Texas Instruments | A DSP GMSK Modem for Mobitex and Other Wireless Infrastructures | Application notes | Texas Instruments A DSP GMSK Modem for Mobitex and Other Wireless Infrastructures Application notes

Texas Instruments A DSP GMSK Modem for Mobitex and Other Wireless Infrastructures Application notes
A DSP GMSK Modem for Mobitex
and Other Wireless Infrastructures
Appliation Report
Etienne J. Resweber
Synetcom Digital Incorporated
SPRA139
October 1994
Printed on Recycled Paper
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Copyright  1996, Texas Instruments Incorporated
Abstract
Mobitex is a packetized wireless 900-MHz wide area network (WAN) that allows mobile/portable
subscribers to transfer data, including e-mail, through the growing national and international network
infrastructure. The network operates with an 8-kbps data rate using GMSK.3 modulation. User terminals
are typically sophisticated portable or mobile devices that encompass one or more applications and all
additional OSI protocol layers necessary to send and receive data on the network. Within the user terminal,
the interface between the radio (physical layer) and other layers is a high-performance Gaussian minimum
shift-keying (GMSK) modem. During transmission, the modem converts packets of network data into
transmit baseband. For receiving, it demodulates similar waveforms into data decisions. The typical
Mobitex modem produces at least part of the physical-layer processing necessary for radio interface.
The cellular industry solution for packetized data is called cellular digital packet data (CDPD). The modem
waveforms used for Mobitex are similar (GMSK), though CDPD uses 19.2 kbps. Core GMSK concepts,
however, still apply; therefore, the modem design described herein can also be used as a basis for CDPD
modem development in the future.
Synetcom Digital Incorporated has developed a DSP-based Mobitex modem that accomplishes the radio
interface. Transmit data in packet form is level shifted and Gaussian filtered digitally within the modem
algorithm so that it is ready for transmitter baseband interface, either via D/A converter or by direct digital
modulation. Receive data at either baseband or intermediate frequency (IF) from the radio receiver is
digitized and processed by the modem —nearly optimally —into data decisions. Packet synchronization
is also handled by the modem, assuring that the next layer sees only valid Mobitex packets. Received signal
degradation from frequency offsets, multipath (Rayleigh) fading, and other effects is anticipated and
addressed in the modem design.
Introduction
About Mobitex
Mobitex is a packetized narrow-band data service operating near 900 MHz (450 MHz in the United
Kingdom), originally conceived by Swedish Telecom and further developed by Eritel, a joint venture of
Swedish Telecom and Ericsson. The service is being offered in the United States by RAM Mobile
Data/ Bell South. Base stations, which typically cover 5–15 mile radii, are arranged in a cellular-like
fashion. Network roll-out has proceeded to the extent that coverage within the top 200 U.S. metropolitan
areas is advertised. At Synetcom Digital Incorporated’s Redondo Beach, California office, five base
stations are audible on an indoor cellular whip, four of which have usable signals.
Other Networks
Mobitex falls into the class of wireless WANs. There is at least one other operational infrastructure, called
Ardis (IBM / Motorola), and several more are anticipated, including CDPD from McCaw Cellular and its
partners.
1
Mobitex Terminal Hardware Architecture
Figure 1 shows a typical terminal architecture. Controller CPU functions typically handle higher OSI
layers, which form packets, provide error coding and scrambling, handle acknowledgments, and control
transmitter and receiver operation.
Figure 1. Typical Mobitex Terminal Architecture
GMSK Modem
Mod
User
Application
Platform
GMSK BB
Controller
CPU
Demod
RX IF
or
RX Baseband
TX
Audio In
TX
RX IF Out
or
Baseband Out
RX
WAN Modems and the Radio Channel
WAN modems are designed to operate with signal distortions produced by multipath frequency offsets and
nonideal radio IF filters. Multipath distortion occurs when a signal reflection causes propagation along
several paths across the link. Different path lengths and reflections produce signal components with
unequal amplitude and delay, which vector sum at the receiver. For fixed links, the vector sum looks like
a superposition of comb filters in the frequency domain. In the time domain with long delays, symbol
energy is smeared; this smearing is known as intersymbol interference (ISI). A null (cancellation) or
significant slope at or near the carrier frequency causes severe distortion to the received signal, which can
degrade bit error rate (BER) performance.
