ANTENNA DIVERSITY RECEPTION IN WIRELESS PERSONAL COMMUNICATIONS

Abstract: An antenna diversity technique for use in wireless personal communications is disclosed. In the so-called microcellular phone environment, fading in signal reception is an important consideration. A new way of performing antenna diversity involves antenna switching during a period of data reception which may result in minimum bit hits to data. The system requires only a single receiving part.

Description: Antenna diversity reception in wireless personal communications. Technical Field and Industrial Applicability The present invention generally is directed to wireless personal telecommunication. In particular, it is directed to a technology of antenna diversity reception of digital signals used in such tele¬ communication. Background Art In wireless personal telecommunication, fading causes the fluctuation of reception level and therefore deterioration in communications. As a method of reducing fading, a diversity reception has been widely performed. For example, if a plurality of antennas are set up at predetermined distances from each other, the probability of all signals from the antennas being simultaneously attenuated is small, hence high reliability is assured. Signal to Interference margins can be increased by as much as 10 dB when two antennas are used rather than one. Various kinds of diversity reception methods are known. U.S. Patent Nos. 4,696,058, issued September 22, 1987 (Tachita et al) and 4,977,616 issued December 11, 1990 (Linder et al), for example, teach diversity receivers which use two or more antennas and one receiver circuit. An antenna selection circuit switches to any one of the antennas in response to received signal strength indication generated by the receiver. U.S. Patent No. 5,097,484, issued March 17, 1992 (Akaiwa), on the other hand, discloses a system which uses two antennas and two receivers. In the wireless personal telecommunication environment, it is normally the cordless fixed part (often called base station) that performs antenna diversity reception because it is more difficult to provide more than one antenna in the cordless portable part (often called microcellular phone, handset or such name). In one system such as one taught in the above-referenced U.S. Patent No. 5,097,484, a link from the base station to a handset is carried out at one frequency and a link in the opposite direction is at a different frequency. In the digital communication environment, it is also possible to use a duplexer which permits two-way communication on a single frequency. U.S. Patent No. 5,067,173, issued November 19, 1991 (Gordon et al) describes such an arrangement. In most cases, if "perfect" diversity is assumed, a system requires a prior knowledge of the received signal strength indication (RSSI for short) at all antennas which can be selected. Such information can only be achieved through the use of dual receivers. In such an arrangement, the RSSI of the signal received on each antenna is continuously monitored, and the best antenna is always selected. This implementation adds considerable cost to a radio receiver. However, without a dual receiver, the unused antenna must be sampled periodically, resulting in bit hits (lost or corrupted bits) or frame hits (lost or corrupted TDD frame) in the received data whenever the "tested" antennas have very poor RSSI. It is therefore desirable to implement antenna diversity while only using a single receiver, and at the same time minimize the effect of bit hits due to antenna switching. Objects of the Invention It is therefore an object of the present invention to provide a process of and apparatus for performing antenna diversity in the reception of digital radio signals. It is another object of the present invention to provide a process of and apparatus for performing antenna diversity in the reception of digital radio signals in which impact to data of bit hits is minimized. It is a further object of the present invention to provide a process of and apparatus for performing antenna diversity in the reception of digital radio signals in which no dedicated time slots in the formats for data stream is required for antenna switching. Disclosure of the Invention Briefly stated therefore, the present invention is directed to a process of performing antenna diversity in the reception of digital radio signals transmitted in time division pulse coded frames, each frame containing a fixed number of coded bits in a specific format. The process comprises steps of receiving the digital radio signals by one of a plurality of antennas and generating the RSSI (received signal strength indication) of the antenna. Further steps include switching the antenna to one of the remaining antennas to receive a predetermined number of bits in each frame starting at a predetermined, numbered bit from the beginning of a frame which has low significance in said specific format. The process continues by deriving the RSSI of one of the remaining antennas, comparing two RSSIs, and choosing the antennas with a larger RSSI for reception of the following digital radio signals. Brief Description of the Drawings For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be made to the following description, taken in conjunction with the accompanying drawings, in which: Figure 1 is a schematic block diagram of a base station according to an embodiment of the invention. Figure 2 is a schematic block diagram of a front end of the base station radio. Figure 3 is a schematic block diagram of a radio