WO1995008874A1

WO1995008874A1 - Digital radio link system and radio link terminal

Info

Publication number: WO1995008874A1
Application number: PCT/FI1994/000426
Authority: WO
Inventors: Jarmo MÄKINEN
Original assignee: Nokia Telecommunications Oy
Priority date: 1993-09-24
Filing date: 1994-09-22
Publication date: 1995-03-30
Also published as: US5764700A; JPH09507004A; DE69431158T2; AU678538B2; AU7699794A; CN1068476C; FI934196A; FI934196A0; EP0720793A1; FI103442B1; DE69431158D1; EP0720793B1; JP3448299B2; FI103442B; CN1133656A

Abstract

The invention relates to a digital radio link and radio link system comprising two radio link terminals (A, B) establishing a fixed radio connection between them. In the system, the radio link terminals transmit alternately at the same radio frequency, each one of the radio link terminals (A, B) comprising (a) baseband modulator means (12a; 12b) in the transmitting branch for generating baseband modulation signals from a digital data stream (TXD); (b) demodulator means in the receiving branch for generating a base-band data stream (RXD) by demodulating baseband signals to be received; (c) means (22, 24) for forming an RF transmitting signal directly from said baseband modulation signals and for forming said baseband signals to be received directly from the RF signal to be received; and (d) timing means (16a; 16b) for connecting the radio link terminal alternately to a transmitting and receiving mode. To achieve a link and a link system that are economically advantageous and as simple as possible, the means comprise a single common quadrature mixer (22) and a local oscillator (24) controlling it, the transmitting and receiving branch being separated at the baseband.

Description

Digital radio link system and radio link terminal

The invention relates to a radio link system according to the preamble of attached claim 1, and to a radio link terminal according to the preamble of attached claim 5.

A radio link system usually operates on the duplex principle, which means that two radio link terminals establish together a two-way connection so that the traffic propagates in both directions simultaneously.

A two-way simultaneous connection is established by using a different radio frequency in the outbound and inbound directions. This, however, causes certain problems, which will be discussed under the following points A to D:

A. In both of the radio link terminals, the transmitter and receiver signals have to be separated from each other so that the receiver will not be over- loaded. In practice, the only way is to use a band division filter (antenna branching unit), which connects the transmitter at one frequency band and the receiver at another frequency band to the antenna. This kind of filter is a very complicated component and thus also expensive.

B. An antenna branching unit requires either channel-associated tuning or tuning that covers only a frequency range portion, which complicates the manu¬ facture of both the filter itself and its spare parts. C. As the equipments at the ends of a radio hop are reverse to each other with respect to frequencies, it is difficult to make them similar in structure. At least, it is necessary to modify the antenna branching unit with respect to the reversed transmitting and receiving frequencies, but the radio parts themselves may be varied in accordance with the reversed fre¬ quencies.

D. As the transmitter and the receiver of the radio link terminal operate at different frequencies, they contain several oscillators operating at dif¬ ferent frequencies and possibly causing interfering mixing results within the radio link terminal.

The following ways, among others, have been used in an attempt to solve the problems described above (references A to D correspond to the points set forth above):

A. In the case of a digital radio link, the band division filter (antenna branching unit) is used all the time, and it has been possible to reduce its high cost mainly only in ways relating to the manufacturing technique. In certain applications, e.g. in a wireless telephone system described in PCT Application WO 84/00455, the duplex operation has been achieved by the use of a single channel where communication takes place alternately in both directions at a rate higher than the normal rate. The rates can be evened out by a suitable elastic buffer, and the antenna branching unit can be replaced with a less expensive switch.

B. and C. The antenna branching unit is replaced in the same way as set forth above under item A.

D. The receiver can be made such that it oper¬ ates on the principle of direct conversion, so that the number of frequency-selective parts and disadvantageous mixing results diminishes. The wireless telephone system disclosed in the above- mentioned PCT Application WO 84/00455 also employs direct conversion.