The actual multipath parameters vary spatially for mobile links. The receiver sees time-varying comb
functions with nulls that traverse the spectrum and momentarily align with the signal frequency, causing
deep fades. Under these conditions, the received carrier-envelope amplitude has been shown theoretically
and experimentally to conform to a Rayleigh distribution. Based on this model, it has been shown that
99.9% of fluctuation occurs within a dynamic range of 40 dB [1].
Typical radio systems allow for some frequency error (tight frequency tolerance is expensive), which may
degrade modem receive performance. Receiver IF and baseband filtering is also never ideal and can
introduce additional waveform distortion from ISI.
The Mobitex modem design described herein anticipates these and other distortions and has been shown
to operate satisfactorily in laboratory simulations of the degradations. Mobile field tests are anticipated to
further qualify modem performance.
Advantages of DSP Modems
Modem DSP code is written to closely approximate the ideal modem architecture —typically, more closely
than an analog implementation approximates it — potentially realizing outstanding modem performance
that is repeatable over time and temperature. The approach is flexible because all modem parameters can
be trimmed in software.
A DSP can assume other chores in the user terminal and may become the platform for additional protocol
layers required for a given network, assuming enough spare MIPS are available, and it may even be
reconfigured to interface with other networks on multiple layers.
2
DSP chips are on the same fast track as CPUs, with smaller feature size, higher speed, lower power, and
lower voltage required with each new generation. Competition among several major corporations has
brought pricing down to levels that compete favorably with discrete analog and ASIC implementations.
Mobitex DSP Modem Characteristics
Code Size and DSP MIPS Requirement
The Mobitex modem code is actually two distinct algorithms associated with half-duplex transmit and
receive functions. The receive (digital demodulator) algorithm is more complex and embodies most of the
important features necessary for a successful modem design. As with all modems, receiver code requires
more processor power, as shown in Table 1.
Table 1. Receiver-Code Processor Power Requirements
ÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁ
Function
Code Size
TMS320C25 MIPS Requirement
Transmit GMSK Modulator
256 words
3
Transmit PN Generator
128 words
1
Receiver Digital Demodulator
500 words
6
Receiver Discriminator†
128 words
4
† Discriminator code is required if the A/D interface is receiver IF.
Bit-Error-Rate Performance
The BER performance of a pair of the Mobitex modems was measured in the laboratory. GMSK IF and
Gaussian noise are summed to create an approximation of the noisy radio channel, representative of weak
receive signals. Signal and noise power levels are calibrated relative to each other and converted to Eb and
No values through bit rate and equivalent bandwidth normalization. The test scenario increments noise in
1-dB steps and captures BER data.
3
Results are plotted against theoretical performance in Figure 2. Performance is quite close to ideal
(< 0.5 dB) over the range of data shown. Transmit GMSK is a continuous 29–1 pseudorandom noise (PN)
code.
Figure 2. Bit Error Rate Versus Eb/No Modem Performance
1
10 –1
10 –2
10 –3
Measured
Theoretical
10 –4
6
4
7
8
9
10
11
12
13
Modulator Design
GMSK.3 Modulation
GMSK has been widely proposed and utilized for mobile radio data communications. In addition to
Mobitex, GMSK is used for GSM (European digital cellular) and CDPD in the U.S. Several characteristics
that make it especially attractive for these applications are:
•
Spectral efficiency (12.5-kHz channels for 8-kbps GMSK.3)
•
Constant RF envelope (efficient class-C amplifiers and hard-limiting receivers)
•
Compatibility with analog FM techniques
•
Reasonable performance (assuming proper modem techniques) in multipath environment
As illustrated in Figure 3, GMSK.3 is generated with Gaussian low-pass filtered bipolar data, applied to
a DC coupled FM modulator, set to a modulation index of 0.5.
