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A natural complement

Thu, 07/31/1997 - 8:00pm
Yvette C. Hubbel, Manager of Business Development, Telecommunications Systems Division; and John Sabat, Jr., Manager of Systems Engineering, Telecommunications Systems Division; Sanders, a Lockheed Martin Company
Introduction

The introduction of 1.9 GHz Personal Communications Services (PCS) has spurred an explosive growth in mobile telecommunications. Predictions are that new wireless voice and data services for business and residential users will complement, and someday possibly replace, today's wired and wireless service. Consistent quality, increased coverage, and the steadily decreasing cost of service are expected to create widespread public demand for PCS.

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The Sanders' PCS-Over-Cable system leverages the upgraded broadband cable television hybrid fiber/coaxial (HFC) infrastructure and enables PCS providers to build flexible PCS networks, as well as introduce service quickly and economically. The system adds wireless telephony to existing cable TV services and is a complement to the cable TV industry's vision of two-way voice, video and data service. It offers several network advantages:

  • Provides total wide-area network service without gaps in RF coverage as an alternative to tower-based deployment for wireless mobility and local loop networks
  • Reduces delays and costs associated with seeking zoning and permit approvals from local municipalities
  • Complements traditional tower-based networks by providing coverage in "holes" left by tower systems and where towers cannot be deployed
  • Flexible hardware placement provides the ability for in-building penetration in office buildings and stadiums, malls, and other large structures.

The CDMA-based PCS-Over-Cable system is currently in production. In July of 1996, Lucent Technologies placed a large-scale production order for Sanders' PCS-Over-Cable equipment to support the deployment of PCS in Southern California for Cox California PCS Inc., a subsidiary of Cox Communications Inc.

Under the Sprint PCS name, Cox launched service in the San Diego portion of the major trading area (MTA) last December. Cox had previously won a pioneer's preference license for the Southern California market. Service is now available to 2.8 million people out of the 20 million covered by the license. As reported in Wireless Week, about 50 percent of the initial service base is covered by CMIs from a geographic standpoint, or 56 percent from a population perspective. [2] This network installation represents the first deployment of the Sanders' PCS-Over-Cable system.

Through its ability to simulcast, the system can be scaled both in geographical area (from hole filling to wide area coverage) and call capacity (from low demand initial buildout for mobility to high demand for mobility and wireless local loop). This technology is equally applicable for future wider bandwidth CDMA signal types and other operating bands such as the 800 MHz cellular band. In addition, protocols other than CDMA are currently being investigated. Depending on customer and market needs, other product variations are being targeted for production later this year.

System overview

The PCS-Over-Cable system, illustrated in Figure 1, is a system which provides the capability for wireless telephony services at PCS air frequencies over two-way upgraded cable TV HFC networks. The system was developed specifically for the IS-95A CDMA standard using patented CDMA technology licensed from Qualcomm Inc.

As shown in Figure 2, the system consists of three primary elements:

  1. Cable Microcell Integrator (CMI)
  2. Headend Interface Converter (HIC)
  3. Headend Control Unit (HECU).

Cable Microcell Integrator

The upgraded cable TV HFC infrastructure serves as the transport medium to distribute the PCS signals between the telephony equipment, located at the cable headend facility, and the CMIs, which are attached to the cable plant at remote locations throughout the service area. Multiple CMIs ("clusters") simultaneously transmitting and receiving the same CDMA carrier are referred to as a CMI "simulcast," typically requiring between two to eight CMI units. A wide service area can be covered by simulcasting the same CDMA carrier over multiple CMIs.

The CMI is the interface transceiver between the wireless mobile user handset or the wireless local loop (WLL) subscriber unit and the HFC network. Depending on the topography, capacity requirements and building type present in the predefined coverage area, the CMIs can be mounted on the cable TV plant approximately every 0.25 to 0.75 miles. Under certain terrain conditions, a CMI's range can be as far as one mile.

A CMI communicates at assigned wireless frequencies via a single transmit antenna and two spatially diverse receive antennas. In the forward link, the CMI converts the cable-based signal carrier it receives from the headend to wireless frequencies and radiates the signal through the transmit antenna to the user handset. In the reverse link, RF signals from the handsets are received by the CMI, converted to a reverse link cable plant frequency, and sent to the headend over the cable TV HFC network.

