Testing N-PCS

There is no doubt that 2-way data messaging and voice paging are the paging technologies of the future. A beep and a text message are no longer good enough. Customers need -- and want -- the simplicity of voice paging and the flexibility to receive and send detailed messages. They want to respond to messages received on their pagers -- with their pagers. They also want to know that they are receiving all of their messages.

Enter narrowband PCS (N-PCS) or 2-way messaging systems. These bidirectional communication systems are based on ReFlex 25, ReFlex 50 and InFlexion protocols that support 2-way messaging between the infrastructure and subscriber units.

For the service providers, N-PCS means a major leap in infrastructure functionality and technology including:

*Reverse channel receivers

*Complex modulators

*Narrowband linear power amplifiers

*Tighter control of transmitter power.

This leap in technology means a major change in infrastructure installation and maintenance test tasks. Likewise, the change in the test task drives new requirements for the associated test equipment. Field service engineers require test instruments with features that are specific to the N-PCS application. In addition, these instruments must meet higher RF performance specifications.

In conventional 1-way paging systems, all communications is from the infrastructure to the subscriber. Therefore, the infrastructure contains only transmitters.

N-PCS systems, on the other hand, use 2-way communications between the infrastructure and the system subscribers. As a result, the infrastructure contains both base-station transmitters and receivers. In other words, N-PCS infrastructure is more like cellular or land mobile radio dispatch systems than conventional paging systems.

PROTOCOLS N-PCS ReFlex and InFlexion systems incorporate several sophisticated techniques to enable new services and better use system channel capacity.

The modulation format for alphanumeric data communications from the infrastructure to the subscriber unit in ReFlex systems is similar to the format used in Flex systems. The highest data rate is 6,400b/s. The communications channel from the infrastructure to the subscriber unit is referred to as the forward channel.

The InFlexion protocol provides voice messaging from the infrastructure to the subscriber unit. In InFlexion systems, two types of modulation are used on the forward channel. Four-level FSK is used to transmit data to the subscriber. Voice is transmitted using a form of linear modulation that drives the infrastructure transmitters to use complex modulators and linear power amplifiers. This is significantly different from conventional 1-way paging transmitters. To enhance channel capacity, N-PCS systems use frequency reuse on the forward channel. This drives the need for tighter power control of the forward channel when compared to simulcast 1-way paging systems.

With both ReFlex and InFlexion protocols, a 4-level FSK modulation format is used for communication from the subscriber device to the infrastructure. The communications channel from the subscriber unit to the infrastructure is referred to as the "reverse channel," and several data rates are used, ranging from 800b/s to 9,600b/s. The reverse channel is time division multiplexed, which allows it to be used by many subscriber devices. As such, the infrastructure receiver is designed to receive data packets at specific time intervals.

INFRASTRUCTURE RECEIVER TEST The N-PCS infrastructure receiver sensitivity test process demonstrates the need for application-specific test capability to support the task.

N-PCS receivers are digital data receivers. The sensitivity figure of merit is typically bit error rate (BER) or a related metric such as packet error rate (PER). It indicates a test setup that can be used to perform the receiver sensitivity test.

The signal generator generates a test signal simulating a subscriber transmission; this signal is input to the receiver under test. In order to simulate a subscriber, the signal generator data frame timing must be synchronous with the timing of the receiver under test. The receiver under test receives and decodes the input signal. It also determines the received BER by comparing the received data pattern to the predefined expected data pattern. The BER is communicated from the receiver under test to the PC via thediagnostic interface. The metric is then displayed to the field-service engineer on the PC. The field-service engineer reduces the RF input level to the receiver until the detected BER is the specified threshold. The associated RF input level corresponds to the receiver sensitivity.

To support the test as described, the signal generator must provide several advanced features. The signal generator should generate 4-level FSK modulation with a deviation accuracy of +/-100Hz with modulation pulse shaping per the protocol specifications. The data pattern transmitted must be operator-selectable to match the receiver under test. Most important, as shown in the test setup, the packet timing of the generated signal must be triggered by the receiver under test or related equipment. Therefore, the signal generator must contain a trigger input. This feature allows the timing of the signal generator to match the timing of the receiver under test.

In the past, paging field-service engineers used general-purpose communications system analyzers that do not contain significant application-specific test features for paging. This type of equipment is not capable of optimally supporting the test described, which requires either several pieces of dedicated test equipment or a customized communications system analyzer.

INFRASTRUCTURE TRANSMITTER TEST N-PCS transmitter testing provides a good example of the RF performance required in supporting test equipment. Some N-PCS transmitters use complex modulators and linear modulators. Transmitter technology is dependent on the supported protocols and the design implementation. An approach to verifying the transmitter hardware is to transmit either one or multiple single sideband tones through the modulator and power amplifier. The transmitted spectrum is analyzed for carrier feed-through, negative image suppression and intermodulation components. This test task typically requires a spectrum analyzer with a narrow resolution bandwidth of 300Hz and 70dB of dynamic range. Some communications system analyzers on the market today meet these performance specifications; many, however, do not.

ENHANCED SYSTEM ANALYZERS The advanced technology used in N-PCS messaging systems drives the need for customized test features and improved RF performance in the test equipment used for installation and maintenance. A similar trend in test-equipment requirements started in the cellular industry about six years ago. It was driven by digital cellular protocols, and test-equipment vendors responded by offering communication system analyzers with custom features, such as error-vector-magnitude measurement capability, to support cellular infrastructure test.

N-PCS is an emerging technology. As the market matures, more test equipment solutions should become available.

Motorola has responded to the test equipment needs of the narrowband PCS (N-PCS) industry by including test features customized for N-PCS infrastructure in its CyberTest system analyzer. The infrastructure system works with most types of paging base stations and includes several customized features that simplify ReFlex 25 and InFlexion equipment testing. The system has a built-in ReFlex/InFlexion reverse-channel signal generator, which was designed to simplify base-receive sensitivity testing. The system includes a wattmeter as well as a modulation analyzer and spectrum analyzer for testing 1-way and 2-way transmitters.

The subscriber test system tests ReFlex 25, ReFlex 50 and InFlexion units, and it provides system simulation functions such as forward channel signal generation and reverse channel decode. The system also performs several page scenarios that emulate voice and data messaging systems. Pagers can be tested while in a functional mode.