TEDS Demodulation Overview

TEDS is based on variable subcarrier/channel bandwidths of 25, 50, 100, and 150kHz, with burst uplink and continuous downlink slot formats. In particular, the Normal Downlink, Normal Uplink, and Control Uplink slot formats feature a 25kHz channel bandwidth with 8 subcarriers; a 50kHz channel bandwidth with 16 subcarriers; a 100kHz channel bandwidth with 32 subcarriers, and a 150kHz channel bandwidth with 48 subcarriers, while the Random Access only applies to a 25kHz channel bandwidth with 8 subcarriers.

The following tables describe the common modulation parameters and parameters specific to each of the slot formats:

Additional topics in this section include the following:

TEDS Modulation Parameters - All Slot Formats

Parameter

Description

RF Subcarriers

8 (25kHz channel bandwidth)
-4, -3, -2, -1, +1, +2, +3, +4

 

 

 

16 (50kHz channel bandwidth)
-8, -7, -6, -5, -4, -3, -2, -1, +1, +2, +3, +4, +5, +6, +7, +8

 

 

 

32 (100kHz channel bandwidth)
-16, -15, -14...-3, -2, -1, +1, +2, +3...+14, +15, +16

 

 

 

48 (150kHz channel bandwidth)
-24, -23, -22...-3, -2, -1, +1, +2, +3...+22, +23, +24

Subcarrier Spacing

2.7kHz

Subcarrier Symbol Rate

2400sym/s

Symbol Filter

Root Raised Cosine (RRC) (alpha = 0.2)

Demodulation

Coherent (Pilot Symbol Assisted)

Frame Rate

32 Slots/Frame

Slot Interleave

Variable

TEDS Slot Format Parameters

The following are parameters that are specific to the different slot formats.

 

Parameters

TEDS Modulation Slot Formats

Normal Downlink

Normal Uplink

Random Access

Control Uplink

Channel Bandwidth

25, 50, 100, and 150kHz

25, 50, 100, and 150kHz

25kHz only

25, 50, 100, and 150kHz

Modulation Type

4QAM,
16QAM
64QAM

4QAM,
16QAM
64QAM

4QAM

4QAM,
16QAM
64QAM

Minimum Search Length

14.167msec (85/6 ms)

14.167msec (85/6 ms)

7.083msec (85/12 ms)

7.083msec (85/12 ms)

Use PvT Off Analysis Time with the minimum Search Length values in the table above to determine the smallest values you can enter for Search Length.

TEDS and TDMA

The modulation selected for TEDS delivers a flexible bit rate in a 25kHz/50kHz/100kHz/150kHz radio channel bandwidths. The basic mode of operation for TEDS is TDMA. The downlink transmission mode is continuous, while the uplink transmission mode is pulsed on a slot by slot basis. Layer 1 reserves certain symbols within the information stream to provide for its operation. These reserved symbols synchronize the radio channel and provide a known reference for performing coherent demodulation of the subcarriers.

TEDS and MQAM Formats

TEDS is a format within the family of Multiple Quadrature Amplitude Modulation (MQAM) formats. MQAM are distinguished by multiple carriers of standard digital modulation, with each carrier called a subcarrier or sub-channel. Each subcarrier is generated digitally, thus all subcarriers are coherent, plus each subcarrier is filtered independently using a Root Raised cosine (RRC) filter with a shape factor (alpha) of 0.2 (adjustable). Note that subcarriers are not orthogonal in frequency, thus MQAM is not Orthogonal Frequency Division Multiplexing (OFDM). The MQAM formats for TEDS are 4QAM (QPSK) (4 states with 2 bits per symbol), 16QAM (16 states with 4 bits per symbol), and 64QAM (64 states with 6 bits per symbol).

TEDS Subcarriers

TEDS features a variable number of subcarriers with corresponding variable channel bandwidths:

Channel Bandwidth

Number of subcarriers

150kHz

48 (-24 to 24)

100kHz

32 (-16 to 16)

50kHz 

16 (-8 to 8)

25kHz 

8 (-4 to 4) 

Subcarriers are separated by 2.7kHz and centered about the center frequency. Subcarriers are designated negative (-) or positive (+) of the center frequency, for example: -4 to -1 and +1 to +4 for an 8 subcarrier. The following images show you an 8 subcarrier designation.

 

The number of symbols in each subcarrier depends on the slot format selected. The table below shows the relationship of symbols to each slot format.

Slot Format

Number of symbols

(per subcarrier)

Normal Uplink

31

Normal Downlink

34

Random Access

14

Control Uplink

14

 

The number of bits in each symbol depends on the modulation type selected. The following table shows the relationship of bits to each modulation type.

Modulation Type

Number of bits (per symbol)

4QAM

2

16QAM

4

64QAM

6

Root Raised Cosine Filter

Each TEDS subcarrier uses a Root Raised Cosine (RRC) filter to process the output of the corresponding subcarrier symbol generator at the transmitting end. The RRC filter ensures minimal system intersymbol interference and maximum frequency domain attenuation outside the subchannel bandwidth, which minimizes interference to adjacent subcarriers. The RRC filter is characterized by the excess bandwidth factor (roll-off coefficient), alpha.

An ideal "brickwall" filter results in an occupied bandwidth that is equal to the symbol rate, in this case, an occupied bandwidth of 5.4kHz (alpha = 0). However, this is not practical because of the roll-off characteristic of the filter. The filter alpha is calculated in the frequency domain by comparing the RRC frequency at the 50% signal attenuation point (W0) to the 100% signal attenuation point (W) using: [W = (1 + alpha) W0] where W0= 2400Hz and W=2880Hz, yielding alpha = 0.2.

See Root Raised Cosine Filter in the Digital Demod section for more information.

Modulation Types

TEDS analysis supports the following modulation types:

See Also

About TEDS Demodulation

Available Features

Setting up a TEDS Measurement

Selecting TEDS Demodulation

Overview of Slots and Symbols (TEDS)

Symbol Constellations

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