El préstamo de ejemplares digitales

The Globalstar system employs Qualcomm’s Code Division Multiple Access (CDMA) scheme to address the system’s multiple access issues. Multiple access is the ability to allow many users to share a common bandwidth. CDMA is one of several common techniques used to address multiple access. Aside from CDMA, FDMA and TDMA are the other techniques used in many communication systems. CDMA is a spread spectrum technology. This means that each user’s data is transmitted across the entire frequency rather than limiting it to a specific time slot or frequency band as with TDMA and FDMA respectively. CDMA operates by assigning each user a pseudo-noise (PN) code. The PN is a long, binary sequence that appears random. While the PN code appears random, it is generated using a specific algorithm that provides orthogonal codes. The PN code is

then used to modulate the data to be transmitted. [Ref. 20]

An example of the system is illustrated in figure 19.

CDMA allows several users data to be transmitted at the same time over the total bandwidth. Each user’s data is essentially stacked on top of the other data to be transmitted. As each user’s data is modulated with a PN code that orthogonal and unique to all other codes, the code isolates each user’s data in the code dimension. This is similar to different frequencies or different time-slots isolating each user in other multiple access schemes. CDMA also allows for better spectral efficiency and increases the ability to reuse frequencies. CDMA techniques also result in a lower average transmission power requirement as each signal is spread across the entire bandwidth. [Ref.

20]

Figure 20. CDMA Modulation Scheme [From Ref. 26]

D. SYSTEM ANALYSIS

Chapter II outlined a series of twelve requirements for CG Coastal Homeland Defense Operations. In order for the Globalstar system to be considered as a viable option, it must meet the minimum of these twelve requirements.

1. Coverage

Globalstar can provide near global coverage and in doing so meets the defined requirements. The polar regions, are the only areas not covered by the Globalstar system. The polar regions are not being addressed as part of costal homeland defense. The system also provides for increased circuit availability by making use of the path diversity attributes of a LEO system. This will equate to better coverage, even when operating in areas potentially blocked by large structures such as a shore-side environment.

2. Accuracy

Globalstar is able to meet the defined requirements for accuracy. Through the use of CDMA technology, the system is able to achieve an increase in bandwidth efficiency and therefore take advantage of forward error correction techniques. This helps to protect against bit error transmissions. CDMA technology and the use of a LEO constellation also allow for a lower transmission power.

This reduction in transmission power provides a substantial reduction in interference to others using the system.

3. Availability

The Globalstar system provides adequate availability for homeland defense operations. One advantage the Globalstar system can provide is that of compatibility with established cellular networks. The standard Globalstar SU provides the capability for the user to access a local cellular network if available. This provides the user with the option of using the cellular network vice the satellite

system for some communications. The system is available for use with AMPS and GSM cellular system.

One potential shortcoming of the Globalstar availability is that the system does carry with it a user capacity for each satellite. Each satellite is only capable of handling approximately 1000 users. This upper limit may prove constraining when dealing with times of extreme use such as generated by a national emergency.

However, these operating limits are offset by the system’s ability to manage its capability by such techniques as path diversity.

4. Cost

Cost is perhaps one of the largest benefits of the Globalstar system. Its costs are much lower than other satellite communication solutions for both usage and equipment. The cost per minute of usage is approximately .80 cents/min per channel. This is in comparison to the nearly $9 per minute per channel for INMARSAT. The other cost savings that Globalstar provides is that of the actual subscriber unit. The LEO constellation works to reduce the required power of the system. This power reduction equates to units that can utilize an omni-directional antenna, vice having to equip each unit with a stabilized, directional antenna. The entire system can be accessed with a SU that is the approximate size of a cellular telephone.

5. Interoperability

Globalstar provides an acceptable level of interoperability. It provides the capability to work as a switched network, providing point-to-point dial-up operations as well as access to PSTN/PLMNs. As mentioned

above, the system also provides easy access to a local cellular network when available. The dial-up capability allows for both ship-to-shore and shore-to-ship fully duplexed operations. It also provides the ability of the unit to access the CG’s Virtual Private Network (VPN) for access to CGDN+.