The CMI communicates with the HIC, and ultimately, with the HECU for the operation, administration, and maintenance (OA&M) functions. It is remotely tuned on command from the HECU to a specific carrier within the applicable frequency block. If the CMI detects a fault, it will immediately disable all outputs, thus never affecting cable TV HFC network performance.

Headend Interface Converter

Each HIC serves as the interface between three sectors of a Base Transceiver Station (BTS) and the HFC cable plant. In the forward link, the CDMA carrier is received by the HIC, which frequency multiplexes the signal onto the cable TV plant at a pre-selected cable TV channel for transport to the CMIs. In the reverse link, the HIC converts the diversity pair to RF signals for output to the BTS.

One HIC can control multiple CMIs in a simulcast for each sector, maintaining full control over CMI operation and frequency assignment. As directed by the HIC, the CMI can tune to any channel assignment within a predefined frequency band, independent of the remotely-selected cable TV channel. The HIC assigns individual CMIs to interface (in simulcast) to a BTS sector, thus establishing the sector footprint. Each HIC provides the cable TV interface at the headend facility for up to three CDMA BTS sectors. Two or more HICs may be combined to support six-sector or larger BTSs. Usually, the BTSs and associated HICs are co-located in the headend with the video and fiber distribution equipment. Alternatively, the BTS/HIC may be remotely located away from the headend using any suitable transport medium (e.g., fiber), allowing their co-location with existing towers or other convenient sites.

The HIC manages CMI operation through a variety of control messages, including frequency assignments, sector assignment, gain control, enable/disable, output power, operating temperature, status query and other critical operational parameters. Special, non-routine queries and commands can also be sent to the CMIs manually via the HIC. Each HIC has an onboard processor to manage all sector operations and end-to-end system gain. In addition, the HIC communicates with the HECU for the CMI/HIC OA&M functions.

Headend Control Unit
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The HECU functions as the control processor, controlling all HIC and CMI OA&M. The OA&M functions include monitoring and control of the CMI/HIC parameters, detection of errors, generating alarms, and collecting, processing and displaying network information. The HECU polls each HIC to collect CMI/HIC alarms, status and statistics. There is no direct link between the HECU and the CMIs; all HECU messages to and from the CMIs are directed through the controlling HIC. The HECU, along with up to 14 HICs, are mounted in a common 19-inch primary rack. An additional expansion rack can be added to increase the number of HICs to 28. Thus, one HECU provides control and status for up to 28 HICs and more than 500 CMIs.

The HECU installed in the primary rack is a Pentium [3] class computer. The HECUs, one for each headend facility, can be remotely accessed in a multiple headend configuration from a Network Operation Control Center (NOCC) over dedicated serial links. The HECU OA&M function can be controlled from one of three sources: locally at the headend, remotely from a centralized operations center, or via a dial-up modem.

CMI frequency translation

Figure 3 illustrates the frequency conversion required for the CDMA signal to be transported between the BTS and a mobile user.

(see figure 3)

In the forward link, three CDMA carriers (*, ß, and *) arrive at the HIC in the 1930 MHz to 1990 MHz frequency range. The HIC maps up to three of these CDMA PCS carriers into a traditional 6-MHz cable TV channel using either harmonically-related carrier (HRC), incrementally-related carrier (IRC), or standard EIA frequency plans. The HIC can place the 6-MHz channel anywhere in the 450 MHz to 750 MHz range.

The CMI receives cable TV signals from the HIC tuned anywhere in the range of 450 MHz to 750 MHz frequency band and selects one of the three CDMA carriers grouped within the channel. It tunes to a CDMA PCS carrier in the range of 1930 MHz to 1990 MHz, upconverts the received signal, and transmits it to the mobile user handset through the single transmit antenna. Multiple CMIs in simulcast extract and transmit the same CDMA carrier. In addition, a control and reference signal pair are used to control and frequency lock the CMIs. These signals are located at 52 MHz and 52.5 MHz, respectively.

In the reverse link, the CMI receives CDMA PCS signals, ranging from 1850 MHz to 1910 MHz, from a handset through two spatially diverse receive antennas. The CMI then combines these signals along with a reverse link control tone into 4 MHz of cable spectrum tuned between 5 MHz and 42 MHz, with a step size of 250 kHz. Multiple CMIs in simulcast combine their received signals for transmission to the HIC/BTS. Where different CDMA carriers are located on separate fiber nodes, the same 4 MHz reverse link plant frequency allocation may be reused, minimizing cable TV spectrum requirements. Under higher capacity implementations, multiple reverse link carriers may be frequency multiplexed on the reverse cable plant. When three CDMA carriers are on the same fiber node, the three diversity pairs would require 12 MHz of reverse link spectrum. The control tone for each diversity pair is positioned directly in the middle of the two reverse link carriers.