Figure 3. Idealized GMSK.3 Generation
Fsym = 8 kbps
F3dB = 2.4 KHz
Bipolar
Data
Gaussian
Low-Pass
Filter
FM Modulator
m = 0.5
8 kbps
GMSK.3
Modulated RF
The .3 suffix on GMSK refers to the BT, or bandwidth, symbol time product. Alternatively, BT can be
expressed as the ratio:
Ftx / Fs = 0.3 for GMSK.3
where Ftx is the transmit filter with a 3-dB bandwidth and 2.4-kHz frequency, and Fs is the symbol rate.
As the ratio increases, more energy at higher frequencies is transmitted, occupying more radio spectrum.
A decrease in ratio below 0.2 attenuates higher frequencies significantly, compromising obtainable
performance.
The eye pattern for GMSK.3 baseband signals is shown in Figure 4. An eye pattern conveys every possible
trajectory in the transmit/receive data baseband waveform synchronized to symbol timing. It is useful
because it can very quickly convey the fidelity of transmit and receive data and is a strong diagnostic tool
in the wireless development environment.
5
Figure 4. Eye Pattern for 8-kbps GMSK.3, 215–1 Length
Pseudorandom Transmit Data{
† Signal observed at the output of the transmit filter
GMSK Modulator Architecture
A block diagram of the modulator DSP implementation is shown in Figure 5.
Figure 5. GMSK Modulator DSP Implementation
TMS320C25 DSP
GMSK Modulator
PN
Gen
Transmit
Data In
1
Data
Select
900-MHz
Transmitter
12-Tap
Transmit
Filter
Baseband
In
1
Level
Shift
1
0101...
Gen
8-kbps
Clock
Out
1
10-Bit
TImer
0 – 8 kHz
TX GMSK
D/A
Low-Pass
Filter
fs = 48 kHz
The present GMSK modulator algorithm accepts data from upper OSI layers that has been packetized, error
encoded, and scrambled according to Mobitex specifications. In most systems, this is accomplished on a
CPU in the application computer or in a separate microcontroller. Ultimately, these functions can occur on
the DSP.
The modulator algorithm either accepts external data or can generate pseudorandom (PN) data with 27–1,
29–1, and 215–1 length codes for transmit test purposes. This feature enables easier bit-error-rate
measurements, eye-pattern checks, and other system measurements during integration with radio gear.
6
The DSP algorithm implements a level shift and digital low-pass filter function on the square data provided
by the other OSI layers or the algorithmic PN generator. A 12-tap (two symbol length) linear-phase FIR
structure forms the transmit filter, which is designed to approximate the ideal Gaussian transmit filter very
closely. The FIR 3-dB point is set to 2.4 kHz for BT = 0.3. The modulator sample rate is 48 kHz, producing
a baseband bandwidth with significant energy out to approximately 5 kHz and virtually no energy beyond
10 kHz.
The modem exists on an evaluation board that contains a 16-bit D/A converter and low-pass reconstruction
filter that attenuates digital spectra beyond fs/2 (24 kHz) to levels near the noise floor. Other
implementations can exploit the latest single-chip CODEC or analog interface circuits, which combine
several D/A and reconstruction filter blocks with A/D converters. A single chip can thus furnish the entire
radio-analog interface. Ten-bit precision D/A converters are adequate for this application.
7
GMSK Demodulator Design
GMSK Demodulator Architecture
A block diagram of the demodulator structure is shown in Figure 6. The upper half of the figure shows an
external interface to a 900-MHz radio receiver. Either a baseband or an IF interface is possible with this
algorithm. The IF interface includes an FM discriminator function in the DSP code.
