Providing service to one million subscribers with a MMDS broadband wireless system
by John Desmond, ADC Telecommunications Broadband Wireless Group

Broadband wireless Internet access is the hot issue of the moment and use of the MMDS frequencies to provide it has been in the spotlight since April of 1999 when Sprint and MCI spent nearly two billion dollars to acquire MMDS licensees. This validated the FCC's decision in September of 1998 which allowed for the first time use of the MMDS bands for two-way communications. MMDS is only one of several technologies that can bring high speed Internet and other broadband services to the home, but as Figure 1 shows, MMDS is almost uniquely suited to bring the benefit of these services to urban, suburban and rural consumers.

One question that has been raised is whether the relatively modest MMDS spectrum, a little more than 200 MHz, could support service to large numbers of subscribers in major metropolitan areas. This article shows how simple frequency reuse and cellularization plans can comfortably service very large numbers of subscribers. Described here is one possible configuration to provide wireless broadband Internet service to one million residential subscribers in a hypothetical large metropolitan region. The actual design to meet this service target will depend on many factors, like terrain and population distribution, which are beyond the scope of this preliminary study. The results, then, should be taken as one example of how this service can be realized, not the only possible approach. In this discussion "downstream" means transmissions from the system to individual subscribers, and "upstream" means return transmissions from subscribers back to the Internet.

Several simplifying assumptions are being used. We assume that:

  1. All subscribers will be offered the same data rates, as described below. This is optimized for Internet access, and represents typical residential service.
  2. Enough downstream MMDS channels are available to provide the necessary download speeds, making the limiting factor for a cell’s performance the upstream channel capacity. (This may not be the case where the service provider is already offering video service.)
  3. Use of the MDS channels for upstream traffic. Based on a total upstream bandwidth of 12 MHz, this study determines the downstream modulation, sectorization, and channel allocation required to balance the upstream performance.
  4. Cell size is in every case driven by capacity, rather than coverage.
  5. Coverage of the city will be approximately uniform, and that cells can be placed in optimum locations.

Service Offering

ADC Telecommunications offers a broadband wireless access system (Figure 2) that permits the delivery of a wide range of rates for wideband data services, giving wireless operators the option to address many types of end users in the same area with optimized service. To simplify this discussion, we will propose in this discussion an upstream and downstream data rate set appropriate for residential Internet service.

The selected residential service provides 256 KBPS in the downstream and 64 KBPS in the upstream. This is somewhat more symmetrical than is usual for a pure Internet offering, but is appropriate for service that will eventually have a significant portion of voice users (VoIP), or future interactive data services. It is a unique feature of the ADC system that a user can seize bandwidth in addition to that which his service set allots him when system loading permits, given that this capability is permitted within the Quality of Service level assigned to the particular subscriber. Because, by definition, this user is seizing "unused" bandwidth, this is not a factor in determining the total capacity of a cell.

Concentration Effect

A unique aspect of this platform is its ability to dynamically allocate bandwidth. In a data-based service, no single user has a need to occupy bandwidth 100% of the time. Therefore, when a user has a high need for bandwidth, a greater portion of the spectrum can be allocated to him for short periods of time. In this way, a service provider can meet the needs of multiple customers, while the apparent bandwidth of an individual customer can seem to be extremely high.

However, multiple users must be managed in such a way that during busy hour, when each user has a high need for bandwidth, users are guaranteed an acceptable quality of service. In this way, an MMDS service provider using this platform can deliver a service superior to wireline-based service, and receive premium revenue. Data concentration ratios must be chosen based on the optimal compromise between quality of service to the end user and total number of users supported, which yields economies to the service provider. Based on industry statistics for peak interval utilization, ADC has chosen a conservative set of concentration assumptions, shown in Table 1. This yields a statistical average data throughput per subscriber.