controller. Figure 4 is a diagram showing various stages of the link connection process. Figure 5 shows frame formats for MUX 1 and MUX 2. Figure 6 is a more detailed illustration of parts of the frame formats. Mode(s) of Carrying Out the Invention In the above-mentioned U.S. Patent No. 5,097,484 there are described two kinds of data formats, either one of which can be used for customer data transmission in the system described. It should be noted here that this patent uses different frequencies for down link and up link. The first format contains a preamble field and data field in a frame. The second format has a further postamble field, in addition to the above two fields. The preambles and postambles are provided for the purpose of the antenna switch. Thus, according to the patent, depending upon the format used, a test antenna is switched on either during the postamble or preamble, both of which should be at least long enough to measure its RSSI. The RSSI of the test antenna is compared with that of the current antenna. The result of the comparison determines either a) to switch to the test antenna as the best antenna, or b) to stay on the current antenna. The antenna switch takes place during the same preamble or during the preamble of the following frame. In U.S. Patent No. 4,977,616, also referred to above, the reception of a signal is continuous but the signal being received is time division multiplexed so that it contains information which is of no interest to the receiver, as well as that of interest. The testing and switching of a new antenna is carried out during the reception of information of no interest. Figure 1 shows a base station according to one embodiment which includes a radio front end 10 with two or more antennas 12 and a radio controller 14 which is in turn made up of a TDD (time division duplex) controller 16 and a processor 18. The base station is connected by wires 20 to switching equipment. Figure 2 is a block diagram of the front end of the radio. Under ETSI CAI (European Telecommunications Standard Institute, Common Air Interface) specifications, a base station handles a multiplicity of channels in the specified frequency band in time division duplex. Two antennas are provided inside the base station housing and there is also a connection port for an optional external antenna. An RF switch 22 permits connection of one antenna to the front end of the radio. A transceiver follows the RF switch and contains various means 24 to switch between a receiving part and transmitting part. The receiving part includes amplifiers 26 and a discriminator/limiter 27 and the transmitting part includes amplifiers 28. Frequency conversion occurs at various stages. In Figure 3, the radio controller is shown. It includes Rx processor 30 and Tx modulator 32 for encoding and decoding. An antenna controller 34 generates antenna control signals which are applied at 29 to the RF switch 22 of Figure 2. Voice/data link, serial control link, and a processor containing memory are also shown as boxes. The antenna controller 34 compares RSSI of various antennas and determines the best antenna for subsequent transmission. Also in the ETSI CAI specifications, there are three formats of the data stream used between a base station and a handset. They are MUX 3, MUX 2 and MUX 1. Referring to Figure 4, there is shown a diagram which illustrates various stages of link set-up. At stage 40, when a handset requests connection it sends a "link request" containing its i.d. and other relevant information to a base station in MUX 3. Upon reception of the "link request" at stage 42, the base station responds by sending back to the handset "link grant" in MUX 2. A handset acknowledges at stage 44 the "link grant" in MUX 2. A multiplicity of packets are then exchanged in MUX 2 to initialize the acknowledged data transfer, base and handset capabilities, and link initialization data. The base station then proceeds to connect voice/data at stage 46 by initiating MUX 1. Subsequent transmission at stage 48 is carried out in MUX 1. Of the three, MUX 3 format is asynchronous and is used only by a handset for a "link request". Because it is asynchronous and only used before a link connection is set up, diversity reception is not essential. MUX 2 format is used by the base station initially for "link grant" and then by the handset. This data format allows both the handset and base station to gain bit and clock synchronization to each other's data transmissions. Finally, MUX 1 is the data format which is used when a bearer channel is desired and contains customer data. Diversity reception is required for MUX 1 and MUX 2. However, unlike the two patents discussed above, it is not possible to create a new field in either format specifically for antenna switching and there are no time slots which carry information of no interest. The present invention makes inventive use of the MUX 2 and MUX 1 formats to attain improved antenna diversity reception. MUX 2 and MUX 1 formats are shown in Figure 5, which also shows alternating transmission (Tx) and reception (Rx), using the same frequency. The duration of Tx and Rx is, in this example, 1 ms and contains one frame of data stream. MUX 2 provides 32 D channel bits, with 10 preamble bits (1-0 transitions for slicing level and clock recovery), and a 24-bit synchronization channel. The 24-bit sync channel is used to allow both base station and handset to verify that they have achieved proper bit synchronization. Since this synchronization pattern is only useful for initial synchronization, and only