Figure 6. GMSK Demodulator DSP Implementation
Low-Pass
Filter
8-Bit
Baseband
Out
DC to 8 kHz = Rx GMSK
A/D
fs = 24 kHz
900-MHz
Receiver
Baseband or IF Interface to Rx
455-kHz to
100-MHz
Rx IF
IF
Out
12-Bit
I
R
A/D
L
fs = 48 kHz
1
Baud
Clock
Out
1
Received
Data
Out
fc = 36 kHz
491-kHz to 100.036-MHz
Crystal Oscillator
A/D Sample
Clock
TMS320C25 DSP
I
Receive
Filter
Transition
Track
Track
Rx
Interface
Select
ACQ
2
B B
VCO
3 or
Integrate
and Dump
6
Decision and
Decision
Feedback
Matched
Filter
I
Correlator
0_
Hilbert
Transformer
90_
FM
Discriminator
Q
Receive
Filter
Q
FM Demodulator Algorithm
8
Acquisition
Track
Threshold
DC
Estimate
Valid
Packet
Bit Sync
Timing Preset
Acquisition/
Track
Control
GMSK Demodulator
Frame
Sync
Detector
The demodulator algorithm employs noncoherent techniques to arrive at each data decision. Two entry
points for digitized data from the receiver are shown in Figure 6.
Digitized IF Processing
As the cost and power consumption of DSP MIPS and associated A/D converters decrease, it will make
sense to locate the A/D converter closer to the antenna, somewhere in the radio IF strip. Traditionally,
digital processing at IF has been applied to expensive military systems in which the highest possible
receiver performance is required. As DSP costs decrease and techniques improve, IF processing may
become standard in wireless applications, where both benefits —cost and performance —are possible. In
anticipation of this next step, a radio IF interface to the DSP demodulator algorithm was created.
Band-limited radio IF (presumed to be at 36 kHz center, 12.5 kHz wide for Mobitex) is digitized at a sample
rate of 48 kHz, realizing a digital down-conversion to a center frequency of 12 kHz. The DSP algorithm
then implements a close approximation of a 0_/90_ splitter that feeds a pair of identical, 7-tap low-pass
FIR receive filters, carefully bandwidth optimized under noise conditions for best overall demodulator
performance.
Digital FM Discriminator
The FM discrimination algorithm maps the frequency of complex IQ samples to a voltage using a
differential estimation technique. Sample-rate decimation by a factor of 2 is also used, yielding subsequent
processing that executes only on every other input IF sample. After decimation, the discriminator
normalizes each sample by I2 + Q2 to wipe off any IF energy variation, due to radio channel fades that fall
out of the receiver’s hard limiting or AGC range. The dynamic range of the normalization algorithm
approaches 40 dB when used with a 12-bit A/D converter.
Normalization becomes a significant issue if the receiver RF/IF chain must have linear or AGC
loop-controlled gain. Certain modulation types require linear receiver performance. In a
multinetwork/infrastructure environment, linearity may be a requirement. The normalization algorithm
exists to cover that eventuality, even though most implementations to date have used hard limiting and
traditional FM receiver techniques.
Baseband Processing
A second entry point to the demodulator algorithm can be selected just after the digital FM discriminator
of Figure 6. The receiver baseband (audio DC to 8 kHz) that carries the data waveform is digitized by at
least an 8-bit A/D converter at a sample rate of 24 kHz. Less precision is required because the receiver hard
limiting and discriminator mitigate most of the envelope fluctuation due to flat signal fading. Processing
beyond this point is identical regardless of which input is selected.
Packet Acquisition
All received Mobitex packets are qualified by an acquisition process that recognizes and exploits
information in the first two data structures of the Mobitex packet, which is shown in Figure 7.
