Controlling base stations over microwave

New wireless applications have caused increased demand for base station and circuit transport equipment. Circuit voice frequency and signaling schemes are necessary in implementing any remote radio system controlled over microwave.

Remote-controlling two-way base stations has been commonplace for decades. Various radio system configurations demand differing control and transmission requirements. Microwave systems are a popular method of circuit transport but require a basic understanding of transmission systems equipment and practices.

Microwave radios The input and output signals of a microwave radio are not directly compatible with discrete two-way radios. Microwave radios provide a method of transport for many circuits at a time-sometimes up to several thousand. The signals carried by the microwave system are a composite, containing information from all combined active circuits. The combining process is called multiplexing. (See Figure 1 below.)

The most common multiplexing schemes are frequency-division multiplexing (FDM) used in conjunction with analog microwave and time-division multiplexing (TDM) used with digital microwave.

Analog multiplex equipment FDM equipment essentially slices up a portion of the HF frequency spectrum into a series of channel slots, each 4kHz wide. Every circuit in the multiplex (mux) system is assigned its own unique slot. Through the multiplexing process, translation and filtering methods convert each circuit into a single-sideband suppressed carrier (SSBSC) signal.

The collection of SSBSC signals (circuits), one stacked above the other with regard to frequency, forms what is referred to as the baseband. The composite baseband signal is applied to, and modulates, an FM microwave transmitter.

Although common carriers use analog microwave links that support thousands of circuits, industrial microwave applications typically do not carry more than 600 channels, and often fewer. Direct-to-line FDM equipment can be purchased on a per-channel basis to accommodate smaller systems in a cost-effective manner.

Digital multiplex equipment TDM is a digital process that converts discrete circuits into a single serial bit stream. The most commonly used scheme in the United States is to convert 24 circuits into a 1.544Mbps data stream, referred to as a T1. Such a T1 multiplexer is often referred to as a channel bank. The composite 1.544Mbps signal allows each circuit a maximum data rate of 64kbps, which is considerably greater than what is required for voice and radio applications.

Currently, digital microwave radios can transport from as few as one T1 to more than a hundred. As the system capacity increases, so do the required microwave transmission data rate and bandwidth.

Voice frequencies (VF) Whether the multiplexing scheme is FDM or TDM, a mux channel modem is required on both ends of each circuit. For radio applications, the modem is the point of termination for the base station on the hilltop, and the controlling equipment (such as a dispatch console) in the main office.

Each mux channel has a VF (audio) response of about 300Hz to 3,400Hz. The mux channel VF receive and transmit connections are referred to as the DMOD and MOD (respectively). They offer a balanced, 600V, two-wire (2W) or four-wire (4W) connection to the associated equipment.

The microwave and mux equipment operate in a full-duplex mode. This means the transmit and receive functions are active at the same time. Used in conjunction with full-duplex base stations and user-end equipment, radio-telephone interconnect companies can provide customers with services that allow them to listen and to speak at the same time.

VF levels Although mux modems operate over a wide range of VF levels, good engineering practice dictates that the maximum transmission level, referred to as test tone level, should not be exceeded. test tone level at the DMOD and MOD of the mux modem are 17dBm and 216dBm, respectively.

Excessive levels are a greater consideration with analog mux and microwave equipment because much of the signal processing requires circuitry to be operating in a linear mode. When levels become excessive, circuitry can be driven into non-linear operation, resulting in intermodulation distortion.

Transmission systems engineers often use the term dBm0 to express the power level of a signal at a particular point in a circuit, referenced to test tone level. For example, if a tone level of 0dBm is measured at the mux channel DMOD, the level can be expressed as either 7dB below test tone level, or just simply 27dBm0. (Remember, test tone level at the DMOD is 17dBm.)

The VF levels of each mux channel modem can be altered by means of variable receive and transmit gain adjustments or internal pad strapping options. For the sake of standardization, mux levels are often set identically for each circuit within the same application (i.e., all radio circuits may be operated at 210dBm0 while all data circuits operate at 213dBm0, etc.). In such cases, any level variations required by different equipment within the same application are accommodated by the use of external pads.