The HIC receives the cable TV signals from the HFC network in the range of 5 MHz to 42 MHz and translates them to RF frequencies in the range of 1850 MHz to 1910 MHz, as required by the BTS. The HIC filters out other reverse link spectrum signals (e.g., cable modems, set-top box control signals).

RF coverage

The PCS-Over-Cable system is a simulcasting system using multiple CMIs deployed over a coverage area. Their location can be determined independently of the call capacity requirements of the service area. The CMI provides a coverage radius of approximately 0.31 miles. The exact range depends on the topography and the number of obstructions (e.g., buildings) in the coverage area. Because of the CMI's small size (approximately 40 lbs., and 18 inches L × 10 inches H × 10 inches W), an RF planner has the flexibility to place them anywhere a cable plant already exists or can be installed economically. Telephone poles, the sides or tops of buildings, billboards, tunnels, subways, bridges, and inside buildings are all likely candidates for CMI installation, depending on the network application.

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Through increased flexibility in installation, the PCS-Over-Cable microcell-based network is less susceptible to RF shadowing. The coverage of a CDMA carrier will be equal to the sum of all the individual CMI coverage areas in the same simulcast. Individual CMI coverage can vary widely depending upon terrain effects a CMI on local high ground can have significantly greater range (more than 1 mile) than one in a low-lying, heavily-wooded area. For a simple flat earth estimate in a suburban environment, the CMI range is about 1,600 feet. Allowing for coverage overlap, three sectors with eight CMIs each in simulcast can provide coverage for a 7.2 square mile suburban area.

Early in the network's life, the PCS-Over-Cable system's simulcast feature allows the PCS operator to minimize initial equipment costs by providing RF coverage at low call density. Initially, CMI coverage is achieved through a simulcast of typically eight units, or 24 CMIs, for a three-sector BTS. In some cases, larger simulcasts may be possible. Over a period of time, CMIs can be remotely reassigned to reduce the number of CMIs in each simulcast to support the additional BTSs incrementally installed in the network. This will increase the call capacity without the need to change CMI hardware or replan for RF coverage.

Centralized call processing

Clusters of CMI transceiver units are distributed throughout an area to provide RF coverage, while call capacity may be varied throughout the area by changing the CMI simulcast number as required. Call processing functions are centralized to allow the separation of coverage and capacity in a given service area. This means that a variety of coverage geometries can represent individual sectors (e.g., CMI strand along a highway) providing added flexibility to the PCS operator. Sector size may be adjusted easily to accommodate the varying demands within different locations of the service area. Traditional tower-based systems are constrained to arranging their sectors within the area limited by the tower's RF footprint.

The PCS-Over-Cable system allows centralization of BTS electronics to minimize the number of BTS units. All of the base stations which perform call processing on the transceiver units are located at one headend facility. With coverage and capacity decoupled, a lower cost per sector six-sector BTS can be used, instead of multiple three-sector BTSs. The same coverage area in a traditional tower-based network would require two separately located, three-sector BTSs, one to support call capacity for each tower's RF footprint. In an initial deployment hardware configuration, PCS-Over-Cable will require less BTS equipment.

PCS-Over-Cable enables the PCS provider to deploy BTS capacity incrementally, on a network-wide basis, as customer demand increases. The system uses the expensive capital asset (BTS) more efficiently than the traditional tower approach does. Thus, a significant time value of capital savings is realized by the PCS provider.

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Greater call capacity is achieved by configuring with low simulcast numbers (less than four CMIs per carrier). This easily concentrates call capacity into smaller geographic areas, which is especially important for D/E/F-band PCS providers because the number of simultaneous carriers that can be transmitted from a site is limited by the available spectrum bandwidth. By contrast, when a traditional tower-based system reaches its capacity limit, additional towers must be constructed between the initial towers.

Centralized call processing also minimizes recurring BTS maintenance costs. In this system, the technician has full access to a large number of centrally-located BTSs without ever leaving the headend facility. This facilitates equipment maintenance procedures. Thus, call processing for a wide-area network is centralized at one headend facility, allowing for flexible, efficient, and economical network management.