9
Figure 7. Mobitex Packet Structure
11
11
16 Bits
11
16 Bits
11
Mobitex Data
00
00
00
Bit Sync Interval
00
Frame Sync Interval
4 ms
Demodulator achieves bit sync and
begins optimal data
demodulation.
Demodulator compares frame sync Rx pattern
to reference pattern. If >1 mismatch, acquisition
attempt is abandoned. Otherwise, ensuing Mobitex
data is demodulated until the packet is complete.
When the demodulator is not tracking and demodulating a qualified packet, an FIR filter-based structure
that implements pattern specific correlation is executed. The correlator searches for the bit sync pattern.
When correlator output exceeds a preset threshold, demodulation begins and frame sync, which is a fixed,
country-specific pattern 16 bits long, is expected. If frame sync does not occur within the next 16 bits with
one bit error or less, the packet acquisition attempt is abandoned and the correlation process is begun again.
In this manner, probability of false acquisition is kept very small, and higher OSI layers in the user terminal
receive data only when qualified packets are present.
Simultaneous to successful correlation, a low-bandwidth tracking-loop algorithm is invoked. Data
transitions (zero crossings) are extracted, and the algorithm attempts to keep crossings aligned by adjusting
the DSP timer register, which ultimately generates sample pulses to the A/D converter. The resulting servo
loop is invoked as long as the qualified packet data is present. This feature is especially important for long
packets and operates reliably even with very weak receive signals.
Also, after each successful correlation, a DC estimate (which is proportional to receiver frequency offset
relative to base station) is extracted from the bit sync sequence and is used to cancel DC offsets in the
baseband demodulation (track) path. The modem performance is made tolerant of frequency offsets in this
manner.
Finally, the correlator triggers an A/D sample timing preset. Correlator output information is examined,
and a precise estimate of correct initial A/D sample phase and frequency is made. The preset timing is
subsequently updated very slowly at each zero crossing with the aforementioned servo loop.
Data Demodulation
After correlation to the packet bit sync pattern occurs, the data demodulation/decision process begins.
Conceptually, the goal of the decision process is simple: every three samples (at 24 kHz) produce either
a zero or one data decision such that the original packet data, prior to modulation, is recovered.
The decision process employs matched filtering (which is identical to transmit filtering),
integrate-and-dump, and decision feedback techniques to minimize the probability of bit errors. The
integrate-and-dump and decision feedback algorithms are especially effective under disturbed conditions,
such as with either fixed or time-varying multipaths, and they also reduce modem sensitivity to ISI induced
by receiver filters.
10
Design Adaptations for CDPD
The CDPD modem requirement is for GMSK.5 radio waveforms at 19.2 kbps. CDPD utilizes cellular
channels that are full-duplex; the packetized protocol can use this characteristic, though a half-duplex
CDPD implementation is also possible. A computer simulation of the transmit eye pattern for GMSK.5 is
shown in Figure 8.
Figure 8. Computer-Simulated Eye Pattern for 19.2 kbps GMSK.5
(Amplitude Versus Time)
6.00
5.00
4.00
3.00
2.00
1.00
.00
–1.00
–1.50
–1.00
–0.50
00
50
1.00
1.50
As compared to Mobitex, the higher baud of CDPD dictates use of a more powerful DSP chip, such as one
from TI’s TMS320C5x family, to support the modem function. Generally speaking, a good estimate for
half-duplex CDPD MIPS required for the GMSK demodulator can be obtained by simply scaling the
6-MIPS benchmark for the baseband-interfaced Mobitex demodulator. A conservative approximation is
based on the ratio of bauds (19.2 / 8 = 2.4). CDPD, therefore, can require up to 14.4 MIPS peak for the
receive modem function.
Digital demodulators can operate with fewer samples per baud than were assumed above. The Mobitex
modem uses an A/D converter to sample IF at 48 kHz or baseband at 24 kHz. The algorithm ultimately uses
three samples per 8-kHz symbol in the data-decision section.
