GB2365281A

GB2365281A - Transmit diversity in an indoor gsm cellular system comprising a base station having at least two antennas having mutual time delay and phase difference

Info

Publication number: GB2365281A
Application number: GB0114031A
Authority: GB
Inventors: Andrew Fogg; John Leslie Haine
Original assignee: IP Access Ltd
Current assignee: IP Access Ltd
Priority date: 2000-07-14
Filing date: 2001-06-08
Publication date: 2002-02-13
Also published as: GB0017427D0; GB0114031D0

Abstract

A cellular radio telecommunications system comprises a basestation having at least two antennas 107, 114. Each antenna is arranged to transmit a downlink signal to one or more mobile subscriber units. The downlink signal is transmitted from each antenna 107, 114 having a mutual time delay and phase difference, the phase difference changing from time to time. Because of the mutual phase differences, if the signals received at a mobile station are mutually in anti-phase during one time slot of the downlink signal, there is a very low probability that they will also be in anti-phase at the corresponding time slot in the next several frames.

Description

<Desc/Clms Page number 1> CELLULAR RADIO TELECOMMUNICATION SYSTEM This invention relates to cellular radio telecommunication systems.

In any wireless system one of the primary limits on effective communications is multipath propagation. When an electromagnetic wave is emitted from an antenna into free space it undergoes reflection and diff-raction from obstacles of all kinds, such as buildings, hills, vehicles, people, walls, and office furniture. Often the direct line of sight from the transmitter to the receiver is obstructed as well, so that the received signal is made up of several components with random amplitude and phase differences which depend on the different paths along which the signals travel. The received signal is therefore dispersed in time, which causes inter-symbol interference with digital modulations. The degree of dispersion is typically measured by the root-mean-square of the distribution in time of the received reflected signals, called the rms delay spread. The phase differences between the different received components may also cause partial or complete signal cancellation. Finally, if the terminal is moving, then each received signal component is subject to a Doppler shift depending on the relative velocity between the terminal and its environment, resulting in a time-varying signal amplitude with very deep nulls. The aggregate impairments caused by multipath are commonly known as multipath fading. In cellular communications systems such as the Global System for Mobile Communications (GSM), multipath propagation is a primary cause of signal degradation. The duration of a single transmitted GSM digital symbol is approximately 4 microseconds, yet the system has to operate in environments where the multipath delay spread can be of the order of 20 microseconds, or nearly 5 symbols. GSM handsets and base stations always therefore incorporate means to compensate for the delay spread, known generically as a multipath equaliser. There is increasing interest in providing coverage by base stations located in confined indoor areas. These may be public areas such as shopping centres, or private domains such as offices or homes. Hand portables are now the dominant type of terminal, and users wish to use their portable telephone everywhere. In the local environment, multipath

propagation is equally prevalent but the relative delay spread is very small. For example, in a room which is 150 metres long, a reflection from the far wall is delayed by only I microsecond. Moreover, handsets being used in buildings tend to move very slowly.

A well-known method of countering multipath degradation is receive diversity. In this method, a receiver is equipped with two or more antennas which are spatially separated. it is well known that if the antennas are in a multipath field but separated by at least one quarter of the operating wavelength ("space diversity"), then the fading envelopes of the signals received at the antennas are essentially un-correlated. The same is true if the signals are received at the same antenna but at different frequencies as long as the frequencies are sufficiently separated ("frequency diversity"). Thus even if one antenna is located at a point where the multipath components cancel, there is a high probability that at least one other antenna is not at such a point. Alternately, if a signal at one carrier frequency is faded then the signal received at a different carrier has a high probability of not being faded. By combining the signals in the correct way it is possible to greatly reduce the fading in the combined signal, that is, to greatly reduce the probability that the combined signal is impaired beyond some critical performance threshold.

A significant benefit of diversity in cellular systems is the reduction of the effect of intra-system interference. Because the probability of the receiver experiencing a fade is significantly reduced by diversity, the transmitter power needed to obtain a desired minimum carrier to interference level over the cell can be reduced. Thus the interference caused in other, distant, cells operating at the same frequency is reduced. Applied throughout a cellular system, diversity can therefore increase system capacity. Space diversity is easy to achieve at the base station because there is no restriction on how far apart the antennas can be placed on the antenna tower. However it is difficult to achieve space diversity at the handset because the terminal itself has to be very small; and also its complexity, size, power consumption and cost will be increased if it is necessary to incorporate an additional receiver to process signals from the second antenna.