Network growth

As subscriber demand increases, network capacity will need to grow to support the additional demand. The PCS-Over-Cable system uses the CDMA protocol to initially build out a service area by deploying a cluster of N CMIs per sector (typically, N = 8 CMIs) to simulcast the first 1.25 MHz carrier over the defined coverage area. Once the distributed antenna system is installed, physical reconfiguration (e.g., addition of new CMI units) is not needed to increase system capacity. The specific number of CMIs required for a given area at initial build-out is dependent on standard RF propagation parameters such as terrain, antenna elevation, gain and the multipath environment. At initial deployment, the typical ratio of BTS sectors to CMIs is 1:8 to provide the greatest coverage to a service area, while keeping the BTS quantity to a minimum.

As capacity demands increase throughout the service area beyond the call processing capabilities present at initial deployment, the given sector serviced by N CMIs may be reassigned so that two sectors can service the same area, each requiring N/2 CMIs. This is accomplished by adding a BTS/HIC pair and reassigning a portion of the previously-deployed CMI units to service the new sectors. This is illustrated in Figure 4. Reassignment is accomplished remotely by the OA&M software, requiring no physical configuration changes to the fielded CMIs. At this stage, the ratio of BTS sectors to CMIs typically decreases to 1:4 to support a medium call demand on the network. The two sectors now each require fewer CMIs than the original single sector, but can still use the first 1.25 MHz carrier. Because CDMA permits a frequency reuse of one, all CMI simulcast sectors are tuned to the same PCS frequency, maintaining soft hand-offs throughout the network.

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Figure Three

Finally, as the service area grows into a high call demand network, the ratio may decrease to 1:2. At this point, the three-sector BTS will support six CMIs, two per sector, within the defined service area. Again, reassignment is accomplished remotely by the OA&M software and, additionally, soft hand-offs are maintained longer throughout the network. At this level of capacity, a tower-based system may require multiple carriers (on different PCS frequencies), which requires hard hand-offs earlier in the coverage area.

Should a service provider be successful enough to require even greater call capacity, a second network of CMIs can be co-located with the first to support a second carrier. Thus, a CMI-based network increases capacity more efficiently than a traditional tower-based network by reallocating sectors geographically and sustaining soft hand-offs over a longer period of time.

The capability of reassigning CMIs within sectors is currently provided in the PCS-Over-Cable system, but in a "static" sense, referred to as static reallocation. In other words, as capacity demand increases, the CMIs are reassigned and then left that way until some time later in the network's life, when capacity increases again and the assignment of CMIs per sector requires re-evaluation.

Sanders is currently developing the concept of dynamic reallocation. Using dynamic reallocation, the reassignment of CMIs within sectors could be achieved automatically and on a continuing basis once the peculiarities of capacity needs in a given service area are analyzed (e.g., traffic patterns) and a daily capacity pattern for the network is determined. This capability is not built into the PCS-Over-Cable system at this time, but could be added through system software changes in the cable plant headend. The CMIs need not be changed to accommodate this feature.

Thus, the PCS system features result in several advantages:

  • Flexible installation for greater and more uniform coverage over wide service areas and diverse terrain conditions
  • Fewer BTSs are required in an initial deployment configuration, and BTSs are added incrementally over a longer period of time as capacity increases in the network
  • Once installed, no physical reconfiguration is required
  • Centralized call processing uses lower cost BTSs and minimizes BTS maintenance costs
  • Network responds to changing capacity requirements through easy, software reassignment of hardware
  • Soft hand-offs are maintained longer in the system network.
System installation and maintenance

The PCS-Over-Cable system is typically installed on aerial cable plant, strung between telephone poles 23 feet above the ground, and spaced 2,100 to 4,200 feet apart. The CMIs are weather-resistant, consistent with common HFC line equipment. Installation and maintenance methods are similar to methods used for cable TV trunk amplifiers and other active line components. CMIs are coupled to the cable plant independently such that currently available video and data services are not affected while the unit is being serviced.

The CMI employs two receive antennas and one transmit antenna for operation. In a typical cable strand operation, small, 6 dBi gain antennas, approximately eight inches in length, are used for both transmit and receive. Antennas can be either omnidirectional or directional (e.g., highway coverage) depending on the specific coverage application. The two receive antennas are installed on brackets which hang down from the strand with the transmit antenna standing upright between them. Separation between the transmit and receive antennas must be at least 40 inches to provide a minimum of 40 dB RF isolation between them. CMIs can also accommodate an underground cable plant by installing the CMI (surface or subsurface) and remoting the antenna to a nearby light pole or other elevated position.