The object of the present invention is to avoid the above-described disadvantages and at the same time provide a radio link terminal and radio link system which can be realized as economically as possible, i.e. with a minimal number of parts while the link terminals are also as similar as possible. This object is achieved by a solution according to the invention, of which the radio link system is characterized by what is disclosed in the characterizing portion of attached claim 1, and the radio link terminal by what is disclosed in the characterizing portion of attached claim 5. The idea of the invention is to apply alternate communication at a single frequency to the radio link system while the transmitter employs direct modulation to a radio frequency and the receiver employs direct conversion from the radio frequency to the baseband so that (a) the transmission and reception use a single common quadrature mixer which is controlled by means of one and the same local oscillator, and (b) the transmitting and receiving branches are separated at the baseband. In principle, the system according to the invention contains no high-frequency frequency- selective parts, such as an antenna branching unit. The link terminal or link system according to the invention does not either contain any separate high- frequency switches, but these can be replaced with an inexpensive baseband switch. In addition, less tuning is required. Savings in costs will be considerable as the transmitter and the receiver of the radio link terminal share the same mixer and local oscillator. As the transmitter and receiver both operate on the prin¬ ciple of direct modulation/conversion and alternate at the same frequency, there is no risk of disadvanta¬ geous mixing results occurring between the transmitter and the receiver. As the system according to the invention does not use two channels but a single channel twice as broad, the spectral efficiency will, in principle, not change. Alternation between transmission and recep¬ tion, however, requires a predetermined recovery time, which in practice deteriorates the spectral efficiency to some extent.

In the following the invention and its preferred embodiments will be described more fully with refer¬ ence to the examples shown in the attached drawings, where

Figure 1 is a block diagram illustrating a radio link system according to the invention;

Figure 2 is a more detailed block diagram illus¬ trating a portion of a radio link terminal according to the invention; and

Figure 3 is a more detailed view of a quadrature mixer according to the invention.

The radio link system according to the invention applies alternate communication at a single frequency to a digital radio link connection transmitting two- way traffic (such as speech and/or data) of at least 0.7 Mbit/s by using direct modulation to the radio frequency in the transmitter and direct demodulation from the radio frequency to the baseband in the receiver. The radio link terminal according to the invention comprises a single quadrature mixer and a single (high-frequency) local oscillator which acts alternately as the local oscillator of the transmitter and the receiver. Figure 1 shows a block diagram illustrating the radio link system according to the invention. The system comprises two radio link terminals A and B, which communicate with each other over a radio path 11. (In Figure 1, the corresponding parts have been indicated with the same reference numerals in such a way, however, that the letter a is inserted after the reference numeral for radio link terminal A, and the letter b for radio link terminal B. ) The transmitting branch of the link terminal comprises an elastic buffer 11, which receives a digital data stream D addressed to the link; and a baseband modulator part 12, which generates a baseband modulation signal for a radio part 13 connected to an antenna 17, the radio part being common to the receiving branch and the transmitting branch. The receiving branch of the link terminal comprises a demodulator 14, which demodulates the baseband modulation signal from the antenna and the radio part; and an elastic buffer 15, which for¬ wards the digital data stream received via the link. The link terminal further comprises a timing unit 16, which causes the link terminal to alternate between the transmitting and receiving modes by controlling the modulator, the demodulator, and the elastic buffers. Both of the link terminals transmit and receive at the same frequency f.

The baseband data stream D entering the link terminal A is applied to the elastic buffer 11a, into which the bits to be transmitted are stored at an even rate. (Storing into the buffer takes place at a sync determined by an incoming clock signal CLK). When the transmission period of the link terminal A starts, the elastic buffer transmits the stored bits, framed in a suitable way, at a rate more than twice the storing rate through the modulator 12a, the radio part 13a, and the antenna 17a, to the opposite end of the radio hop. (In the figure, the framed data stream to be transmitted is indicated with reference TXD and the respective clock signal with reference TCK. ) At the opposite end of the radio hop, the received signal passes through the antenna 17b and the radio part 13b to the demodulator 14b, which performs demodulation (makes a decision on the received bits) and applies the bits into the elastic buffer 15b. (In the figure, the received framed data stream is indicated with the reference RXD, and the respective clock signal with the reference RCK. ) The elastic buffer disassembles the frame structure of the radio link, and the bits are retrieved at an even rate. In the reverse direc¬ tion (from terminal B to terminal A), the operation is similar but it alternates in the time domain with the first-mentioned transmitting direction.

Transmitting and receiving periods alternate at an even rate, and speed differences between the bit streams are evened out by conventional techniques. A predetermined recovery period is needed between the transmitting and receiving periods to give the equip¬ ment time to set and to cover the propagation times of signals and echoes propagating on the radio path. The gross bit rate is thus clearly more than twice as high as the net bit rate.

Figure 2 shows more closely the radio link ter¬ minal according to the invention. For the sake of clarity, the elastic buffers 15 are not shown. Accord¬ ing to the invention, the radio part 13 comprises a single quadrature-type mixer 22 and a local oscillator 24 controlling it.

