IES20000064A2 - Improved Spread Spectrum Communication - Google Patents

Abstract

A method of constructing signature sequences for spread spectrum communication techniques, which is not limited as the sequences constructed are not restricted to certain lengths. The method includes the steps of constructing a Structured Code basis set and interchanging subsequence's of the basis set in a predefined order to provide sets of sequences including a basis set which constructs both conventional Structured Code families in a simplified manner and new families of Structured Codes with other lengths thereby increasing the window of available sequences with good correlation properties for use in spread spectrum communication systems.

Description

Improved spread spectrum communication The present invention relates to spread spectrum communication and more particularly, to spread spectrum communication techniques and applications using signature sequences.

Spread spectrum communication techniques are used for information carrying signals in a iety of communication systems because of their ability to reduce the effects of certain ismission impairments. Many multi-user communication techniques suffer co-channel irference, multiple access interference and intersymbol interference. The use of spread ctrum transmission and reception attenuates these interference types. meal Area Networks (LANs) there is an increasing desire to extend or replace wired Ns in order to increase functionality and maximise the number of potential applications such systems. This trend facilitates the growing need for wireless access. This wireless ess allows mobile computer users to remain in contact with a given corporate LAN over rt distances. Currently available systems provide such connections using either radio or ~*~ared communication technology. For certain system requirements, this communication is adequate. However, the application of such technologies to wireless LANs is relatively new and therefore can prove to be both expensive and unreliable. Furthermore, the data transmission rates achievable are relatively low which significantly limits the number of applications to which the systems may be applied. Coverage of a large area using infrared technology is particularly expensive, even more expensive than the radio communication equivalent. While point-to-point or line-of-sight infrared technology is cheaper than radio technology, it is unsuitable for most wireless LANs.

The main constraint on using any wireless LAN is interference. Infrared transmission particularly suffers from inter-symbol interference produced by multipath propagation effects. Achieving full coverage in an operating environment, while keeping within the limits of eye safety presents another problem to system designers, as does contending with interference produced by natural and man-made light sources that might be present. A further problem that occurs when designing infrared receivers for such systems is that the receivers must provide the required sensitivity and bandwidth at minimal cost.

Achieving full room coverage while avoiding multipath propagation are conflicting requirements in a wireless infrared LAN and full room coverage is essential if reliable communication to and from any point within the room is needed. In order to achieve full room coverage, it is necessary to diffuse the transmitted infrared radiation. While diffuse transmission will reflect off walls and ceilings to fill the room the signal power reaching a given receiver is usually very small necessitating the use of very sensitive receivers. The problem of sensitivity is compounded by the necessity to detect a weak information-bearing signal in strong interference. As a result of these problems there are few commercial infrared wireless LANs or associated systems available on the market.

Commercially available systems therefore tend to use elaborate structures and circuitry, which are expensive when compared with radio technologies. While there is a dearth of infrared technologies that support multi-user communication, there are numerous infrared technologies that support directed line-of-sight transmission. The most commonly available of these is the infrared serial port link based on the Infrared Data Association (IrDA) Standard. IrDA links can operate at data rates up to 4 Mb/s and are used in relatively inexpensive IrDA access points. However, IrDA links are only guaranteed to operate over a one metre range and are limited generally to one-to-one communication. IrDA is primarily intended as a replacement for a single wire-connection and is not intended for multi-user access. Notwithstanding this limitation, the popularity of IrDA clearly illustrates the enormous demand for reliable wireless technology. The best way to achieve multi-user links is to flood the operating environment with infrared light. While this enables multiple-users to connect to a network from anywhere in the operating environment, significant power is lost in such a diffuse environment thereby compromising the signal-to-noise ratio (SNR) at the receiver.

The use of structured codes for such communication is known. Previously known methods construct Structured Codes by deselection of a seed set of sequences containing m sequences each of length w, construction a plurality of cosets from the seed set; construction of a plurality of subsets each obtained by concatenating the sequences of a coset according to a predefined order; and construction of a set of Structured Codes by concatenating subsets of sequences.

The sequences thus constructed may be used to benefit the performance of communication systems based on spread spectrum techniques, however, the construction method described is limited in that the sequences constructed are restricted to certain lengths.

It is an object of this invention to construct in addition to conventional Structured Code families new families of Structured Codes with other lengths. In effect, a new method of constructing Structured Codes to greatly increases the window of available sequences with good correlation properties for use in spread spectrum communication systems.

It is also an object of the present invention to seek to provide a method and apparatus for the generation of improved signature sequences for spread spectrum communication, which will overcome all of the aforementioned problems.

Accordingly, the present invention provides a method of constructing signature sequences for spread spectrum communication techniques, the method comprising the steps of constructing a Structured Code basis set and interchanging subsequences of the basis set according to a predefined order to provide sets of sequences including the basis set which can be used in a communication system.

Preferably, the method includes the step of selecting a seed set and a mask set of sequences.

Ideally, the number of sequences in the seed set is prime or a power of a prime.

Ideally, the method further includes the step of performing modulo addition or biphase multiplication between the seed and mask sets of sequences to produce a basis set.

The construction of sets of Structured Codes by the re-ordering of subsequences of the basis set according to a predefined order.

The invention will now be described, which shows, by way of example only, one embodiment of a method and apparatus for the generation of improved signature sequences for spread spectrum communication in accordance with the invention.

