TDD signal analysis using the Hongke SAF handheld spectrum analyzer

Comparison of FDD and TDD transmission

Frequency Division Duplex (FDD)

FDD requires two independent communication channels (i.e., transmit frequencies) separated by a guard band to minimize co-channel interference (Figure 1). Good filtering, duplexers, and possibly radio shielding are necessary to ensure that the transmitter does not desensitize adjacent receivers. FDD radios continuously transmit in both directions over the radio link to provide full-duplex capability.

Figure 1. FDD requires two symmetrical spectrums for uplink and downlink.

Time Division Duplex (TDD)

TDD uses a single frequency band for both transmission and reception by allocating alternating time slots for transmission and reception operations (Figure 2). The information to be transmitted—whether voice, video, or computer data—is in serial binary format. Each time slot can be either 1 byte long or a multi-byte frame.

Figure 2. TDD alternates sending and receiving station data over time. The time slots may vary over time.

In some TDD systems, the alternating time slots are of equal duration or have equal download and upload time slots, however, TDD systems do not have to be 50/50 symmetrical. Systems can be designed to operate asymmetrically (e.g. 75/25 or 90/10 is common) or dynamically based on traffic conditions.

TDD spectrum scanning with Spectrum Compact

Time-division duplex radio systems transmit rapid physical carrier changes in the time domain. In order to be able to display such signals on the screen of the Spectrum Compact, the Spectrum Compact must be configured accordingly.

1. Disable "Signal ID" mode under TOOLS → SETTINGS

2. To ensure maximum reading speed, use 100MHz SPAN. 0OMHz span can be set under the "SPAN" menu by pressing the "MIN SPAN" button. Using a SPAN value larger than 100MHz will result in a longer scan refresh cycle, increasing the time required for accurate measurements.

3. Due to the frequency hopping nature of TDD signals (e.g. OFDM modulated carriers), it is necessary to accumulate the Spectrum Compact scans over a period of time. This can be done using the "MAXHOLD" or "CUMULATIVE" traces under the "TRACE MODE" menu on the main screen. SAF recommends selecting at least 100 scans, regardless of the SPAN selection.

MAXHOLD trajectory

The blue trace shows the highest level detected since the start of the sweep. During each sweep, only the frequency point with the highest power level is updated. Clicking the MAXHOLD button repeatedly resets the MAXHOLD trace. The blue counter below the grid shows the number of sweeps since the most recent MAXHOLD mode started. Above the grid, the power levels of the CENTER and MARKER (if activated) frequencies are indicated in blue.

Figure 3. Display of WIFi signal readings in “MAXHOLD” tracking mode

CUMULATIVE trace

The power level of each frequency for all previous sweeps since the start of the sweep is displayed in green. During each sweep, only the frequency points for which no levels were previously saved are updated.

Figure 4. Display of WIFI signal readings in “Accumulation” tracking mode

A list of TDD applications where Spectrum Compact can be used include:

Interference and available channel detection;

Relative power observation between different TDD signals;

Channel bandwidth determination;

Traffic intensity survey of TDD signals (e.g. using frequency hopping systems);

Use point-to-point narrow beam xternal antenna to find TDD signal source.

Example

In the next two figures, a spectrum scan of a 5GHz point-to-point (PtP) radio unit using TDD transmission is shown and detailed. Note that in most cases it is not possible to observe an accurate PtP TDD signal by connecting a spectrum analyzer directly to the radio unit, as most TDD radios only detect TDD signals after the link between a master and slave (PtP) or access point and client (Point-to-Multipoint or PtMP) is established.

Figure 5. PtP 5GHz TDD radio signal scan using “MAXHOLD” tracking mode

Delta channel bandwidth for a TDD signal. Since Spectrum Compact is set to 100MHz SPAN, each grid cell is 10MHz wide. In this example, the signal occupies 2 cells, which means the channel under investigation is 20MHz wide.

Figure 6 shows the same situation as Figure 5, but using the "Accumulation" tracking mode. The "Accumulation" tracking mode allows a more precise view of how the scanned spectrum is occupied and how the signal energy is distributed within the spectrum.

Figure 6. PtP 5GHz TDD radio signal scan using “CUMULATIVE” tracking mode