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In general, CMIs can be installed wherever cable TV (or dedicated) coaxial cable exists. CMIs can be installed in subways, tunnels, airports, bridges, office parks, inside office buildings, malls, hospitals, etc. They can be installed on rooftops or on the sides of buildings if cable plant is nearby or if it is economical to run dedicated cable where existing cable TV infrastructure does not exist. Locating the CMIs to achieve optimum RF coverage enables the RF planner maximum flexibility when planning out the defined service area.

Simulcast feature

The PCS-Over-Cable system uses the CDMA standard developed by Qualcomm. The real advantage in using CDMA is the increased capacity it provides, as well as its robustness in the presence of multipath interference. Thus, with CDMA, better reception is sustained, and more links may be packed into a limited bandwidth than might be possible with any other available access scheme. [4]

CDMA employs hard, soft and softer hand-off procedures to support the mobile user's need to change channels within a given coverage area. Using CDMA, carriers are distinguished from each other either by different PCS carrier center frequencies or different pseudorandom noise (PN) timing offsets. A hard hand-off occurs when the mobile user transitions between two base stations supporting two different carrier frequencies. A soft hand-off results when a mobile user moves between two base stations using the same carrier frequency but different PN offsets. Soft hand-offs are preferred to hard hand-offs because the mobile can be simultaneously serviced by multiple base stations, thereby preserving audio quality through selection diversity even under highly variable RF propagation conditions. A softer hand-off results when a mobile user moves between sectors within the same cell. [5]

Simulcast in the PCS-Over-Cable system, illustrated in Figure 5, adds another form of hand-off which is transparent to both the BTS and the mobile user. In the CDMA waveform, simulcast appears to the BTS and the mobile as multipath, which both the BTS and mobile unit are designed to use to their advantage.

Figure 6 shows an example of the effect of simulcasting upon the time-based correlator for a CDMA receiver. The relative delays for each correlation peak are the sum of propagation delays along the cable plant to different CMIs and the RF propagation delays from the individual CMIs to the mobile handset. Differences greater than 2 μsec are easily resolved by the correlator and are easily met during the layout of the CMI network.

The limit on the number of CMIs in simulcast is established by noise summation on the reverse link. The total reverse link system noise figure increases as 10 log(N CMIs in simulcast). The reverse link range allows the CMI to use forward link transmit power levels less than that of a tower (2.5 watts). With the strand-mounted CMIs typically at 23 feet in height and at lower transmit power levels, the coverage area of a single CMI must be less than that of a tower. Through the use of the simulcast feature, however, CMI coverage can be as large as the footprint of a tower. In fact, if CMIs were operating at a height comparable to towers, the coverage area of a CMI simulcast would be greater.

Network architecture

The cable TV plant provides the network infrastructure, illustrated in Figure 7, through which all communications and statusing to and from the CMI cluster is achieved. The network also provides a means by which the PCS signal is translated to and from the base station and transponder. In general, the function of the HIC/CMI system in the forward link is to downconvert the PCS telephony signal to be compatible with the cable TV system and send the signal through a combiner for addition with broadcast video signals, cable modems, and other advanced services (e.g., video-on-demand (VOD), near video-on-demand (NVOD), impulse pay-per-view).

After the signal is combined, it is then sent through the laser transmitter and converted to light for transmission over fiber optic cable to the fiber nodes. At the fiber node, the signal is converted back to coaxial medium for transmission out to the neighborhoods where the CDMA signal is extracted and re-radiated in its original form through a simulcast of distributed microcells.

In the reverse link, the CMI serves as a distributed reception array servicing a single PCS sector. Once received, the telephony signal is converted to the appropriate cable frequency by the CMI and added onto the cable plant along with the other cable services and transmitted in the reverse path to the fiber node. At the fiber node, the signal is converted to light for transmission over fiber to the laser receiver located at the headend. Here, the combined signals are converted back to an electrical medium and distributed to all of the reverse path headend equipment. Once extracted, the HIC sends the PCS telephony signal to the BTS for demodulation and to the switch for further routing. If the destination is a wireline network, the call will be routed to the PSTN.