The new method of constructing Structured Codes begins with the construction of a Structured Code basis set followed by the interchanging of subsequences according to a predefined order. For the purposes of this specification, basis set is taken to mean a foundation set of sequences from which a set of Structured Codes can be constructed. A Structured Code basis set possesses the same properties as the Structured Codes constructed from it. That is, a set of Structured Codes including its basis set are very closely related and hence behave in a similar manner with respect to set cross-correlation. In this sense the basis set provides an additional set of sequences, which can be used in a communication system such as Multilevel Code Keying (MCK) or Multilevel Bi-Code Keying (MBCK) in order to improve the bandwidth efficiency of those schemes.

The method for constructing or generating signature sequences will now be presented. In general, sequence elements may be binary or non-binary, real or complex, quadriphase or polyphase etc. with corresponding mathematical operations being performed accordingly.

In the present disclosure binary sequences are used by way of example. A binary sequence with elements e {1,0} is mapped into a biphase sequence with elements € {-1,1} by using the assignment (l-»-l) and (0—>1). Then, modulo-2 addition between binary sequences is achieved by biphase multiplication between the corresponding biphase sequences. In order to facilitate explanations, the following notation has been adopted throughout. A biphase sequence {a·} has elements at s {-1,1}. However, for simplicity in the description of sequence generation hereinafter, the biphase notation a-, e {-,+} is used where symbol refers to "-1” and the symbol "+" refers to "+1". Furthermore, we denote a sequence of length w by {a,·} = (a0, a„ a2, ... awA) while {a(m)} denotes a set of sequences {a/0)}, {α,α)}, {α/2)}, ... {α/'"'0} each of length w. The sequences belonging to the set {a(m)} are usually periodic in w, however, this need not always be the case.

The procedure for constructing a Structured Code basis set is described below. Let denote a seed set of m sequences {ίζ/0)}, {ω/0}, {a/2)}, ... {a/”'0} each of length w as illustrated in Table 1.

Let {6(b)} denote a mask set of n sequences {α/0)}, {<2,·(1)}, {α/2)},... {a/”'0} each of length v as illustrated in Table 2.

When constructing a set of Structured Codes the integer m must either be prime or the power of a prime. Then the Structured Code basis set is constructed by taking the Kronecker product of by {b(n)} as illustrated in Table 3. This procedure generates a set of K=mn sequences each of length N= vw.

In order to construct sets of Structured Codes from the basis set the following procedure is followed. From Table 3 we identify the sequence subsets bf’{a(m)} for 0 That is starting with k = 0, the sequences of b0m{a(m)} are ordered according to the 1st column of the addition table, the sequences of h,(0){a(m)} are ordered according to the 2nd column of the addition table and so on until j = v-1 modulo m. Then for k = 1, the sequences of b0(1){a(m)} are ordered according to the 1st column of the addition table, the sequences of b^){a(-m)} are ordered according to the 2nd column of the addition table and so on until j = v-1 modulo m. This procedure is repeated until k-n-1.

When v < m the re-ordering of sequences is completed using the first v columns of the addition table. When v = m the re-ordering of sequences is completed using all the m columns of the addition table. When v > m the re-ordering of sequences is completed by repeating the columns of the addition table modulo m. In general, the re-ordering can start at any column in the addition table and either ascend or descend modulo m. However, since the initial order of each subset is assigned the natural order of the integer residue class Zm = {0,1,2,.../72-1} then re-ordering commences with column 1 of the addition table for convenience. In previously known methods of generating structured codes, it has been shown that a number of distinct addition tables can be constructed for each value of m. Then a distinct set of Structured Codes can be constructed for each distinct addition table available.

The procedure for constructing sets of new Structured Codes is illustrated by way of example when m = 4 and n = 2. Firstly, a seed set {a(4)} containing 4 sequences each of length 4 is selected as illustrated in Table 4. Also illustrated in Table 4 is the mask set {b{1)} which contains 2 sequences each of length 2.

Construction of the basis set is illustrated in Table 5 and is obtained by taking the Kronecker product of {a(4>} by {ό(2)}.

Four sets of new Structured Codes can be constructed corresponding to the basis set and the addition tables {0 1 2 3, 1 0 3 2, 2 3 0 1, 3 2 1 0}, {0 2 3 1, 2 0 1 3, 3 1 0 2, 1 3 2 0} and {03 1 2,3 02 1, 1 203,21 3 0}.

Table 6 illustrates the four orthogonal sets of Structured Codes constructed.

The four sets of Structured Codes presented in Table 6 can be used in an MCK or MBCK modulation scheme where the basis set is used to supplement the number of available sets of Structured Codes.

The sets of Structured Codes illustrated in Table 6 are of length 8 which represents a new length not previously constructed. When {h(,°} is equal to the transpose of by transpose of {a(m)} is meant the interchange of the rows and columns of {a(m)} when viewed as a matrix, then the Structured Codes generated by the new method described in this document are the same as those generated by the previously known method for the conditions that m is either a prime or the power of a prime.

Without loss of generality, the new method of constructing Structured Codes can use a plurality of masking sets since by taking the repetitive Rronecker product of masking sets will produce one larger masking set which can then be applied as illustrated in this document.

It will of course be understood that the invention is not limited to the specific details as herein described, which are given by way of example only, and that various alterations and modifications may be made without departing from the scope of the invention.