a subset of these bits need be tested after synchronization has been achieved, the diversity process uses the last 9 bits of the sync pattern to test the RSSI of the new antenna. Thus the figure also includes a diagram of antenna operation. Figure 6 is a detailed representation of MUX 2 and MUX 3 at around the time of antenna switch. The last half of the sync bits are shown as S11-S23, followed by the first D bit. At the S13 bit, the antenna is switched to a new test antenna which measures RSSI for 9 successive bits starting at S14. Thus, at S22, the result of comparison between RSSI of the test antenna and that of the current antenna determines whether to use the test antenna as the best antenna or to stay on the current antenna for the following transmission. If so determined, the current antenna is switched back on to receive the signal starting at S23. Therefore, the diversity process during MUX 2 is able to continually sample the RSSI of all antennas, with bit hits affecting only an unused part of the synchronization marker. The diversity process allows only an unused antenna to be tested at every frame, as this was determined to be a sufficient rate for the fading rates expected in bipedal motion. However, using the same process, all unused antennas could be tested during every 1 ms receive frame, at the expense of more unused synchronization bits or faster hardware sampling circuitry. Since all antennas are being tested at a rate (every 2 ms with 2 antennas; 4 ms with 3 antennas) much faster than the fading rate (maximum rates are approximately 20 ms), the process for MUX 2 antenna diversity results in virtually perfect diversity switching. The MUX 1 format is also shown in Figure 6 and contains no synchronization information, and as such assumes that bits and clock recovery are stable. In MUX 1, a hit to any one of the received bits will result in an error. Therefore, the purpose of the MUX 1 diversity process is to minimize the "perceived" impact of these bit hits. In the ETSI CAI specifications, B channel holds 16 adaptive differential pulse code modulation (ADPCM) data samples, each of which is 4 bits in length. These samples, which represent differential changes in linear digitally encoded voice, have a predefined ordering of bit positions, from most significant bit (MSB) to least significant bit (LSB). Since these samples represent changes in voice data, it follows that changes in LSB values will have less impact on the resulting linear digital voice than do changes in MSB values. The MUX 1 diversity process uses this fact. Figure 6 shows corresponding bit positions between MUX 2 and MUX 1. In the figure, the 9th nibble of B channel is made up of 9β 3 9β 2 9β ! 9BQ with 9β 3 being MSB and 9β 0 being LSB. Likewise, the 10th nibble is made up of i«B 3 ιθβ 2 lOβj ιθβ 0/ with iθβ 3 being MSB and ™Bo LSB. Therefore 9β ! corresponds to S13 and HB 0 to S22. Antenna switches, to test RSSI on unused antennas, are performed during the B channel so that if a bit is corrupted during the antenna switch, the corrupted bit will be a LSB bit of an ADPCM nibble. To this end, the MUX 1 diversity process is able to switch antenna during the B channel, test the RSSI of unused antennas, and either remain on the new antenna or switch back to the previous antenna. The entire process of the implementation used in the base station requires a maximum sample time on the new antenna of two nibbles (approximately 100 μs), and if the tested antenna has a lower RSSI than the current antenna, then only two ADPCM samples will be lost. It is important to note that if the RSSI on the sampled antenna is less than a mute threshold (a predefined threshold which specifies the limit of the base station to receive data with a bit error rate, BER, less than 0.1%), then the two nibbles are zeroed, which minimizes the changes in the linear digital voice decoded from the ADPCM data. The "perceived" errors in the data stream can be further minimized. Therefore, in another embodiment, the MUX 1 diversity process limits the rate of antenna testing, and thus antenna switching (and B channel bit hits), to once per 5 receive frames (every 10 ms). This limit ensures that the BER of the B channel does not exceed 1%, due to selection of an antenna with a lower RSSI than the current antenna. In yet another embodiment, various values of RSSIs are monitored and stored. They are used to switch antennas to more precisely follow the changing characteristics of fading and other circumstances. Therefore the following values are stored: "filtered RSSI" (a long term average RSSI); "peak RSSI" (the highest RSSI sample since the last antenna switch); and instantaneous "sampled RSSI". In MUX 1 of this embodiment, if the "sampled RSSI" drops by a margin called "RSSI Delta" below the "filtered RSSI", then a new antenna will be sampled, limited to a maximum rate of once per five frames (e.g. 10 ms), unless the "sampled RSSI" is less than a perfect diversity threshold (a predefined level at which the probability of dropping the link becomes quite high), in which case an antenna is sampled at every frame (e.g. 2 ms). In this manner, priority shifts from minimizing user perceived bit hits, to maintaining the link by ensuring that the best antenna is always selected. In addition, and not limited by a 10 ms maximum sample rate, if the "sampled RSSI" drops by a margin called "peak RSSI Delta" below the "peak RSSI", then a new antenna will be sampled and "peak RSSI" will be updated. This absolute RSSI trigger catches both very fast fades as well as very slow fades.