6. Latency

As discussed earlier, Globalstar eliminates the latency issues normally associated with GEO communication systems. This reduction of latency, to that of a typical terrestrial network, allows common networking protocols such as TCP/IP to be employed. There is also not distinguishable delay in any voice conversations. This is also in contrast to a GEO service provider.

7. Reliability

Globalstar meets the requirements for reliability. It has solutions designed with the maritime environment in mind and has proven reliable through several years of operations. It operates on proven technology and should provide a stable and reliable communications link in the future as technologies improve and the system is updated.

8. Capacity

Globalstar provides less than marginal system capacity for coastal homeland defense operations. The current architecture can only provide services for approximately 1000 users per satellite. However, the overall system capacity has been increased with the use of CDMA technology and path diversity. There is always a potential of the system exceeding its capacity of potential users during

times of extreme usage such as a national or regional emergency.

9. Ease of Use

Globalstar provides an easy to use system. For point-to-point calls, it is no different than a standard cellular telephone. The same is true for a network connection. The service can be accessed as any other dial-up service, so there are no additional training requirements involved.

The proliferation of cellular communications would ease the transition of the Coast Guard to a Globalstar solution.

10. Security

Globalstar meets the security requirements for operation. While the system is a wireless technology, the potential exists for transmission interception. This is offset by the use of both CDMA technologies and Globalstar’s system encryption. CDMA is a spread spectrum transmission technology that makes it difficult to intercept or monitor transmissions. Globalstar also provides an additional system encryption on. This add-on features a 128-bit encryptiadd-on key. The device can alternatively be programmed with Advanced Encryption Standard (AES) or Data Encryption Standard (DES) digital cryptographic algorithms. This provides end-to-end security for both satellite voice and data services. [Ref.

18] Globalstar’s use of a bent-pipe architecture minimizes the risk of signal manipulation and traffic analysis. This architecture also allows for user installed link encryption.

11. Maintainability

Globalstar provides a marginal maintainability solution. The space segment is fairly maintainable as it operates in-orbit spares for all satellites. The SU terminals were designed to reduce the power and therefore the size of the unit. This eliminates the need to maintain a stabilized, directional antenna. The coastal homeland defense operations will be conducted mostly by units 87’ in length and smaller. The Globalstar system’s reduced size provides an advantage to units with limited deck space.

The only potential downfall of the size reduction is the potential for the SU to be lost overboard during at-sea boardings.

12. Throughput

As a single channel service, Globalstar provides only marginal throughput. The defined requirement is 128 kbps.

Globalstar’s current architecture can provide only 9.6 kbps per channel. This is further reduced to only approximately 7.4 kbps per channel once FEC and bit interleaving is added to the overhead.

The potential exists to overcome Globalstar’s limited throughput. Qualcomm, Inc. is currently developing solutions to increase data rates to 144 kbps with actual throughput of approximately 128 kbps. These solutions essentially join 16 separate data channels. The separate channels are joined at the transceiver and are transmitted over a single carrier using unique CDMA technologies.

While these solutions meet the requirements of 128 kbps, there are some potential problems implementing them.

Currently, the technology is only experimental. Also,

Qualcomm, Inc. has not yet incorporated this technology in any maritime solutions; it is only being researched for aviation and land mobile applications. In order for the CG to take advantage of this type of technology, they would have to pursue the design, testing and initial implementation of such a system. This could prove quite expensive, but very beneficial in the long run. As Qualcomm, Inc. has not yet deployed the technology; there is no pricing data available as of the time of this research.

E. CONCLUSION

Globalstar can provide only a partial solution. While it provides some advantages over other systems, it still falls well short of the data rates required for coastal homeland defense operations. To overcome the low data rates, a solution to multiplex several channels must be implemented. This may cause problems, as the solution will have to be proprietary to the CG. One benefit the Globalstar system provides is increased security. The system is able to provide both link and full system encryption. This is an advantage that other systems do not provide. The system addresses several items of concern when compared with a GEO system, but it is unable to fully meet the connectivity requirements of a full solution.