Deploying Multiple DA Applications

Author: Tony Burge, RuggedCom, USA

Multi-Application Options

The various wireless technology options discussed will be compared (or contrasted) based on a subset of the requirements discussed previously (interface support, redundancy and robustness are manufacturing-specific, not technology-specific): Throughput; Range; Latency; Security; QoS (Prioritizaton); Uplink Biasing; Ecosystem

The two top levels of wireless technologies are public carrier or private infrastructure. Private infrastructure refers to utility-owned, closed-loop wireless networks.
Table 3 is a high level guideline to the benefits and deficiencies of each technology. This chart is the sole opinion of the author based on a decade of experience.

Public Carrier (PC): Public carrier infrastructure provides pervasive coverage in most populated areas, significant throughput, moderate security, and response times under 100 ms. By using approved devices it provides an ecosystem of products from various vendors. However, the concern remains whether public infrastructure can facilitate guaranteed prioritized service. Many carriers are working on features to support prioritized service-usually with a higher rate of pricing. If a utility is in an area with pervasive public carrier coverage with a guaranteed level of service at a fixed price, this would alleviate the initial capital expenditure of a private infrastructure. This is often not the case.  From a business case perspective, PCs focus on individual consumer-based users that make up the majority of the carriers customer base. Also of note is broken chain of custody for data in a third party network operations center and the embedded downlink biasing included in the technology (instead of uplink biasing).
In summary, public carriers have evolved to meet many needs of power utilities and require limited capital expenditure. They provide moderate throughput, pervasive coverage, acceptable latency, and adequate security features, but the need for additional security, channel availability during emergencies, and the ability to pass sufficient data in the uplink remain areas of concern.

Private Infrastructure: One new concept to this article is introduced at this point - deployed frequency. Frequency is not a feature necessarily, because each technology may be deployed within various frequency bands. As a general rule of thumb, and all things being equal, the lower in the RF electromagnetic spectrum a technology is deployed, the better the propagation and building penetration characteristics. For most wireless data communications, the range of point-to-multipoint frequencies available is 140 MHz to 5.8 GHz.

Narrowband - Often deployed in frequencies from 140 MHz to 900 MHz, a narrowband solution provides excellent RF propagation characteristics. Added to this is the very narrow channel size (hence the term narrowband) allotted by regulatory agencies (ETSI, IC, FCC, etc.)

The channel size allocated is usually limited to 25 kHz and often down to 6.25 kHz. While this narrow channel with high power allocations and operating in the lower portion of the RF electromagnetic spectrum provides excellent range, throughput is significantly limited - up to 50 kb/s. Even with excellent range, throughput is sufficient only for the most essential communications.
Since narrowband licenses are protected by regulatory agencies, the transceiver does not need to compete for channel usage, which provides very deterministic and low latency communications. With such low throughput, the ability to support higher level security and prioritized services are also limited, because these features require overhead  to operate. All narrowband options known to the author are proprietary; therefore, the ecosystem of products available is limited to a single manufacturer.

Mesh Wi/Fi - Broadband mesh technology is most often deployed in unlicensed bands with some deployed in designated bands for public safety or other entities. This technology is often based on IEEE 802.11 standards to some degree, which means that throughput is very high, but range is significantly limited. For a wide area network deployment, the limitation of range results in a high level of required infrastructure. Even though mesh technology facilitates node-to-node relaying or hopping, this comes at a cost: reduced throughput and increased latency at each node. Some manufacturers overcome this limitation by providing a multi-radio solution in a single box. This solution becomes more expensive with the same limitation of range (even though throughput and latency are improved). Each manufacturer implements different mesh algorithms, so the ecosystem of products is usually limited to a single vendor.

Broadband mesh technologies provide high throughput and sufficient security to support multiple applications; however, the amount of infrastructure required often makes this technology significantly higher to deploy than other technologies.

 Proprietary Broadband - This category refers to proprietary RF designs or modifications of standards, such as 802.11, that improve range in an attempt to balance throughput and coverage.  Proprietary broadband solutions have been successfully deployed for over a decade for power utilities. These solutions provide good throughput balanced with good range and acceptable latencies. Most are deployed in unlicensed or lightly licensed bands, which puts additional responsibilities on manufacturers to include interference mitigation features to facilitate deterministic communications.

