE device. CPE devices, such as set-top boxes, cable modems and home gateways, had previously supported only one upstream/downstream band combination. By using this dual-band architecture, CPEs can comply with the DOCSIS 3.1 cable industry standard, and MSOs have the flexibility to offer their customers new and expanded services. A dual-band architecture eliminates an extra step in giving customers higher speeds. At the flip of a switch, a cable service provider can repartition its frequency plan when it desires in order to better match the data consumption needs.

Before the introduction of these products, no switch had been released that met the linearity requirements necessary to support a dual upstream/downstream band architecture. To create this full-frequency architecture, the switch is placed directly at the cable modem (CM) F-connector between the filters and cable input. This switch must maintain frequency duplexing established by the two low-pass and high-pass filters and must deliver high linearity to avoid corruption of the downstream band. Additionally, this switch must comply with the stringent DOCSIS 3.1 CM integrated spurious emissions requirements of –50 dBmV. Such a low spurious level requires the switch harmonic performance to be greater than –115 dBc. An example product that exceeds this specification is the Peregrine PE42723 switch where the second harmonic is –121 dBc and the third harmonic is –140 dBc at 17 MHz. Figure 2 shows the second and third harmonic for this switch when the input power (PIN) is 65 dBmV.

Figure 3

Figure 3 Integrated spurious emissions of the SOI switch in a DOCSIS 3.1 module, measuring –50.3 dBmV.

Figure 3 reveals the integrated spurious emissions of the switch in a Murata DOCSIS 3.1 module. The switch was tested with a Keysight signal generator and spectrum analyzer using a DOCSIS OFDMA upstream waveform, an input power of +71.8 dBmV, 24 MHz BW and utilizing 4,096 QAM. The integrated spurious levels were measured across the low end of the downstream band at 56, 62 and 90 MHz center frequencies and over a 4 MHz integration bandwidth. Even with this over-stressed condition of being driven by 71.8 dBmV and delivering more than 69 dBmV to the F-connector, the integrated spurious level for this module remained below the –50 dBmV requirement.

Switches that can meet these stringent requirements are manufactured on the UltraCMOS® process, a patented variation of silicon-on-insulator (SOI) technology on a sapphire substrate. Sapphire is a near perfect insulator, offers excellent RF and microwave properties and has a mature supply chain. UltraCMOS silicon-on-sapphire (SOS) chips feature low defect density for simpler construction; dielectrically isolated transistors for excellent power handling and multiple thresholds; inherent CMOS logic levels; and high ESD ratings. Nonlinear effects from the substrate and metallization are an advantage in meeting strict linearity requirements. The sapphire substrate demonstrates consistent behavior with respect to insertion loss vs. input power. Further, a sapphire substrate demonstrates excellent characteristics with respect to second and third harmonics versus input power. Specifically, a through-line measurement on a sapphire substrate at a fundamental power level of +18 dBm (+65 dBmV) results in harmonic levels of –102 dBm for the second harmonic and –122 dBm for the third harmonic, translating to –120 dBc and –140 dBc, respectively. From a substrate perspective, merely meeting these requirements represents a substantial challenge for other technologies.

Technology Transitions from Concept to Reality                                           

Introduced in 2013, DOCSIS 3.1 implementation is an ongoing process. In previous generations of the DOCSIS standard, implementation has taken several years to go from a concept to deployment. However, the cable industry has made a collaborative effort to rapidly develop and deploy DOCSIS 3.1 technology. ABI Research predicts that nine million broadband subscribers will be using DOCSIS 3.1 equipment by 2017. This represents over one percent of total fixed broadband subscribers worldwide.3

In January 2016, CableLabs announced that five cable modem products had received DOCSIS 3.1 certification. Cable modems from CastleNet, Technicolor, Askey, Ubee Interactive and Netgear proved that they could comply with the latest specifications. This product certification announcement was an important milestone for CableLabs. Compared to previous DOCSIS generations, the time from specification development to product certification occurred in half the time.4

Figure 4

Figure 4 Dual upstream/downstream band architecture with full frequency coverage. The switches must provide high linearity (SWB), high isolation and linearity (SWA) and low insertion loss (SWC).

MSOs are also preparing their systems for the new standard. In December 2015, Comcast announced that they had successfully tested a DOCSIS 3.1 modem at a Philadelphia home. This initial test marked the world’s first DOCSIS 3.1 modem on a customer-facing network and proved that DOCSIS 3.1 technology could work on Comcast’s existing HFC network. While the technology trials started in Philadelphia, Comcast plans to expand testing to other locations in Pennsylvania, Atlanta, Georgia and Northern California.5

As the worldwide cable industry transitions from DOCSIS 3.0 to 3.1 technologies, MSOs need to future-proof their CPE devices with high flexibility in addition to backward compatibility. One of the main benefits of these switches is that they support both DOCSIS 3.0 and 3.1 requirements, allowing for a simple and cost-effective transition to the new standard. In fact, this is a key selling feature for cable modem vendors. Of the five DOCSIS 3.1 certified cable modems, these switches are designed into three of the five cable modems—the three that feature a switchable band-select feature.

DOCSIS 3.1/3.0 Dual-band Architecture

Beyond the F-connector switch, there are other critical switch sockets in a DOCSIS 3.1/3.0 cable modem dual-band architecture. Figure 4 shows a dual upstream/downstream band architecture at full frequency. In this example, high performance switches must be placed at sockets SWA, SWB and SWC. As discussed, the switch at the SWB socket must generate near zero out-of-band emissions to mitigate harmonic spurious falling in the DS1 legacy band. These switches can be placed in the SWB switch socket, which is between the cable drop and the filters. Socket SWA in the upstream band requires high isolation and high linearity. Finally, switch socket SWC in the downstream band requires a switch with low insertion loss.

Figure 5

Figure 5 Dual-band architecture with an optimized upstream band using high isolation switches (SWA and SWB).

Optionally, Figure 5 depicts a dual-band architecture with an optimized upstream band. In this example, the switch can be placed in the SWB switch socket, which is after the filters on the upstream band. Switch socket SWA on the upstream band requires high isolation.


The ability to meet DOCSIS 3.1 standards is crucial for the cable industry. There are currently few switches that are able to meet the DOCSIS 3.1 linearity and harmonics requirements and enable a dual upstream/downstream band architecture, but new UltraCMOS switches from Peregrine Semiconductor are able to meet the stringent requirements. This ability to support dual-upstream/downstream bands in the same CPE device is a critical enabler in making DOCSIS 3.1 a reality.


  1. “Visual Networking Index (VNI) – VNI Forecast Highlights,” Cisco, 2016, [Accessed: 31- March 2016].
  2. “Featured Technology” CableLabs, 2016, [Accessed: 31- March 2016].
  3. “ABI Research Forecasts 9 Million Broadband Subscribers to Use DOCSIS 3.1 Equipment by 2017,” ABI Research, 2016, [Accessed: 30-March 2016].
  4. “Five New CableLabs Certified DOCSIS 3.1 Cable Modem Products Usher in a New Generation of Broadband Services,” CableLabs, 2016, [Accessed: 31-March 2016].
  5. “World’s First Live DOCSIS 3.1 Gigabit-Class Modem Goes Online in Philadelphia,” Comcast, 2016, [Accessed: 31- March 2016].