For CDPD, it is estimated that if two samples per baud are used, approximately 0.7 dB of performance is
sacrificed. The associated baseband sample rate is 38.4 kHz, and the corresponding MIPS requirement is
approximately 10 (33% less than the 3 samples-per-baud case).
CDPD’s GMSK.5 uses a higher BT factor (0.5). The immediate result is an eye pattern that is less filtered
than shown in Figure 4. Overall modem receive performance is correspondingly improved. Adjustments
of constants in the current decision feedback algorithm are necessary to optimize performance, though the
current constants (based on GMSK.3) will operate surprisingly well.
CDPD transmit baseband eye pattern has been simulated and is shown in Figure 8. The Gaussian transmit
filter 3-dB frequency is 9.6 kHz. The transmit and receive Gaussian digital filter is adjusted for the new
bandwidth.
11
Transition of GMSK Modem to TMS320C5x
Work has begun to translate the existing ’C2x code to a ’C5x processor. The GMSK modulator and portions
of the demodulator algorithm are currently able to execute successfully on TI’s EVM system. The
translation is very straightforward, using TI’s DSP assembly conversion utility (DSPCV.EXE), and the
utility is able to convert ’C2x source code (.ASM) files directly to ’C5x source code files. A minor amount
of manual intervention is necessary after running the utility. This intervention is associated with memory
directives that do not have exact equivalents between the two processor families.
Conclusions
Packet networks such as Mobitex or CDPD generally operate with a sophisticated protocol that allows for
error detection, limited error correction, and, if all else fails, packet retransmission. All data is eventually
received successfully across the link. High-performance modem techniques are employed to meet overall
network performance requirements because inferior modems can generate unnecessary traffic, requiring
repetition of missed data.
The Mobitex modem code exists on a 16-bit fixed-point TMS320C25, which is an entirely adequate
platform for the core modulation/demodulation algorithms implemented. No issues associated with the
16-bit fixed-point precision were encountered. In general, no applications are envisioned in which
floating-point processors or wider fixed-point registers are necessary for wireless modems anticipated for
future implementation.
The existing code is portable to the Texas Instruments TMS320C5x family, which will ultimately offer
3.3-volt, 40-MIPS operation, suitable for battery-powered portable operation. The fully implemented IF
interface Mobitex modem algorithm requires 10 MIPS for demodulation. The ’C5x family and similar
processors from other manufacturers open prospects for other layers of wireless protocol executing on the
same DSP, with ultimate partitioning of DSP and controller-processing responsibilities dictated by
DSP/processor cost, memory requirements, speed and power consumption, and interface issues. All new
designs should weigh these issues carefully.
The DSP chip offers flexibility beyond Mobitex. Multiple wireless infrastructures, including CDPD, can
ultimately be accommodated on the same processor, which, in fact, may be necessary for long-term product
survival. As wireless/PCN industries take shape, the emphasis will likely be on flexibility. Systems that
are incompatible starting at the lowest link/physical layers will dictate that user radio/modem devices be
capable of loading and executing new modem and control (protocol) code as needed. A single user terminal
can thus interface with multiple infrastructures.
Code Availability
The associated software is available for licensing from Synetcom Digital Incorporated, 1426 Aviation
Boulevard, Suite #203, Redondo Beach, California 90278.
12
References
1.
Feher, Kamilo, Advanced Digital Communications Systems and Signal Processing Techniques,
Prentice-Hall, 1987.
2.
Hirono, Masahiko, Miki, T., and Murota, K., “Multilevel Decision Method for Band-Limited
Digital FM with Limiter-Discriminator Detection”, IEEE Transactions on Vehicular
Technology, August 1984, pp. 114–122.
3.
Mobitex Interface Specification, Revision 2A, RAM Mobile Data, Woodbridge, New Jersey,
February 1993.
4.
Cellular Digital Packet Data System Specification, Release 1.0, Book III, Volume 4, July 19,
1993.
13
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