Up to three CDMA sectors of the BTS telephony equipment interface to the cable plant through a single HIC unit. A CDMA sector/HIC can interface to single or multiple cable plant fiber nodes, depending on the differences between fiber node coverage and the desired coverage of a single CDMA sector. Any number of CDMA sectors/HICs may be combined for operation on a single fiber node or group of fiber nodes. A typical installation will have dozens of CDMA sectors with their associated HIC units connected to dozens of fiber nodes simulcasting the same PCS frequency.

With multiple fiber nodes, separate CDMA carriers from different HICs/BTSs may re-use the same 6 MHz cable TV channel allocation on the cable plant. The same cable TV channel will be used across the entire cable network, while the CDMA content on each fiber node in that channel allocation will be different. This is also true for the efficiencies of reverse cable spectrum frequency allocation.

Headend wiring topology

The interface between the PCS headend equipment and the cable TV HFC network treats the reverse and forward links slightly differently. On the reverse link, each sector is treated independently, with each diversity receive pair requiring 4 MHz of reverse link cable bandwidth anywhere in the 5 MHz to 42 MHz region. The telephony call capacity per geographic area determines the number of fiber nodes per PCS sector, or the converse, the number of PCS sectors per fiber node. Typically, based upon RF propagation and coverage, there will be only a few CMIs per fiber node in some cases, just one.

The simulcast of CMIs which constitute the entire sector will span multiple fiber nodes. Because the same reverse link cable TV frequencies can be reused for different PCS sectors, usually only 4 MHz of reverse link spectrum is required to support a PCS-Over-Cable network. Only when a fiber node splits across the coverage area of two or more PCS sectors will more than 4 MHz of bandwidth be required. In most cases, this situation can be avoided by considering the layout of the cable plant when deciding upon the boundary regions of the PCS sectors.

Because three CDMA carriers fit within a single cable TV channel, the forward link interface between PCS headend equipment and the upgraded cable TV HFC network is considered three carriers at a time. On the forward link, the entire group of three carriers is broadcast across the sum of the fiber nodes that comprise the three carriers. Each CMI selects one of the three forward link CDMA carriers for transmission. There is a single control and reference signal pair that is shared across all CMIs within the three simulcast sectors. A single HIC is used to generate the three CDMA carrier grouping, along with the control and reference pair, and it supports a single three-sector BTS. The control and reference pair from one HIC is independent from those of other HICs. Six-sector BTSs can be simply supported by the use of two HICs in parallel, while still maintaining the same three-carrier grouping described above.

Through frequency reuse across the cable plant HFC network, multiple HICs can interface to the cable plant, all using the same cable TV channel, with each one carrying different CDMA carriers. These carriers can be either the same PCS frequency with different PN offsets or different PCS frequencies. The relationship between PCS frequencies, PN offsets, forward link cable TV frequencies, and reverse link cable TV frequencies are all independently and remotely selectable from the headend control unit. These control functions can also be accessed from a site remote from the headend such as a NOCC, allowing multiple headends to be controlled from a single centralized location.

Cable plant spurious requirements

CDMA, as transported by the PCS-Over-Cable system, makes few demands upon the dynamic range and ingress performance levels required of the cable plant. The forward link is straightforward because the range of power levels between a CDMA pilot and a fully loaded carrier is only 7 dB. Each fully loaded carrier operates at 15 dB below video reference. Spurious signals within the CDMA signal resulting from the cable plant need only be 40 dB below video reference, well within the operating characteristics of present HFC networks. The CMIs incorporate all the requisite filtering needed to reject the other video and digital signals coexisting on the cable plant to meet the transmission purity requirements of both the FCC and J-STD-019 ("Recommended Minimum Performance Requirements for Base Stations Supporting 1.8 to 2.0 GHz CDMA Personal Stations").

The CDMA reverse link is resistant to cable plant ingress levels because CDMA is a spread spectrum signal, and it is designed to operate at or below noise. The only requirement is that ingress power be at least 10 dB below the noise floor of the simulcast of CMIs when placed upon the cable plant. The cable plant needs only to maintain a 30 dB noise/ingress free range. As with the forward link, the reverse link has requirements imposed upon it by J-STD-019. The standard includes some high dynamic range desensitization requirements (e.g., two interfering tones 70 dB greater than the CDMA signal power at 1.25 MHz and 2.0 MHz frequency offsets) as well as linear operation with CDMA signals as strong as -65 dBm. The CMI's reverse link receivers not only translate the PCS carriers to cable plant frequencies, but filter out the interfering signals and perform the necessary gain control to meet both requirements without placing the dynamic range burden on the cable plant, nor on the fiber link transceivers within it.