Signaling Most circuit applications also require the transmission and reception of supervisory information to perform a particular function related to circuit operation. In a phone circuit, supervisory information may indicate an off-hook condition to the central office, while in a radio circuit, it may activate a transmitter. The transmission and reception of supervisory information within a circuit is achieved by means of signaling.

In-band signaling is accomplished by use of one or more frequency specific tones that fall within the VF passband of the mux channel. Presence or absence of a particular tone frequency within the circuit activates or deactivates the needed function. For example, tone remote base stations use a specific in-band tone signaling scheme that can activate a transmitter, change operating channels and disable receive CTCSS decoder functions.

Out-of-band signaling is achieved by transmitting and receiving supervisory information over the circuit, but from outside the normal VF passband. In FDM systems, out-of-band signaling is accomplished by insertion of a signal on either the edge of the channel’s frequency slot or within the slot but outside the VF passband (such as a 3,825Hz tone). Presence or absence of the signal determines the state of the supervisory function. In TDM systems, signaling is determined by the logic state of dedicated signaling bits.

Whether analog or digital mux is used, a mux modem has two primary connections dedicated for signaling functions. These are referred to as the E-lead and the M-lead. The M-lead is the transmit signaling lead. When the mux modem M-lead is activated, signaling is transmitted to the far-end of the circuit. Reception of the far-end M-lead activates the local E-lead. Digital mux has the capability of providing two E- and M-leads, which proves to be advantageous in certain applications, as shown in Figure 8 on page 36.

Main distribution frame (MDF) The VF and E- and M-lead connections from each circuit are cabled from the mux equipment rack to terminal blocks, typically located on the inside wall of the site. Most signal-carrying site equipment (including any external pads) is cabled to the same general location. The collection of all equipment terminal blocks can then be found in the same portion of the building, on the same wall and is referred to as the main distribution frame (MDF). With all equipment signal connections available in one location, jumpers (referred to as cross-connects) can be made from one piece of equipment to another by connecting wires between the respective blocks. In this manner, the mux channels are cross-connected to their respective pads and base stations.

Multiplexed signal connections FDM systems require coaxial cables to carry the multiplexed receive and transmit baseband signals between the multiplex equipment and the microwave radio. On the other hand, T1 signals can use inexpensive telephone wire (twisted pair). (See Figure 2 on page 24.) Cable specifically designed for higher data rate applications, such as Category V, allow spanning of greater distances between the mux and microwave equipment. It is a good practice to have the transmit and receive pairs on separate cables, or at least in separate bundles if the same cable is used.

Practical applications Figure 3 on page 26 offers an example of a remote-controlled base station using E- and M-lead signaling. Here, the dispatcher can press a button on the console to key the hilltop transmitter. The button causes switch contacts to close routing battery voltage to the mux channel M-lead. Activation of the M-lead is detected on the hilltop end, causing a contact closure of the hilltop mux channel E-lead relay. The E-lead relay contacts apply a ground to the base station PTT line, keying the transmitter.

As the dispatcher speaks into the microphone, voice audio leaves the console and, after appropriate padding, is applied to the mux channel MOD. The voice is recovered on the hilltop-end from the mux channel DMOD and then routed through a pad to the base station exciter. Because the transmitter has been keyed, the dispatcher’s voice will now be broadcast over-the-air. If a radio transmission from a mobile or portable field unit is received by the hilltop base station, it is routed through the receiver to a pad, then on to the mux MOD connections. The recovered signal is delivered from the DMOD of the office-end mux, through a pad to the console speaker amp. The dispatcher now hears what is being received by the hilltop radio.

Some base stations may require a specific keying voltage that originates within the radio itself. Figure 4 on page 28 illustrates such a situation. On receipt of signaling from the dispatcher, the voltage obtained from the base station is routed into the mux channel F-lead, then back out the E-lead to the radio PTT line.

