Seminar Topics: CDMA Wireless Data Transmitter

MULTIPLE ACCESS METHODS

In order to accommodate multiple transmitters with one data receiver and achieve design flexibility and extensibility, a multiple data access scheme needs to be implemented. This can be achieved by assigning each transmitter a distinct feature eg.,certain frequency a time slot or a code .Accordingly , three design options were considered in the present design :frequency division multiple access[FDMA],time division multiple access [TDMA]and code division multiple access [CDMA] techniques

A. FDMA technique

In FDMA,each transmitter has a frequency exclusively assigned to it. This enables transmitter to send data at any time regardless of other transmitters. The frequency ranges can be transmitted simultaneously. The frequencies should be non-overlapping. If overlapping one of the overlapping range is shifted to another range with same bandwidth.S ince absolute real time monitoring systems are usually not needed in production environment,a transmitter will need to send data only at specified intervals,leaving the assigned frequency unused most of the time.

B. TDMA technique

A single carrier frequency can be used by several transmitters, if it is ensured that at no time more than one transmitter sends data simultaneously . Accordingly, each transmitter is assigned a time slot for signal transmission .The time slots need to be synchronized between all the transmitters in the system. This requires that for each transmitter, a separate receiver is implemented, and the receiver must be ON at all times, increasing the power consumption of the circuitry.

C. CDMA technique

It is a digital cellular technology. It uses spread spectrum technique. CDMA doesn’t design a specific frequency to each user. Each channel use fully available spectrum. Individual conversions encoded with pseudo random digital sequence. It is a military technology. It was used in second world war in jamming transmission. Using this all stations are permitted to transmit over the entire frequency all the time. Multiple transmissions are separated using coding which makes an assumption that when multiple signals combined together they will not get gargled. They add linearly.

Each bit is sub-divided into ‘ m’ short intervals called chips. Each station is assigned to a unique m – bit code called chip sequence. To transmit a 1-bit the station will be transmitting the chip sequence and for a o-bit the station will be transmitting 1’s compliment of chip sequence. No other patterns are permitted. We use bipolar signals with 1-bit being +1 and 0-bit being -1. Suppose a station is having a chip sequences. An important assumption to be noted is that all chip sequences must be pair wise orthogonal is the normalized inner product of any two different chip sequences must be zero ie, S.T=0 and S.T=0. where T another stations chip sequence.

Recovery of data: to recover the data being transmitted the receiver must know, the chip sequence of the sender in advance. So for recovery process, the normalized inner product of transmitter and receiver is found. If S is the received chip sequence and C is the chip sequence of sender, three cases are.

i. S.C = 0 ( C has not transmitted anything )
ii. S.C = 1 ( C has transmitted a 1-bit )
iii. S.C = -1 ( C has transmitted a 0-bit )

This has been divided into frequency – hopping [FH] and direct –sequence [DS] CDMA .In both a unique code has been assigned to each transmitter. This code is a pseudo –noise sequence, meaning that it will appear to be noise unless it is known to be a code. So, the transmission can only be decoded by a receiver that knows the codes used.

Both versions of CDMA require a much higher bandwidth than an uncoded transmission. In FH – CDMA, the higher bandwidth is divided into small bands, each having the bandwidth of uncoded transmission. While sending the data, the used band is changed according to the assigned code. In DS-CDMA the code bits have a higher frequency than the data bits, employing an integer multiplication factor .This factor was chosen as 4 now, based on the available on-chip memory in DSP used. Hence, the original bit stream is multiplied by code bit stream. Since the data is transmitted over a broad frequency band at each fixed point in time ,the receiver can decode the data even if a part of the bandwidth is superimposed by an unrelated transmission. Hence the DS-CDMA has an advantage over FH-CDMA ,because the data can be transmitted even when a particular frequency is not usable due to noise .

Figure 3. data coding of 110 bits using cdma technology

As an example of DS – CDMA , the top section of fig.3 shows the first 12-bits of a pseudo- noise sequence and the middle section of the fig shows the first three bits of a data that are to be coded .The data bits have the duration of 4micro seconds .The CDMA code bits have a duration of one quarter of that of the data bits ,according to the selected spreading factor 4.The data bit stream is multiplied by the code bit stream and the result is shown in bottom part of the figure .

