The present invention relates to communications methods and apparatus, and more particularly, to communications methods and apparatus in which signals are encoded according to codes such as modulation and channel codes.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 typically has allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system's service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity. Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using “spread spectrum” techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Many wireless systems transmit information using variable encoding schemes in which transmitted information is coded, e.g., modulated and/or channel coded, using a code that is dependent on the nature of the information being transmitted. For example, in systems conforming to the GSM standards, provision is made for signaling within the same bandwidth or payload space as normally used for speech. This signaling channel is denoted the fast associated control channel (FACCH), and is operated in a “blank and burst” fashion. In parallel to the FACCH signaling channel there may also be a signaling channel that is sent outside the payload space for speech, denoted the slow associated control channel (SACCH). In order to save bandwidth, the bit rate of the SACCH typically is relatively low. The FACCH typically can be operated at a higher data rate, as all the voice payload space is typically used. However, transmission of the FACCH will blank the speech signal at times.
A mechanism for indicating whether a data block contains speech or FACCH signaling has been provided in the GSM protocol. A data block corresponding to 20 ms of speech is interleaved over 8 slots. Each slot contains a synchronization word, which is located in the middle of the slot. One bit immediately preceding the syncword and one bit immediately following the syncword is assigned to be the indication of the type of payload, i.e., speech or FACCH. As there are 8 slots for each block of data that either entirely contains speech or entirely contains FACCH signaling, there are in total 16 bits for the voice/signaling detection. The mobile station will typically make a majority decision based on the voice/signaling indication bits (the “stealing flag” (SF)). If the payload is being used for speech, all the bits of the SF are set to zero, and if a FACCH is being transmitted, all the bits of the SF are set to one, thus coding two-valued information (“speech” or FACCH) according to a (16,1) code.
This coding and interleaving can provide increased reliability when operating over an error prone radio channel. If frequency hopping is enabled, each slot will be received with in the best case independent Rayleigh fading, which can improve the detection performance of the SF at low vehicular speed.
When designing IS-54, the idea of using stealing flags was rejected. In IS-54, each 20 ms speech segment is only interleaved over two slots, and there is no provision for frequency hopping. In order to achieve performance in IS-54 that is comparable to the performance provided by SF signaling in GSM, the amount of redundancy (coding) would tend to relatively large, potentially reducing the payload and providing a less than desirable speech coder rate.
Instead of using a stealing flag, the discrimination between speech and FACCH for IS-54 is typically performed by examining the payload. For example, U.S. Pat. No. 5,230,003 to Dent et al. describes a technique for determining whether speech or FACCH signaling is sent by exploiting the difference in channel coding of these two signals. Another alternative is to decode according to both hypotheses and then examine the cyclic redundancy code (CRC) check result for both decoding results, as both the speech frame and the FACCH signaling frame are typically CRC coded before channel coding to allow the receiver to verify whether its demodulation and decoding processes resulted in correct recovered data.
The technique described in the aforementioned U.S. Pat. No. 5,230,003 allows the receiver to make a decision about which hypothesis is most likely before the full frame is decoded, thus potentially reducing complexity. If no CRC fields or SFs are provided, this technique may be the only option for determining the coding of a variably coded signal. For example, this may be the only viable technique for differentiating amongst a plurality of speech and signaling signals in the TIA/EIA IS-95 technology.
The current proposal for the Medium Access Layer (MAC) and Physical Layer protocol for packet data in ANSI 136 (referred to as “MANGO”) allows the transmitter to change the modulation and channel coding for each transmitted slot. If channel conditions are favorable, the least amount of redundancy (coding) and the modulation type with the highest constellation point (bits/symbol) preferably is selected. Depending on channel conditions for a specific receiver, the transmitter can select the mode (coding & modulation) to achieve the highest number of net bits in a slot for the given channel condition which produces a desired level of accurate transmission. The receiver typically determines for each received slot (or burst) which mode the transmitter is using.