Tone-remote base stations are common. Figure 5 on page 30 shows the hilltop end of such a circuit. Signaling functions such as keying, channel switching and disabling the receiver CTCSS decode function are accomplished through the use of different tone frequencies. The tones are contained within the normal passband of the circuit and therefore do not require use of E- or M-leads.

Figure 6 on page 32 is an example of a remote-controlled data radio. As in the previous example, E & M-Leads are not necessary because the supervisory information is embedded within the data being transported. The hilltop mux modem receives data originating from the host computer on the office-end. The VF DMOD signal is padded, then applied to the receive side of a 4W data modem. The modem converts the analog VF receive data signal to a digital (usually RS-232) format. The digital data is then cabled to the base station controller (BSC). The BSC develops a PTT function from the receive data signal and deviates the transmitter with an analog data modulation scheme. The transmitted data is received by mobile data units in the field. In the uplink direction, recovered data from mobiles and portables in the field is sent back to the BSC, modem, pad and mux MOD. The multiplexed data transmission is transported back to the office host computer via microwave.

The most common scheme for transmitting the hilltop receiver squelch gate status back to the office is through the use of a status tone. Since typical status tone frequencies of 2,175Hz and 1,950Hz fall within the bandpass of a mux circuit, the use of E- and M-leads is unnecessary. It is possible however, to accomplish the same thing with E- and M-leads, as shown in Figure 7 on page 34. The active squelch gate voltage level must be compatible with the mux channel M-lead operating voltage range. This is often not the case and a Carrier Operated Relay (COR) must be added between the base station and the M-lead to convert the voltage levels appropriately. The COR is activated when the base station receiver unsquelches. Battery from the mux is routed through the COR contacts and applied to the M-lead. Activation of the hilltop M-lead in turn operates the office-end E-lead. The office-end E-lead connection can be run to an E and M style voter or used simply to light a lamp on the dispatcher console indicating re ceiver activity for that hilltop.

Figure 8 on page 36 is a simplified illustration of the VF and signaling connections for a simulcast EDACS trunking system.This application uses a mux modem with two E-leads. One E-lead is used to key the transmitter (PTT), while the other switches the station GETC between analog and digital modes (A/D).

Circuit level alignment Circuit level alignment should be performed not only after the circuit is first installed, but also periodically to ensure proper performance. The most common piece of test equipment used in the VF alignment process is a transmission impairment measuring set (TIMS). This device transmits VF tones over a wide range of frequencies and levels. In its receive mode, it offers critical measurement of tone levels and frequencies in several impedances, bridging or terminating. The more sophisticated TIMS can run circuit parameter tests such as impulse noise, signal-to-noise ratio, noise-to-ground, PAR, distortion and others.

Figure 9 on page 37 and Figure 10 above illustrate a 4W, E and M mux circuit between a dispatch office and a hilltop. The letters A through G indicate test points at which level measurements can be made. Actual circuit levels may vary, depending upon the application, however these figures serve as a general example of how and where to make level measurements from one end of the circuit to the other.

Figures 9 and 10 assume that the dispatch console is set up to send and receive a maximum level of 0dBm, with average voice levels being 10dB lower(210dBm0). Using a constant-level test tone makes the alignment process much easier than with signals such as voice that vary significantly with time. For this reason, the circuit is broken at the output of the console and a TIMS is inserted to inject a 210dBm (210dBm0) tone at point A.

The typical frequency sent by a TIMS for circuit testing and alignment is 1,004Hz. This tone is sent to the mux channel MOD, via a 216dB pad. The pad adjusts the level to hit the MOD at 226dBm. Because test tone level at the MOD is 216dBm, the 226dBm tone level is still210dBm0 (10dB below test tone level). This 210dBm0 level is maintained throughout the length of the circuit. The tone level at point B should be verified with a TIMS. If the TIMS is clipped across the balanced pair (paralleled) it should be set to the bridging mode. It is placed in the terminating mode if the circuit is broken and connected directly into the TIMS.