For the present design pseudo –noise sequence has a bit length of 31 bits. This result in 33 orthogonal sequences and subsequently 33 transmitters to be accommodated within transmission system. For installations requiring more transmitters, longer code sequences need to be used. This can be readily implemented, as the modification is only needed at the receiver side.

CDMA provides multiple access communication capabilities. In CDMA each user is provided with an individual pseudo –noise code .It has near -in far –end property.

TRANSMITTER DESIGN

Although several wireless transmitters are commercially available on the market, they were not appropriate for the intended bearing condition monitoring application because of their fixed design geometry and packaging forms it is difficult for them to be structurally adapted and integrated into the bearing system to be monitored. Second they contain a receiver circuitry to allow for fault duplex data and communication and error correction. However, such a circuitry requires significantly increased power supply and additional space to physically accommodate the extra components. Given RF transmission alone would account for over 30% of power consumption of entire circuitry, and the receiver section drains more power due to higher number of components, commercial transmitters were not energy optimized in view of power consumption and battery life, and were therefore not appropriate for this.

The digital data transmitter developed in this study transmits a data stream at 2-s intervals, with bandwidth up to 3KHz. A 12-bit resolution is needed to provide proper data resolution. To allow for a generic solution, the analog to digital converter TLV2543 was selected, which provides an interface connection for up to 11 sensors .It has power down mode CDMA encoding and QPSK modulation for data transmission are done by DSP TMS320VC549 [low power consumption and power saving modes]. Sampling every 2-s long data interval with a sampling rate of 6KHz and 12-bit resolution would result 72Kbits/S. To store this amount of data on to the DSP on-chip memory with a 16-bit data width without wasting memory space ,the 12-bit data samples were stored as direct bit strings without any 4-bit prefix .Thus every four 12-bit samples were stored in three DSP memory locations of 16-bit width each ,occupying9Kwords. For CDMA coding there is a total memory need of 36Kwords.

To ensure data transmission integrity over a wireless link, a checksum is added to data sample transmitted ie, recalculated at receiver end .If a mismatch is identified, a retransmission is requested from transmitted. Since transmitter does not include a receiving section on its circuitry it cannot perform inquiry based data retransmission. To improve transmission integrity, each data sample is transmitted three times consecutively. On receiver these transmissions are subsequently compared with each other for consistency. For enhanced transmission integrity, a signal-filtering algorithm can be implemented on the receiver end.

To enable consecutive data transmission, all the data needed must be stored in DSP memory at the same time. As DSP has only 32K word memory space which must hold data, a loop-up table for data modulation and provide memory mapped registers. It is necessary to compress data. This was accomplished by using Lampel –Ziv 1977 data compression algorithm. The algorithm uses dictionary based compression that recognizes repeating patterns in the data and places these patterns in to a dictionary. Compression may vary between 1 to 90%. As the signal contain repetitive periodic waveforms a compression ratio of 50% is expected. The compressed data are CDMA coded and QPSK modulated. For data modulation, the coded bits are sent alternately into two streams. This process is simplified and accelerated by a look-up table. The final sine wave output is stored in another look-up table.Based on phase shift an offset for access to sine look-up table is then selected. In fig 4 a flow chart of data transmission sequence is shown.

The frequency of sine input to RF interface is 78.125KHz. The sine wave is approximated with 16 samples per wave by DSP. The DAC requires 16bit data transmission for each sample to be converted. So a clock frequency of 1.25 MHz is needed for serial communication between DSP and DAC. To convert this signal to carrier frequency a mixer has been implemented which uses the carrier from a local oscillator. The up-converted signal is subsequently transmitted into air via antenna. The RF interface thus was implemented using minimum number of discrete components. No filters were used at base band and for RF frequency to reduce circuit complexity and power consumption. To further minimize CDMA spreading and QPSK modulation were implemented through software.

To ensure program safety for data compression the program is stored in flash memory chip AM29LV400. The flash memory stores sine wave table and pseudo-noise sequence need not be recalculated. In fig 5, the principle design of a transmitter is shown.

The design has been prototyped on a conventional PCB using SMT. The overall size of transmitter using discrete components and SMT packaging, measured about 3.6´5.8 cm, which is less than half the size of a credit card fig 7. The complete system is shown in fig 8.