During the receiving period of the link ter¬ minal, a signal to the antenna 17 is mixed to the baseband in the quadrature-type mixer 22, to the local oscillator input of which a signal LO from the local oscillator 24 is applied. Mutually substantially quadrature baseband signals I and Q obtained from the mixer are passed through baseband amplifiers 21 to the demodulator 14, which makes the decision on the received bits and combines the signals into a single data stream RXD. The baseband amplifiers 21 also contain lowpass filters determining the frequency selectivity of the receiver and possibly also equal¬ izers. Upon transition to transmission after a recep¬ tion period, the baseband amplifiers 21 are dis¬ connected from the mixer, and a signal from the modu¬ lator 12 is applied to the I and Q ports of the mixer, the shape of the signal depending on the modulation applied. (Generally speaking, the modulator performs the required baseband processing, including the division of the data stream TXD into two portions (I and Q), encoding, filtering, and conversion of the data into analog form. ) The disconnection of the baseband amplifiers, however, is optional, as trans¬ mission and reception take place at different times, and so signals received during the transmission period are of no importance. Logically, however, there is a switch indicated with reference SW1 between the mixer and the baseband amplifier. If the switch is also implemented physically, this can be done e.g. by an analog FET switch of the model 4052 or 4066. From the point of view of the invention, however, it is essential that the transmission and reception branches are separated at the baseband.

If the link utilizes PSK- or QAM-type modu¬ lation, voltages corresponding to these are applied from the modulator 12 as the I and Q signals, and fine control of frequency (not shown in Figure 2) possibly applied from the demodulator 14 to the local oscil¬ lator 24 is disconnected or frozen.

If FSK-type modulation is used, a DC voltage is applied to the mixer 22 as the I and Q signals, and the frequency of the local oscillator is modulated by a voltage (indicated with reference Vm in the figure) obtained from the modulator. For this period of time, an AFC voltage possibly obtained from the demodulator to the local oscillator 24 is disconnected (or frozen). Combining FSK-type modulation with the above- described system constitutes the preferred embodiment of the invention, at least economically. This is because the maximum undistorted output power will be obtained in this way, in addition to which the de- modulator can be implemented by using a circuit arrangement based on measuring differences between the direction angles of consecutive displacements that have occurred in the I/Q plane of the signal point. This demodulator solution, which is interference- resistant and advantageous in costs, is described more fully in Finnish Patent Application FI 932 519, which is referred to for a more detailed description.

In principle, the mixer 22 can be any known (passive, quadrature) mixer structure. However, in order that the mixer 22 would not cause any needless attenuation at the transmission stage, the mixer is implemented according to the preferred embodiment of the invention by switching it for the duration of the transmission period into a state where its conversion loss is at its minimum. In this kind of alternative, the quadrature mixer may be e.g. a mixer realized by FETs and controlled so that it will act as an active mixer at the transmission stage and as a passive mixer at the reception stage. Figure 3 shows this kind of passive/active mixer, which in principle comprises two separate mixer blocks controlled at the different phases. The local oscillator signal LO is applied through an input capacitor Cin to a first power divider 31, from which it is further divided as such (with the same phase) to a second power divider 32, and with a phase shift of 90 degrees to a third power divider 33. The second and third power dividers each control their own FET block FI and F2, respectively, which are mutually similar so that the port of the respective power divider corre¬ sponding to the phase shift of 0° is connected to the gate of the first FET, and the port corresponding to the phase shift of 180° to the gate of the second FET. The first FET block FI forms the terminals of the I signal so that the source electrode S of the first FET forms the 1+ terminal of the signal, and the source electrode S of the second FET forms the I- terminal of the signal. (Signal I- is equal to inverted signal I+. ) Correspondingly, the second FET block F2 forms the terminals of the Q signal so that the source electrode S of the first FET forms the Q+ terminal of the signal, and the source electrode S of the second FET forms the Q- terminal of the signal. (Signal Q- is equal to inverted signal Q+. It is also to be men- tioned that the signals 1+ and I- are indicated in Figure 2 with the same reference, I, and the signals Q+ and Q- with the same reference, Q. ) In both of the blocks, the RF-ground is formed for the source elect¬ rode of the respective FET by means of the capacitors Cl, C2 and C3, C4, respectively. In both of the FET blocks, the drain electrodes of the FETs are inter¬ connected, and this common terminal is connected to a fourth power divider 34, which combines the mixed signals and applies them through an output capacitor Cout and the RF port of the mixer to the antenna and, respectively, divides the signal to the mixer blocks in the reception. In principle, the fourth power divider is not absolutely necessary in the mixer.