When the dispatch console transmit button is pressed, battery voltage is applied to the mux channel M-lead to activate signaling. A dc voltmeter can be used to verify presence of the signaling voltage at the M-lead connection (Point B).

The tone is then multiplexed through digital or analog methods. Because the signal at point C is multiplexed, a TIMS will be of no value to verify VF tone levels. If FDM (analog) equipment is used, and the channel baseband frequency slot is known, a frequency selective level meter (SLM) can make the level measurement. On the other hand, if TDM (digital) equipment is used, a T1 tester with DS0 drop and monitor capabilities can be used. Presence or absence of circuit signaling may be verified with these same test instruments. VF level discrepancies should be eliminated by adjusting the mux channel MOD level adjustment.

The multiplexed signal is injected into the microwave radio and transmitted to the hilltop. Once received at the hilltop, the multiplexed circuit VF tone level (point D) may again be verified using the same means as at point C. If the tone has not maintained the 210dBm0 level, the discrepancy is being introduced by the microwave radio equipment and should be corrected. This is a likely scenario with analog microwave equipment. The transmit deviation may be incorrect on one end, or the receiver demodulator output level may be wrong on the other. Adjustments can be accomplished within the microwave radios to correct these problems. This type of level discrepancy should not occur when using a digital microwave equipment, however. Here, VF levels within multiplexed circuits are a function of the multiplex equipment only.

At the hilltop mux DMOD (point E), a level of -3dBm should be realized to maintain the -10dBm0 circuit levels. A -7dB pad reduces the tone level to provide compatibility with the particular base station line input level requirements (in this case 210dBm). The base station antenna port should be terminated into a communications monitor to check the transmitter deviation level without broadcasting the test tone over-the-air. Transmitter keying is accomplished when the office-end M-lead is detected by the hilltop E-lead circuitry. Activation of the hilltop E-lead applies a ground to the PTT line, keying the base station.

The communications monitor is set to the base station transmit frequency. The modulated tone level should result in 2/3 of the maximum allowable deviation when the transmitter is keyed. Discrepancies in transmit deviation levels are eliminated by adjustment of a line input level or exciter level adjustment within the base station itself.

Figures 9 and 10 illustrate transmitter deviation of 63.3kHz, which assumes the maximum transmitter deviation is 65kHz. The maximum transmitter deviation adjustment should be set prior to the circuit alignment process.

This completes the alignment process in one direction only. Figure 10 shows how alignment would be accomplished in the opposite direction. The communications monitor is set to the base station receive frequency and put into the signal generate mode. Modulated with a 1kHz tone to 2/3 maximum deviation (63.3kHz), sufficient RF signal (typically 1,000mV) is injected into the receiver to produce a clean sounding tone with no audible noise.

A TIMS (in the bridging mode) can be clipped across the balanced pair at point B to verify the tone level out of the receiver. The line output level adjustment of the base station should be set for 0dBm. The tone is then sent through a pad to adjust the level for 210dBm0 at the MOD (an absolute level of 226dBm), which can be verified by a TIMS at point C. Once multiplexed, the tone may be monitored at point D by a SLM or T1 tester with DS0 drop and monitor capabilities, depending on whether analog or digital mux is used. The multiplexed signal is injected into the microwave transmitter and transported to the office-end.

After being demodulated through the microwave receiver on the office-end, the multiplexed signal at point E can once again be checked for proper levels, as at point D. The output of the mux channel DMOD can be adjusted to produce a 210dBm0 signal (23dBm) while monitoring with a TIMS at point F. Finally, the tone is padded appropriately to hit the terminated TIMS at the level required for proper console operation (210dBm).

Summary Emergence of new wireless applications has resulted in increased demand for both base station and circuit transport equipment. A complete understanding of how these and existing remote systems operate not only requires knowledge of two-way radio but transmission systems as well. Familiarity with circuit VF and signaling schemes is basic to the successful implementation of any remote radio system controlled over microwave.