Since these solutions are proprietary, the ecosystem of solutions is limited to a specific vendor for a specific implementation. Also, QoS is often implemented to a minimum level. Proprietary broadband solutions are field-proven; however, the proprietary nature locks a utility to a specific vendor and the throughput is, at most, adequate to support multiple services.

Broadband over Standards - Wireless broadband based on fully interoperable standards provides the benefits of high throughput, good range, high security, and often full implementation of QoS.  Broadband over standards, such as IEEE 802.16e, is deployed in licensed, lightly licensed, and unlicensed bands, so there is flexibility of deployment in areas where regulatory allocation of certain frequencies is limited.

Since this category is based on standards, the ecosystem of solutions available is larger and not limited to a single vendor per implementation (providing that appropriate interoperability testing and certification have occurred). In certain geographical regions, bandwidth is limited to such a degree that deploying these wider-band solutions (e.g., channel sizes of 3.5 Mhz) is not feasible. If frequencies are available, this category provides high throughput with highly secure, prioritized service and moderate coverage.

Technology-specific Considerations: There are many emerging features that facilitate advanced interference mitigation and improvements in throughput. The following list describes a few of these features the author recommends to research when selecting a wireless broadband solution.  A key point regarding broadband technologies that implement the features described in this section is the ability to sustain good range even when deploying with wider occupied channels (i.e., greater throughput).  With the advanced features described in this section, range (which is a function of receive sensitivity) is able to be maintained - this translates to high throughput with better range than available using other wide-channel, point-to-multipoint solutions.  Table 4 is representative of receive sensitivities (measured in dBm) obtainable using these technologies.

Scalable Orthogonal Frequency-Division Multiplexing Access (S-OFDMA): S-OFDMA facilitates sub-channel utilization (permitting a connected subscriber unit to use a subset of the occupied channel), the ability to access the same channel in adjacent cells or base station locations (providing more options in channel selection), and, most importantly, divides a large channel into multiple subcarriers to act as “narrowband” channels. Improving on OFDM technology, “Scalable” refers to keeping the subcarriers relatively small even when the occupied bandwidth is large. For example, the OFDM technology of 802.16d always used 256 subcarriers regardless of channel size. With S-OFDMA, the number of subcarriers can be increased to 512, 1,024, or 2,048; thereby, keeping the relative size of each subcarrier narrow. Implementing scalable subcarrier algorithms permits the use of wider channels (providing higher throughput) even in areas of in-band interference" (Figure 3.)

Forward Error Correction (FEC):  Sophisticated FEC algorithms are implemented to assist in rebuilding partially distorted messages on the receiver. Based on signal quality, very minimal FEC is incorporated, which provides higher throughput. Conversely, where signal quality is not as strong, higher order FEC is incorporated to facilitate successful message transmission. The FEC rates may be set to adapt automatically, along with modulation settings. If a portion of the subcarriers are distorted, FEC is often able to rebuild the message without requiring retransmission, which facilitates robust RF communications even in the presence of interference.

Hybrid Automatic Retransmit reQuest (Hybrid ARQ): In some cases, messages are unable to be successfully received or recovered via FEC.  In these cases, Hybrid ARQ provides a comprehensive retransmit request capability at the radio transceiver level without incurring the additional latency costs of waiting for a router in the network to perform the retransmit request. In short, Hybrid ARQ reduces latency and provides more deterministic communications in the presence of interference.

Multiple-in, Multiple-out (MIMO):  OFDM technology assists in minimizing the effects of multipath interference; however, MIMO adds the benefit of being able to take advantage of multipath interference to further enhance RF robustness.  MIMO Matrix A facilitates improved coverage by transmitting the same data on both transmission polarities. MIMO Matrix B facilitates improved throughput (where signal strength is sufficient) by sending unique data on both polarities. Advanced MIMO technology provides the ability to auto select the appropriate MIMO Matrix profile based on real-time signal characteristics. 

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