According to the preferred embodiment of the invention, this mixer structure known per se is util- ized in such a way that the timing means 16 determin¬ ing the transmission/reception alternation control the mixer by connecting a bias voltage +Vdd to the drain electrodes of the FETs through a choke L2 and the fourth power divider 34 at the transmission stage so that the FETs act as active (amplifying) mixers. Upon transition to the reception stage, the timing means 16 determining the transmission/reception alternation disconnect the bias voltage, and so the FETs act alternately as conductive switches, and the mixer acts as a passive mixer. Through a choke LI, a bias voltage is applied to the gates of the FETs; this voltage is continuously on (during both transmission and recep¬ tion). The mixer structure shown in Figure 3 is advantageous in the sense that it operates in both directions and it can be easily used as active in the transmission direction, which is particularly advan¬ tageous, as the link terminal according to the inven- tion has no separate amplifiers on the RF side.

When an FSK-type signal is used, attenuation caused by the mixer can also be prevented by providing the mixer block with a switch which switches the mixer for the duration of the transmission into a state in which the local oscillator signal passes directly through the mixer at the greatest possible power. Even in this case, the timing means 16 determining the transmission/reception alternation control the state of the mixer. Even though the invention has been described above with reference to the examples of the attached drawings, it is obvious that the invention is not limited to it, but it can be modified within the inventive idea disclosed above and in the attached claims. The more detailed structure of the equipment will vary in various ways, depending on the modulation method used, for example.

Claims

Claims :

1. Digital radio link system comprising two radio link terminals (A, B) establishing a fixed radio connection between them, wherein the radio link ter¬ minals transmit alternately at the same radio fre¬ quency, each one of the radio link terminals (A, B) comprising baseband modulator means (12a; 12b) in the transmitting branch for generating baseband modulation signals from a digital data stream (TXD); demodulator means (14a; 14b) in the receiving branch for generating a baseband data stream (RXD) by demodulating baseband signals to be received; frequency-conversion means (22, 24) for forming an RF transmitting signal directly from said baseband modulation signals and for forming said baseband signals to be received directly from the RF signal to be received; and timing means (16a; 16b) for connecting the radio link terminal alternately to a transmitting and re¬ ceiving mode so that when one of the radio link ter¬ minals is in the transmitting mode, the other radio link terminal is in the receiving mode, c h a r a c- t e r i z e d in that said frequency-conversion means comprise a single common quadrature mixer (22) and a local oscillator (24) controlling it, the transmitting and receiving branch being separated at the baseband.

2. Radio link system according to claim 1, c h a r a c t e r i z e d in that the quadrature mixer (22) operates under the control of the timing means (16a; 16b) alternately in two different modes, the timing means (16a; 16b) being connected to control the mixer so that it is in an active mixing state for the duration of the transmission period and in a passive mixing state for the duration of the reception period.

3. Radio link system according to claim 1, wherein FSK modulation is used, c h a r a c t e r- i z e d in that DC-voltage signals are applied to the mixer (22) as quadrature signals (I and Q), and the frequency of the local oscillator is modulated by a voltage (Vm) obtained from the modulator means (12a; 12b).

4. Radio link system according to claim 1, wherein FSK modulation is used, c h a r a c t e r¬ i z e d in that said timing means (16a; 16b) are connected to control the local oscillator signal so that it passes directly through the mixer during the transmission period.

5. Digital radio link terminal comprising baseband modulator means (12a; 12b) in the transmitting branch for generating baseband modulation signals from a digital data stream (TXD); demodulator means (14a; 14b) in the receiving branch for generating a baseband data stream (RXD) by demodulating baseband signals to be received; frequency-conversion means (22, 24) for forming an RF transmitting signal directly from said baseband modulation signals and for forming said baseband signals to be received directly from the RF signal to be received; and timing means (16a; 16b) for connecting the radio link terminal alternately to a transmitting and re- ceiving mode so that transmission and reception take place at different times; c h a r a c t e r! z e d in that said frequency-conversion means comprise a single common quadrature mixer (22) and a local oscillator (24) controlling it, the transmitting and receiving branch being separated at the baseband.

6. Radio link terminal according to claim 5, c h a r a c t e r i z e d in that the mixer is realized by FET switches, and that said timing means (16a; 16b) are connected to control said FETs so that they are in an active mixing state for the duration of the transmission period and in a passive mixing state for the duration of the reception period.

7. Radio link terminal according to claim 5, wherein said baseband modulator means (12a; 12b) comprise an FSK modulator, c h a r a c t e ri z e d in that DC-voltage signals are applied to the mixer (22) as quadrature signals (I and Q), and the fre¬ quency of the local oscillator is modulated by a volt¬ age (Vm) obtained from the modulator means (12a; 12b).

8. Radio link terminal according to claim 5, wherein said baseband modulator means (12a; 12b) comprise an FSK modulator, c h a r a c t e r i z e d in that said timing means (16a; 16b) are connected to control the local oscillator signal so that it passes directly through the mixer during the transmission period.