It is also difficult to apply conventional frequency diversity to a cellular system because the amount of available spectrum is limited. Also, whilst frequency diversity can be applied even if the receiver only has one antenna, two receiver chains are normally required.

However, in a TDMA system such as GSM it is normally possible to use frequency hopping (FH). With FH, the frequency used by a given timeslot in successive frames changes pseudo-randomly. If the signal received during the slot by the terminal (or by the BTS in the reverse direction) is at a frequency which is faded in one frame, it is relatively unlikely that it will also be faded at the frequency used in the next frame. The forward error correction and interleaving means used in GSM can then often correct errors in the faded slot based on parity information in previous and following slots. Frequency hopping also reduces the susceptibility of the system to co-channel interference because cells use different hopping sequences and therefore whilst co-channel interference may occur in one frame it is unlikely in succeeding frames. Frequency hopping is likely to be much less effective in combating multipath in an indoor environment, because the relative delay spread will be very short, which increases the correlation between fading at different channel frequencies.

One method of obtaining the benefits of diversity without impacting receiver complexity is to use transmit diversity. This is for example possible in cordless telephone systems des-ii-i-'e-d according to the DECT standard. DECT uses time division duplexing, in which the base station and handset transmit alternately in time on the same frequency. The base station is equipped with at least two antennas and can select which is used for transmission or reception at any given instant. While the base station is receiving from the handset it determines which antenna provides the best reception. It then uses the same antenna to transmit signals back to the handset. Because both ends of the link operate at the same frequency and the radio link is reciprocal the same antenna will also give best performance for the other link direction. However transmit diversity is not applicable in the GSM system because the downlink (base - handset) and uplink (handset - base) are at different frequencies and their fading characteristics are therefore generally un-correlated.

Another method of providing apparent transmit diversity is so - called quasi - synchronous transmission which has been applied in private mobile radio (PMR) networks which have to provide a good signal to mobile.radios which can roam over a very wide area which cannot practically be covered from a single base site. The operators of such systems often have only a single frequency channel available and cannot therefore apply handoff such as

is commonly used in cellular networks to ensure continuous coverage. Such PMR systems are for example used by emergency services.

In quasi - sync, different base stations transmit in overlapping areas on very slightly different frequencies, which are nevertheless well within the overall channel bandwidth. A mobile receiver in an area where the transmissions overlap receives the sum of two multipath fields each at a slightly different frequency. However, the frequency difference is indistinguishable from Doppler shifts caused by vehicle movement. The result is that the signal received is effectively improved as the total energy received is the sum of the energy from the two base stations. Many of the benefits of transmit diversity are obtained without the need for the transmitters to have any knowledge of the paths to the receiver. It is known however that in quasi-sync systems it is necessary to ensure that the common modulating signal fed to all the base sites have to have their relative time delays equalised to avoid serious distortion of the received and demodulated signal at the mobile, since PMR terminals do not incorporate any form of multipath compensation.

It is an object of the present invention to obtain many of the benefits of transmit diversity in an indoor GSM cellular system without requiring any modifications to the standard GSM hand-portable telephone.

In particular, it is an object of the invention to improve the coverage provided by an indoor base station without having to increase its output power. The invention thereby seeks to decrease the effect of interference from an external transmission at the same frequency on the performance of a handset covered by the indoor base station; and to reduce the interference caused by the indoor base station to handsets outside the building.

According to the present invention there is provided a cellular radio telecommunication system comprising a base station having at least two antennas, said antennas each arranged to transmit a downlink signal to one or more mobile subscriber units, wherein said base station is arranged so that signals transmitted from said antennas have a mutual time delay and phase difference, said phase difference changing from time-to-time.

Because of the mutual phase differences, if the signals received at a point are mutually in anti-phase during one time slot, there is a very low probability that they will also be in anti-phase at the corresponding time slot in the next several frames.

Preferably the phase difference between the at least two signals transmitted from the antennas changes periodically. Alternatively the phase difference may change on a random or pseudo random basis.

Alternatively or additionally, the transmitted signals comprise a plurality of time slots, or bursts, and the phase difference changes at the beginning of each successive slot. The phase difference may lie within the range of 00 to 360'. Preferably the antennas are mutually spatially separated by distance sufficient to ensure that any multi-path fading in the transmitted signals as received by at least one mobile subscriber unit within the coverage of at least said two antennas is decorrelated.

