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 |
|---|---|
|
8 (25kHz channel bandwidth) |
|
|
|
|
|
|
16 (50kHz channel bandwidth) |
|
|
|
|
|
32 (100kHz channel bandwidth) |
|
|
|
|
|
48 (150kHz channel bandwidth) |
|
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 | |
|
25, 50, 100, and 150kHz |
25, 50, 100, and 150kHz |
25kHz only |
25, 50, 100, and 150kHz |
|
|
4QAM, |
4QAM, |
4QAM |
4QAM, |
|
|
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 Time Division Multiple Access: A technology for digital transmission of radio signals between, for example, a mobile telephone and a radio base station. In TDMA, the frequency band is split into a number of channels which in turn are stacked into short time units so that several calls can share a single channel without interfering with one another. Networks using TDMA assign 6 timeslots for each frequency channel. TDMA is also the name of a digital technology based on the IS-136 standard. TDMA is the current designation for what was formerly known as D-AMPS.. 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 Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS.). The MQAM formats for TEDS are 4QAM (QPSK Quadrature phase shift keying) (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:
- 4QAM (2-bit, 4 symbol - Quadrature Phase Shift Keying - QPSK)
- 16QAM (4-bit, 16 symbol - Quadrature Amplitude Modulation)
- 64QAM (6-bit, 64 symbol - Quadrature Amplitude Modulation)
See Also