Table 1: Data Concentration Assumption Set

Residential Service     Equivalent Traffic/Subscriber
Peak interval utilization 4%   Upstream: 2.56 Kb/sec
Concentration 25 : 1   Downstream: 10.24 Kb/sec

 

Total Upstream Traffic Capacity in a Sector

The receive antennas used at the headend are not omnidirectional. Sector coverage antennas are used for added antenna gain, which serves to improve the link budget in the upstream and therefore hold down the power required (and cost) of the customer premises equipment (CPE). The total cell upstream data capacity will be the per-sector capacity multiplied by the number of sectors. The sectorization plan to be used will be determined by the total cell capacity required, the number of upstream channels available, the deployment limitations imposed by the terrain to be covered, and practical antenna technology.

Let’s first look at a simple scheme where four 90o beamwidth antennas are employed (Figure 3) and one 3 MHz block of the 12 MHz MDS radio spectrum is used in each sector. For a reliable upstream design, again in the interest of link budget margins, ADC recommends the use of QPSK modulation. Using QPSK modulation in the upstream path, a bearer data efficiency (actual subscriber data, excluding overheads) of 1.13 bits/Hz can be obtained. In each sector, a total of 3 MHz of radio spectrum is used. So for each sector, the total data throughput is 3.39 Mb/sec. . The number of residential subscribers that can be supported in one sector, then, using the 64 KBPS data rate and the concentration as shown in Table 1, would be 3.39Mb/s divided by 2.56 kb/s/subscriber or approximately 1300 subscribers/sector. 5200 subscribers can served by each four-sector cell.

Sectorization Plan For One Million Subscribers

The sectorization plan that is actually proposed for this network uses eight 45 sectors; in this design, each upstream radio channel is used two times per minicell. This configuration will support 10,400 subscribers under normalized traffic and propagation conditions.

An attractive feature of an eight-sector implementation is the ease of growth from an initial four sector design as traffic demands increase. In the eight-sector configuration, as shown in Figure 2, the minicell upstream capacity increases to its ultimate 10,400 subscribers, because each of the eight sectors still serves 1300 subscribers. This doubling of capacity can be accomplished without a visit to the subscribers' premises, as there is no need to change CPE antenna alignment or anything else at the subscriber's location.

 

 

The eight-sector implementation is shown in Figure 4.

 

For the mini-cell architecture, Figure 4 represents the maximum achievable sectorization scheme using available antenna technology. It employs eight sectors and reuses every channel twice within every cell.

Due to the directivity of hub and subscriber antennas, the dominant source of intercell interference is from co-aligned sectors that at their far border experience a signal to interference distance ratio of 5. For a propagation coefficient of 3, conservative in an urban environment, this results in a S/I of 21 dB. Users suffering from on-boresight undesired interference will need to use 16-QAM or QPSK modulation for the downstream signal. Fortunately, as ADC's experience in deployment in this band shows, terrain factors result in a large percentage of the covered sector to have quite high S/I allowing for even 64-QAM downstream modulation. A key point to bear in mind is that the S/I is very unpredictable in practice, thus the ability to program the modulation to work at a variety of S/I realizations is a fundamental advantage of the ADC system.

Equipment Requirements

Approximately 100 of these 8-sector cells will provide the necessary upstream capacity for one million subscribers.

To determine the downstream spectrum and equipment required for this service plan, it is necessary to determine the data capacity of a 6 MHz downstream radio channel. Following the same procedure as before, it can be seen that the statistical downstream load per subscriber is 10.24 Kb/sec.

Table 1: Data Concentration Assumption Set

Residential Service     Equivalent Traffic/Subscriber
Peak interval utilization 4%   Upstream: 2.56 Kb/sec
Concentration 25 : 1   Downstream: 10.24 Kb/sec

A major benefit of the ADC broadband wireless system’s design is the ability to select QPSK, 16QAM, or 64QAM downstream modulation on an individual subscriber basis, and to easily modify this selection as needed without visiting the customer’s location. As the minicells will be very small and distances will be short, it is probable that most subscribers will be able to use 64QAM modulation in the downstream path, although as mentioned above, a minority may need to drop down to lower modulation orders where interference problems exist. With 64QAM, the bearer data efficiency is 3.6 bits/Hz. For 16 QAM, the number is 2.37 bits/Hz, and for QPSK, 1.13 bits/Hz. The downstream capacity of a 6 MHz channel, and the equivalent number of subscribers statistically supported by the channel are:

Table 2: Downstream Channel Capacity

Modulation Bits/Hz 6 MHz Channel Capacity # of Subscribers
64QAM 3.6 21.6 Mb/sec 2109
16QAM 2.37 14.2 Mb/sec 1387
QPSK 1.13 6.78 Mb/sec 662

In a typical urban minicell deployment, where reflections may cause severe interference, it may be necessary to provide an even mixture of QPSK, 16QAM and 64QAM modulation orders. In this situation, the number of subscribers served by 6 MHz of spectrum can be calculated to be approximately 1400 subscribers

In this case, the minicell would require 7.5 6 MHz radio channels to support 10,400 subscribers.