Claims: WHAT IS CLAIMED IS: 1. In a process of performing antenna diversity in the reception of digital radio signals transmitted in time division pulse coded frames, each frame containing a fixed number of coded bits in a specific format, the process including steps of generating the RSSI (received signal strength indication) of one of a plurality of antennas, switching the antenna to one of the remaining antennas to derive its RSSI and choosing the antenna with a larger RSSI for reception of the following digital radio signals, the invention being characterised in that: the above step of switching the antenna to one of the remaining antennas is conducted to receive a predetermined number of bits in each frame, starting at a predetermined, numbered bit from the beginning of the frame which has a low significance in the specific format. 2. In the process of performing antenna diversity in the reception of digital radio signals transmitted in time division pulse coded frames, according to claim 1, wherein each frame contains a fixed number of coded bits in at least two specific formats, the first format consisting of D bits and B bits and the second format of D bits, preamble bits and sync bits, the step of switching further comprising steps of: (a) when the digital radio signal is in the first format, switching the antenna to one of the remaining antennas to receive a predetermined number of B bits in each frame, starting at a B bit which has the least significance in decoding, and (b) when the digital radio signal is in the second format, switching the antenna to one of the remaining antennas to receive a predetermined number of the sync bits in each frame. 3. The process of performing antenna diversity in the reception of digital radio signals according to claim 2, wherein when the digital radio signal is in the first format, the step of switching the antenna to one of the remaining antennas is conducted at every preselected, numbered frame only. 4. The process of performing antenna diversity in the reception of digital radio signals according to claim 1, 2, or 3, comprising further steps of storing more than one value of RSSI and switching the antenna to one of the remaining antennas at every preselected, numbered frame only, depending upon the values. 5. The process of performing antenna diversity in the reception of digital radio signals according to claim 1, 2, or 3, wherein there are no less than three antennas and the step of switching the antenna to one of the remaining antennas is carried out in sequence. 6. The process of performing antenna diversity in the reception of digital radio signals according to claim 4, wherein there are no less than three antennas, the step of switching the antenna to one of the remaining antennas is carried out in sequence. 7. In an antenna diversity reception apparatus for a base station in the wireless personal communications system which uses digital radio signals transmitted in time division pulse coded frames, each frame containing a fixed number of coded bits in a specific format for data transmission and management, the apparatus comprising a plurality of antennas, a digital radio signal receiving part, an RF switch for switchably connecting one of the plurality of antennas to the receiving part, and an RSSI measuring means for measuring RSSI (received signal strength indication) of any one of the plurality of antennas which is connected to the receiving part, the invention being charaterised in that the apparatus further includes: an antenna controller for controlling the RF switch so that one of the remaining antennas is connected to the receiving part for a predetermined number of bits in a frame which bits are sufficient to measure the RSSI and have small significance in decoding for data transmission and management, and the antenna controller further selecting one antenna as the best antenna according to the measured RSSI for the following digital radio signals. 8. The antenna diversity reception apparatus for a base station in the wireless personal communications system, according to claim 7, wherein each frame contains a fixed number of coded bits in at least two specific formats, the first format consisting of D bits and B bits and the second format of D bits, preamble bits and sync bits. 9. The antenna diversity reception apparatus for a base station in the wireless personal communications system, according to claim 8, further comprising storage means for storing more than one value of RSSI, and the antenna controller controlling the RF switch according to the stored values.

Inventors: SMITH ROLAND ANDREW, SMITH, ROLAND, ANDREW
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1993
Main classification: H04B7
IPC codes: H04B7