U.S. Pat. No. 5,757,813 to Raith describes provision of variable coding (modulation and channel coding) for a packet data operation. Two methods of signaling the coding used are described. A separate field indicating the current coding may be provided outside of the field carrying the payload in each slot. This field typically has a predetermined format, including channel coding format and modulation type. Once this field is recovered at the receiver, the rest of the slot can then be decoded based on the information in this field. Another approach involves providing respective different syncwords for respective ones of the coding modes. The receiver correlates or compares the received waveform during the time of the syncword with each possible candidate syncword. The candidate syncword exhibiting the greatest correlation provides an indication to the receiver which coding is applied to the payload-bearing part of the slot.
The first technique described above, i.e., providing a separate field indicating the coding, has been proposed for MANGO. FIGS. 3–6 illustrate respective slot formats for different modulations (π/4-DQPSK and 8-PSK) on downlink and uplink channels. Referring to FIGS. 3–4, the downlink slot format includes synchronization SYNC, coded data frame type/coded superframe phase (CDFT/CSFP) and packet data channel feedback (PCF) fields. The CDFT field includes three data frame type (DFT) bits that indicate the modulation and channel coding. The DFT information, together with five-bit superframe phase (SFP) information, is encoded in a (12,8) code, i.e., the eight information bits are protected by four check bits. In the uplink, as shown in FIGS. 5–6, the DFT information is sent separately from the superframe phase SFP information. Three bits of information of DFT information is encoded into a (6,3) code, i.e. 3 redundancy bits are added to the 3 bits of information.
The second technique described above, i.e., using variable sync words, is used in the Enhanced Data Rate for Global Evolution (EDGE) physical layer protocol of the GSM-based packet data set of protocols known as General Packet Radio Service (GPRS). In EDGE, respective first and second syncwords are used to indicate whether a current “block” is transmitted with 8PSK or with GMSK modulation. A receiver receiving such a signal may correlate the received signal with two candidate syncwords, and use the correlation results to determine which demodulation to apply to further process the data in each block. Similar to GSM speech, a block constitutes four slots.
If a coding indication field outside of the payload is used, improper decoding may be applied if this coding indication field is incorrectly recovered. An error check, e.g., a CRC check, may be conducted after demodulation and channel decoding. If an error has been made regarding the actual coding applied, this check can indicate an erroneously received frame. In some systems, an automatic repeat request (ARQ) protocol can then cause re-transmission of the corrupted frame. However, re-transmission of frames may result in lower net data throughput and increased delay of delivering the payload. Thus, erroneous decoding the signaling field may have a negative effect on performance.
Referring to the above-described MANGO example, when decoding the CDFT field, the SFP information is typically known to the receiver. Thus, the (12,8) code can be effectively treated as a (7,3) code, allowing improved CDFT decoding performance. An example of a decoding process that takes benefit of a known SFP value is described in the aforementioned U.S. Pat. No. 5,751,731. However, even if such an enhanced decoding technique is used, the error performance in decoding the DFT information may be unsatisfactory.
FIG. 7 illustrates results of a simulation run for a non-dispersive Rayleigh channel for a vehicle speed of 8 km/hr at a frequency of 900 MHz. The simulation assumes a perfect Nyquist condition, the x-axis represents the carrier to noise ratio, and the y-axis represents the word error rate (WER) of an equivalent (7,3) Hamming code. The shortened (7,3) Hamming code is capable of correcting one error bit. For statistical simulation, a word error is declared if the number of errors exceed the error correction capability of the code, i.e., in this case, a word error is declared in error if more than one bit error is detected. Hard decision decoding is used for the demodulation, although soft decision decoding could result in slightly better performance. FIG. 7 shows that for the channel conditions of interest, the DFT is incorrectly decoded with a probability of about 1–10%. Thus, about 1–10% of the frames may need to be re-transmitted, which may result in a commensurate reduction in throughput.