Downstream Sectorization Plan

To simplify the frequency planning required to design and deploy a system, it is very useful to use the same sectorization plan for upstream and downstream signals. For the implementation requiring an even mix of QPSK, 16QAM, and 64QAM links, an RF bandwidth of 6 MHz is required per sector. The resulting number of subscribers that can be served is:

(6 x 8) x 1386/6 = 11,088 subscribers with an even mix of modulation orders

This would be best achieved using a single 6 MHz radio channel in each sector. This configuration results in a reasonable balance with the 10,400 subscribers determined by upstream capacity.

As RF channels are used twice in each minicell, a total of four MMDS 6 MHz video channels would be needed for this network implementation

Spectrum Alternatives

This analysis has shown that a realistic plan for deployment of wideband data services to one million subscribers in a typical large metropolitan area requires approximately one hundred minicells, and the use of four of the thirty-one MMDS video channels for downstream transmission. This is based on the assumption that all upstream transmission will be in the 12 MHz of the MDS 2.15-2.162 GHz band, and that it is critical to minimize the use of MMDS video channels.

ADC has designed its system to be extremely flexible, and easily scalable. As part of ADC’s product rollout, it is planned to offer an option to use inband MMDS channels for upstream late in 2000. If it is possible to make use of additional MMDS channels, possibly in conjunction with a move from analog transmission to digital video compression, the network complexity can be greatly simplified.

Table 2 below shows how the number of minicells required can be reduced if more spectrum can be made available for data transmission. It is based on using an even mix of QPSK, 16QAM, and 64QAM modulation in the downstream, and QPSK in the upstream link. As in the basic example discussed above, upstream and downstream capacity is balanced, with the limiting factor being the upstream link.

Table 3: Implementation with Added Spectrum

Upstream

Channels

Downstream

Channels

Upstream Capacity

(Mb/sec)

Downstream Capacity

(Mb/sec)

Subscribers

Per Minicell

Minicells

Required

2 4 27.1 113.6 10593 95
4 8 54.2 227.2 21186 48
8 16 108.5 454.4 42372 24
16 32 217.0 908.8 84744 12

 

Cost-Effective Growth Strategy

Considerable savings in real estate, equipment and backhaul costs are obviously possible where more spectrum is available. A less obvious benefit is in minimizing truck rolls to subscribers’ homes as the network grows to the one million subscriber level. If spectrum is available, a network of 12 four-sector minicells can be established at the beginning of network deployment. With the initial available spectrum (2 channels up / 4 channels down), each cell will support up to 5,200 subscribers as discussed above, for a network capacity of 62,400, and coverage of the entire city. Splitting the sectors to provide eight sectors per minicell will double the system capacity with no additional spectrum. Adding additional spectrum will grow the system up to the final capacity. None of these changes will require a visit to the subscribers’ locations to realign their antennas to a new cell. Once the eight-sector configuration is established, the addition of more channels will cause no changes in the co-channel interference level.

Most plans for implementing broadband wireless using MMDS focus on a staged implementation, starting with one or more large diameter "supercells", and adding minicells for growth. As shown here however, given equipment flexibility in modulation and channel size, an immediate implementation of a minicell structure can have advantages in serving a potential large population of users. The key is in knowing what capacity must eventually be attained and a sophisticated design for growth.

About the Author

John Desmond has thirty years of experience in the digital telecommunications field, and for the last nine years has been exclusively involved with wireless access technologies. He currently holds the position of Senior Manager, Broadband Wireless Access Market Development, with ADC in Minneapolis.