Conclusion

The ongoing upgrade of the traditional cable architecture to a two-way HFC architecture has provided cable operators the luxury of adding new broadband communications services to their cable plants through significantly increased capacity. Thus, the modern cable TV distribution plant, with upgraded fiber architectures already in place, is a good platform for PCS. This diligent upgrade effort over the last decade has placed the cable industry in a strong position to provide PCS service to its customers with minimum additional investment. [6]

Sanders' PCS-Over-Cable system is a self-compensating, closed-loop, transparent extension of the BTS utilizing modern, highly reliable HFC networks. Leveraging the existing broadband infrastructure, the system allows carriers to build uniform, wide-area coverage quickly and economically.

Using PCS-Over-Cable, RF coverage and call capacity are more independently controlled than in traditional tower-based networks. With a CMI network, the simulcast feature enables greater and more uniform RF coverage through increased flexibility in installation and permits a network to be tailored to the capacity demand of the service area. Centralized call processing provides quick response to call capacity changes, resulting in optimum network performance. The centralization of BTS electronics at one headend facility minimizes the number of BTS units, as well as recurring BTS maintenance costs. Initially, the system can be inexpensively built out to meet coverage and low initial call capacity requirements. Later, the same physical CMI hardware configuration can still be used to support increased capacity network conditions.

As call capacity demand increases over time, only the headend equipment needs to be expanded. Network growth is achieved more efficiently through software reassignment of CMI hardware while sustaining soft hand-offs over the life of the network. The potential addition of dynamic reallocation to the PCS-Over-Cable architecture will add even more flexibility to tailor the system automatically to ever-changing capacity demand.

Sanders' PCS-Over-Cable system provides a PCS provider with a network platform that will accelerate service introduction to any area including those facing difficulties with the construction of towers. The PCS-Over-Cable system eliminates time-to-market delays and the costs associated with regulatory issues such as land acquisition, permitting, zoning restrictions, and construction delays found in the conventional tower site acquisition process. For the first time, carriers have an alternative to traditional macrocell networks. The PCS-Over-Cable system is now available to provide wide area wireless coverage for both mobility and wireless local loop applications.

Biographies

Yvette Hubbel holds B.S.E.E. and M.S.E.E. degrees and is currently a manager of business development in the Telecommunications Systems Division of Sanders, a Lockheed Martin Company. Her current technical interests include mobile communications and wireless networks.

John Sabat also holds B.S.E.E. and M.S.E.E. degrees and is currently a manager of systems engineering in the Telecommunications Systems Division of Sanders. His current technical interests include communications and signal processing systems.


References
[1] PCS-Over-Cable is a trademark of Sanders, a Lockheed Martin Company.
[2] Fred Dawson, "Cox Eyes More Use of Cable Network," Wireless Week, 1/13/97.
[3] Pentium is a registered trademark of Intel Corporation.
[4] Fred J. Ricci, Personal Communications Systems Applications, Upper Saddle River, N.J., Prentice Hall Inc., 1997, p. 39.
[5] William C. Y. Lee, Mobile Cellular Telecommunications: Analog and Digital Systems, Second Edition, New York, McGraw Hill Inc., 1995, p. 531.
[6] Richard Mueller, "Modern Cable TV Networks, The Ideal Platform for PCS," Proceedings of the IEEE National Telesystems Conference, Atlanta, Ga., pp. 059 – 064, Piscataway, N.J., IEEE Service Center, 1993, p. 064.

Acknowledgments
The authors would like to extend special thanks to the Telecommunications Systems marketing group for its support of this article.

 

Since the beginning of cellular phone service, consumer demand for wireless services has far exceeded the most optimistic market forecasts. With the recent introduction of six new PCS carriers into each market, competition for the wireless subscriber is expected to become fierce. High quality and cost-effective networks will play a critical role in differentiating individual carriers from their competitors. In this article, Sanders, a Lockheed Martin company, describes a network architecture and system solution, called PCS-Over-Cable [1], which will make the "anywhere, anytime" PCS vision a reality today, independently, or as a complement to conventional tower-based systems.

The system allows carriers to quickly and economically construct wireless networks. Developed for the high voice quality, high capacity Code Division Multiple Access (CDMA) protocol, the system gives a PCS carrier the capability to construct wireless, mobility and local loop networks while tailoring network capacity for high call demand areas.

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