RTX improves wireless (DECT) data capacity

RTX improves DECT technology from 9Kbit/s to 56Kbit/s

RTX’s latest DECT OEM project with the Australian Satellite TV Distributor Foxtel proves one of the technology’s many possible uses.

Wireless engineering experts made it feasible

Foxtel realized that installing telephone cables is expensive and often undesired by the end user, and so needed a 56Kbit/s wireless solution. This was not possible with DECT in a normal configuration, but RTX’s position as the unchallenged leader within DECT made it possible to extend from the normal 9Kbit/s configuration to 56Kbit/s. The project, the wireless phone jack (WPJ), uses standard components from National Semiconductor, which due to large scale production made the cost of materials very competitive.

Highly reliable connection – avoiding bit errors by 16-bit CRC

The DECT air interface implementation must provide the system with an error-free channel in order to avoid destroying modem performance, and must support the bit-rate required by the digital signal processing unit. Errors in the DECT digital channel occur as either bit errors or frame errors. To avoid bit errors, all payload data is protected by 16-bit CRC, and optionally by a FEC (Forward Error Correcting) code enabling the receiver to correct a number of bit errors.

DECT double slot

The system uses standard DECT double slots for data transport, with the slot format illustrated below. It is a standard DECT double slot format as specified in EN 300 175-2, with a special B-field format. The S-, A-, Xand Z-fields are standard DECT implementations, as specified in EN 300 175-3.

Beyond the standard DECT, RTX has extended with following features

The B-field consists of a 16-bit header field which is protected by a 16-bit CRC,and a 720-bit data field protected by both a 16-bit CRC and a 32-bit FEC code.

DECT R-CRC

The 16-bit CRCs which protect the header and data component of the B-field packet use the standard DECT R-CRC according to EN 300 175-3. S-field A-field B-field X Z

Advanced A-field setup

Connection setup and release use the advanced A-field setup procedure for a multibearer connection as specified in EN 300 175-3, and encryption activation follows the standard multi-bearer encryption activation procedure.

Frame errors are reduced

Frame errors are minimized by the use of semi-dual slot diversity and an ARQ mechanism, which allow the system to retransmit lost frames.

Delay and storage buffer

To enable the support of retransmission, a 100ms delay buffer has been implemented in the RX chain, and a 100ms storage buffer has been implemented on the TX side. The delay buffer absorbs retransmitted data from the air interface to the analog two-wire interface, and the storage buffer is used for data storage in case retransmission is required.

High modem speeds

In a standard DECT audio connection, the maximum modem data transmission speed is 14.4Kbit/s, and often only 9.6Kbit/s is achieved. This is due to the ADPCM coding scheme, the >20ms delay and the limited audio signal bandwidth. RTX has overcome these limitations by modifying DECT signal processing. The audio bandwidth and signaling revolution has been improved.

Solution for the delay problem

Introducing a digital signal path between two analog signals adds a delay. Using an air interface-based transmission, as in this case, adds to the delay and makes it too long for the echo cancellers of the connected modems to handle. This is a well-known problem with a well-known solution: digital echo cancellers are added where two-line analog signals are converted to digital, or vice versa.

Why use DECT?

DECT is usually used at home or in the office in voice applications, but the use of DECT is far more extensive. The data application can be in a small, handheld device for measuring products, or in a system, where cable installation is either very expensive or impossible.

Limited interference and power consumption

DECT uses a license–free, standardized frequency band where interference is limited. DECT also has very low power consumption. With a line-of-sight range of 300 meters, it is ideal as a point-to-point application in a local data network.

Echo canceller is improved significantly

Some important improvements have been added to the standard (normal practice) echo canceller to enable it to work correctly with modem signals: - The echo canceller FIR filter only adapts to those specific parts of the modem signals intended for adjusting echo cancellers in the transmission path. - The double talk detection threshold has been reduced dynamically. This secures the filter from losing its correctly adapted coefficients through wrong update decisions. Apart from these two main additional features of the echo cancellers, fast signal detectors and careful trimming of parameters and levels are important for successful implementation.

The Wireless Phone Jack is based on LMS

In the WPJ, a FIR filter-based LMS, adaptive echo canceller is used. The TX signal passes through the unit unchanged. But it is still used for signal input to the echo canceller FIR filter, for input to the wide-band-signal (wbs) filter and for signal level detection.

Optimizing echo canceller performance

The TX signal level is also used for scaling the FIR filter update factor, mu. This is done to achieve the best possible performance from the echo canceller. The RX signal is altered by summation of the echo canceller FIR filter output. The ratio between RX and TX signal levels decides whether or not the FIR filter can be updated. The actual value of the RX/TX ratio is calculated by the ratio control unit, based on the difference between the RX signal levels before and after the echo canceller summation point.

Conclusion

RF transmission has been made reliable by protecting payload data with CRCs and buffering data in order to allow for retransmission of lost data. Furthermore, the standard DECT frame format has been altered to accept the increased data rates supported by the WPJ.

Audio bandwidth

Audio signal bandwidth has been extended by increasing the sample rate. This enables even V.92 modem signals to be transmitted, as all the required signal bandwidth is at hand.

Rebuilding the echo canceller

The standard DECT echo canceller has been rebuilt to provide more efficient damping of the echo by utilizing the embedded modem protocol training signals and freezing the cancellation filter during data transmission. With cancellation ratios of 45 to 50dB, the echoed signal becomes insignificant, and the RF channel delay is even allowed to be in the 100ms range, which is necessary for more reliable RF connections.

Best performance ever seen

In all, this implementation gives the best performance ever seen for V.34, V.90 and V.92 analog modems over a DECT radio connection.