Preferably the mutual time delay between the at least two signals transmitted from the antennas is within a predetermined maximum delay chosen such that a mobile subscriber unit including an equaliser can demodulate the aggregate signal received from the at least two antennas. Preferably the predetermined maximum delay equals the maximum allowable delay spread in accordance with GSM standards. By setting the predetermined maximum delay to be no larger than the maximum allowable delay spread specified in GSM standards the predetermined delay of the signals mimics the delay spread of a multiple path signal in a conventional GSM system that can be demodulated by a standard equaliser within a mobile subscriber unit.

Preferably the base station is arranged to generate a first downlink signal to be transmitted from a first one of the antennas and then taking this first downlink signal the base station introduces said time delay and phase difference to generate a second downlink signal that is transmitted from a second one of the antennas.

Preferably the downlink signals are phase shift key modulated. Additionally, the base station may be arranged to modify a digital baseband signal prior to modulation to thereby

generate the phase difference at the time of modulation. Additionally, for example in a GSM implementation, the baseband signal is differentially encoded and the base station arranged to modify the baseband signal by altering the voltage level of the encoded signal. Additionally, the downlink signal may be GMSK modulated.

Alternatively, the baseband signal may be EDGE encoded and said base station modifies said baseband signal by manipulating at least one symbol of said signal.

In a GSM implementation of the invention, the auto-correlation ftmetion of the GSM signal is relatively narrow in time, so that two versions of the same GSM carrier which are sufficiently separated in time have low correlation. If both the signals are received at the same receiver, it is relatively unlikely that they will destructively interfere because they are not highly correlated. The conventional equaliser of the GSM receiver will then optimally estimate the modulation signal from the combined signal which appears similar to a conventional GSM signal with multipath.

The invention will now be described in a typical embodiment, as shown in the attached Figures.

Figure I is an example block diagram of the base station transmitter scheme for GMSK modulation; Figure 2 shows the un-modified and modified GSM downlink burst structures for GMSK modulation; Figure 3 shows the un-modified and modified GSM downlink burst structures for EDGE modulation.

In Figure 1, digital baseband circuits 10 1 generate the conventional GSM BTS data stream as described in GSM Specifications 05.04 and 05.02. This is differentially encoded by the encoder 102 whose output is the modulo-2 sum of the present and previous bit of the baseband data stream. The differentially encoded binary stream is processed in items 103,

104, 105 106 and 107. First it is level shifted by 103 so that a binary I is translated to a negative level and a binary zero to an equal positive level. The resulting bipolar signal is filtered by the Gaussian filter 104 which has impulse response defined in the above GSM Specification, and applied to the Voltage-Controlled Oscillator (VCO) 105 where it frequency modulates a carrier at the transmit frequency (with frequency hopping if appropriate) with a modulation index of 0.5. It is well known that a single binary "I" from the differential encoder will then cause a total phase excursion of -90' of the resulting carrier signal; whilst a single binary "0" causes a phase excursion of +90'. Finally, the Power Amplifier (PA) 106 generates the necessary transmit power and feeds the Antenna 107.

In the second block comprising items 108, 109, 110, 111, 112, 113 and 116, a delayed version of the GSM signal with periodic phase shifts is generated. First a time delay TI is inserted by digital delay line 108. Next the signal is level-shifted as before by 109. Now however, a random signal 115, generated by the Random Generator 116, is added to the level-shifted signal at a defined time. The timing and characteristics of the random signal are considered later.

Following addition of the random signal, the waveforin is Gaussian filtered by 111; applied to the VCO 112; and finally amplified by the PA 113 and radiated by the antenna 114. Figure 2 shows the signals in various parts of the system shown in Figure 1, derived from the GSM standards documents.

At the end of the nominal burst period the signal 201 has 3 tail bits which are specified to be binary zeros as shown, followed by an inter-slot guard period of 8.25 bits. The signal is specified to be all I's during this guard period, but this is arbitrary as the GSM handset is not required to demodulate these bits. The differentially encoded data is shown as 202, and it can be seen that the transition from the tail bits to the guard bits is differentially encoded as a single binary 1, which will cause a -90' phase shift of the transmitted signal.

The delayed, differentially encoded signal is shown as 203 (assuming 2 symbols delay; and the level shifted signal as 204 (corresponding to the output of the block 109 in Figure 1). The Random Signal 115 is as shown as 205 in Figure 2. For most of the time, the signal is zero, but during four bits of the guard period, during which the level shifted signal 205 is at +v, the random signal assumes a constant value -dv, where dv takes a value at random between 0 and v.

The effect of the random signal is to reduce the carrier phase shift during the four bit periods of the guard period. Without the random signal, the carrier phase will advance by 90'A = 360' during this period. In the extreme case, where the random signal takes the value -v, there will be no net phase shift during the 4 bit periods. Therefore, the relative phase of the carriers transmitted from the antennas will be different by a value between 0 and 360' by the start of the next burst.

The above description of the method applied to GSM assumes that the GMSK modulator uses a Gaussian filter and frequency modulator. Most implementations of GMSK modulators use digital signal processing techniques to accurately synthesise the required phase without requiring a precise frequency modulator. It is however evident that the above method may be simply modified to cover any form of implementation of the modulator and the patent will cover these by extension. For example, another common implementation of the modulator contains a phase accumulator which digitally adds up phase increments indexed from a look-up table according to the current set of input symbols (four or five depending on the Gaussian filter aperture) and the sample phase. If the accumulator can also accept one-shot input from a phase offset register programmable from a micro-controller, this can be used to introduce random phase steps of any desired magnitude. The current phase value in the accumulator is then used to look up sine and cosine sample values in a further lookup table, and these sample values converted into analogue form to produce the desired signal.

Figure 3 shows the burst structure for an EDGE modulated signal according to the above GSM standards. EDGE uses 8-PSK modulation with a bit rate which is three times higher than conventional GSM. Each triplet of 3 bits maps to a symbol which has the same

duration as the GSM GMSK symbol. EDGE does not apply differential encoding. The signal 301 shows the bit sequence for the tail bits of the burst and the guard period for normal EDGE. It can be seen that instead of 8.25 I's the guard period consists of 24.75 I's. Because EDGE does not use differential encoding and the EDGE modulator provides means to rotate the carrier phase by any amount up to 1800, the required phase shifts can be introduced by direct manipulation of just the first symbol (first three bits) of the guard period. Signals 302, 303, and 304 show the appropriate patterns required, taken directly from Figure 2 of GSM 05.04, for phase shifts of 90' and 180' compared with the un-shifted carrier generated by 301. Again the phase offset applied can be periodic or selected pseudo-randomly.

Again, physical implementations of the EDGE modulator can take several forms but the method described can be easily modified to these.

In both the methods described above two separate signals are transmitted. However, in each case the method can be generalised to three or more signals, where each signal has a different time delay and different phase sequence.

Claims

CLAIMS 1. A cellular radio telecommunication system comprising a base station having at least two antennas, said antennas each arranged to transmit a downlink signal to one or more mobile subscriber units, wherein said base station is arranged so that signals transmitted from said antennas have a mutual time delay and phase difference, said phase difference changing from time-to-time. 2. A cellular radio telecommunication system according to claim 1, wherein said phase difference changes periodically. 3, A cellular radio telecommunication system according to claim 1, wherein said phase difference changes randomly. 4. A cellular radio telecommunication system according to any preceding claim, wherein said transmitted signals comprise a plurality of time slots and said phase difference changes between successive slots. 5. A cellular radio telecommunication system according to any preceding claim, wherein said phase difference is within the range 0' to 3 60'. 6. A cellular radio telecommunication system according to any preceding claim, wherein said antennas are mutually spatially separated by a distance sufficient to ensure that fading in the transmitted signals as received by said at least one mobile subscriber unit is de-correlated. 7. A cellular radio telecommunication system according to any preceding claim, wherein said mutual time delay is within a predetermined maximum delay such that a mobile subscriber unit having an equaliser can demodulate the aggregate signal received from said at least two antennas.

<Desc/Clms Page number 11>

8. A cellular radio telecommunication system according to claim 7, where said predetermined maximum delay equals the maximum allowable delay spread in accordance with GSM standards. 9. A cellular radio telecommunication system according to any preceding claim, wherein said base station is arranged to generate a first downlink signal to be transmitted from a first one of said antennas and introduces said time delay and phase difference to generate a second downlink signal to be transmitted from a second one of said antennas. 10. A cellular radio telecommunication system according to any preceding claim, wherein said downlink signals are phase shift key modulated. 11. A cellular radio telecommunication system according to claim 10, wherein said base station is arranged to modify a digital base band signal prior to modulation to thereby generate said phase difference at modulation. 12. A cellular radio telecommunication system according to claim 11, wherein said base band signal is differentially encoded and said base station modifies said base band signal by altering the voltage level of said encoded signal. 13. A cellular radio telecommunication system according to claim 12, wherein said downlink signal is GMSK modulated. 14. A cellular radio telecommunication system according to claim 11, wherein said baseband signal is EDGE encoded and said base station modifies said baseband signal by manipulating at least one symbol of said signal. 15. A cellular radio telecommunication system substantially as described herein with reference to, or as shown in, the accompanying figures.